DPMS large frame AR308 Polishing a turd (trouble shooting with solutions)

[357Max]

During this covid lock down I decided to try chasing down the accuracy and reliability issues I've been having with a newly built 80% LR/DPMS 308.

Initially I started this thread looking for some input on a strange bolt issue. I'm now updating it to be sort of a build/trouble shooting large frame DPMS thread. I've seen so many of these started and never run to ground. This will be soup to nuts polishing the turd!

It shoots a consistent 3-4moa 5 shot group @100YDS no matter the ammo. typically 3 shots will be within about 1.5 moa & then at least 2 flyers (mostly lateral).
Tried Win White box 147 to break it in, then FGMM 168, Hornady 178 BTHP, & Lake city M118 173grn.

Shooting done off bench with Atlas bipod and rear bag.

The reliability issues have been bolt override where the case is ejected & next round is partially chambered & smashed against feed ramps with bolt ontop of case (like a short stroke, but it's not).



Build specs: Includes some initial issues (This will be updated as parts were changed).
LR 308 DPMS Gen 1
All parts were new.

• American made tactical DPMS 80% lower
• Davidson Defense LR-308 Billet Upper Receiver .308 Ar (DPMS High Profile Style) This one matches lower and has forward assist in correct location.
• Barrel - Bergara 18" 308 Win. 1/10 SS mid length gas with .750 GB, 5/8-24 thread
• VG6 Gamma 300 BLK 2 port muzzle break with shim kit
• SLR Sentry 7 .750 adjustable gas block 4 screw clamp on.
• melonited mid length gas tube from SLR
• Centurion CMR 12" hand gaurd . Barrel nut did not fit over barrel extension shoulder (opened nut ID .040) Barrel nut also bottomed on upper before tightening barrel (shortened barrel nut .060) Makes me wonder about Barrel/extension. Shouldn't have had to modify barrel nut from Centurion?
• Std upper completion parts (dust cover & forward assist)
• Aero lower parts kit less FCG
• Trigger is Larue MBT-2s 2.5lb 1st and 2lb second stage
• BCG is Toolcraft nitrite with small firing pin & double ejector (didn't want double ejector but that's the only way to get small firing pin for 6.5 conversion later.
• Heavy Buffers XH carbine buffer 8.5oz
• ArmaLite #EA1095 rifle length buffer spring
• Magpul UBR Gen 2 stock with A5 length tube
• Noveske branded Geissele super charging handle
• Wilson Combat/BCM mod 3 grip
• trigger shims.com for front take down pin - lower shimming
• DPMS 10 round steel mags, Magpul 10 rnd mag, Magpul 20 rnd mag, & Magpul 25 rnd mag. Note: there are slight differences between the magpuls.
• Athlon Ares BTR 2.5-15 x 50 in Eotech rings

During inspection to try and figure out accuracy issue. I noticed the barrel extension was imprinting the lugs on the bolt.
I suspect the Toolcraft bolt has a heat treat issue.
FYI - not bashing Toolcraft I have another one of there BCG's in a 6.8 SPC2 and like it. Their Customer service is very good. I sent them the 2 pics below and they replied within the hour with a return shipping label & said it would be replaced.

Anyone have experience with this? Do you think the imprint on the lugs not meshing up perfectly with extension would cause flyers?

The dual ejector BCG was not my first choice, but I wanted the small firing pin & that is the only way I could get it from Toolcraft. I don't think the dual ejectors serve any purpose and may also hurt accuracy.

Could it be bolt thrust?
Chamber polished to smooth (doesn't look to smooth)?
Still scratching my head, hell it's just a 308 with 120 rounds of factory ammo through it. (updated down thread)

Did some searching on the web and found one other guy that tried one of these large frame Bergara barrels in 308. He was shooting 3-4 moa and gave up on it.
I'm also suspicious of the barrel because of the modifications required to make the Centurion barrel nut fit Bergara's extension. (also updated down thread)

The barrel extension lugs look perfect, but I'm not putting a new bolt back on this barrel. Hell I can't in good conscience sell the barrel cheap in the PX.
Guess I should give Craddock a call and order a barrel. (again updated down thread)

Head space was checked, barrel scoped and slugged, gas turned down, tried single feeding, tried 2 different scope/combos, nut and bolted everything with a fat wrench, etc. Still 3-4moa with lateral flyers. WTF
Recoil is very soft in my opinion, although I have no other large frame to compare it to.
I can ease the bolt into battery with just spring pressure riding the charging handle down & no hang ups whatsoever.

 

[Craigm63]

I have a 16” Bergara barrel that shoots pretty well. Keep in mind it was a $100 barrel when I bought mine, QC may not have been the best. They do have a sub MOA guarantee if I recall correctly.

I know that does not answer any of your questions so I will leave it at this, I chased accuracy issues for a year or so with a lower priced barrel. This was a different LR .308 and I would say that it is worth it to just go ahead and replace the barrel to save the time and frustration.

 

[JMGlasgow]

That bolt is soft, and potentially dangerous. Send it back and get it replaced.

 

[mudpig]

Soft bolt is definitely an issue with only 120 rounds thru it. I'd guess that the barrel nut is not fully securing the barrel tightly enough into the receiver. The barrel extension to receiver fit may be sub-par as well. I've found that lapping the front face of the receiver takes a lot of the inconsistencies out as well.....

 

[bfoosh006]

At the moment, I also am going to say a soft bolt.

If the fit was part of the issue, ( or short headspace ) IMHO the bolt wouldn't fit with a loaded round... since it would be to "hardened" to cooperate.

Any marks on the face of the bolt, or the face side of the lugs ? Pic ?

 

[thehun]

Your extension might be out of spec too...

This is why I don't frankenstein AR10s and just buy factory.

 

[357Max]

Good thoughts so far.
Yes Bolt is going back to Toolcraft........their CS is top notch.
I forgot to mention in OP the face of the receiver was lapped.
Had to cut Centurion barrel nut .060 shorter to keep it from bottoming out when torqued to 75lb.

Found another target from 6 months ago. Compare to the one above and you can see the pattern (lateral flyers).

 

[357Max]

Another thought. What is the difference between an Armalite extension vs DPMS? Bergara told me this was DPMS, but I don't know what the difference is if any?

 

[357Max]

At the moment, I also am going to say a soft bolt.

If the fit was part of the issue, ( or short headspace ) IMHO the bolt wouldn't fit with a loaded round... since it would be to "hardened" to cooperate.

Any marks on the face of the bolt, or the face side of the lugs ? Pic ?

 

[thehun]

Good thoughts so far.
Yes Bolt is going back to Toolcraft........their CS is top notch.
I forgot to mention in OP the face of the receiver was lapped.
Had to Centurion cut barrel nut .060 shorter to keep it from bottoming out when torqued to 75lb.

Found another target from 6 months ago. Compare to the one above and you can see the pattern (lateral flyers).

View attachment 7310831

Why on earth do you need to torque the barrel nut to 75 ft/lbs????

 

[redneckbmxer24]

That bolt is definitely trash but I wouldn’t put the accuracy issues past the barrel or optic either.

 

[357Max]

Why on earth do you need to torque the barrel nut to 75 ft/lbs????

Torque range is 30-80........Why wouldn't you?
I torqued it 75 because I felt that would be appropriate (my opinion). Extension is also bedded to receiver with Loctite 620 (forgot to mention that in OP)
Barrel nut is smooth so no need to chase timing of gas tube on this. It was one of the reasons I picked the Centurion hand guard.
I've seen where the service rifle guys play will torque and test 35-80 chasing that last little bit & seems 50-80 is where they settle. That ain't me I'm chasing 3-4moa
The whole point of torque in this case is to put the rim of extension into tension against the receiver face with a 18" lever in my case hanging off.
A lot of the torque is wasted just overcoming thread friction & that friction is even greater on a large frame AR vs AR15.

 

[groves]

Good choice on using the SLR Rifleworks gas block. Have you adjusted it down to where it barely locks to the rear on the last round? The bolt wear could be a result of the bolt trying to unlock too early.

 

[357Max]

That bolt is definitely trash but I wouldn’t put the accuracy issues past the barrel or optic either.

Yes I thought optic initially. The target I added in post #9 is from a few months ago with a different optic (Steiner M332). After the first outing with the rifle a & disappointing performance I took the Steiner off and found the Quick detach base to optic was barely snug. Lock tighted (Blue 242) and torqued it back on with a Fat wrench. Target in post 9 was after re-torquing Steiner. I then switched to the Athlon Ares BTR used for target in OP.

As for the barrel. I'm leaning more and more to it being the culprit. Crazy thing is that I built this rifle in a master AR armorers class where I had access to gauges that are unobtainable (probably every Mil armorers gauge ever made). The level of checking done on this thing was far beyond a typical build because it was a learning experience.
Here is a list of what was checked specific to the barrel front to back:

• gauged muzzle threads for proper pitch/size
• the muzzle thread pitch gauge had a precision 308cal hole in it so when screwed on you then slid in a precision ground rod to check bore alignment of muzzle threads.
• We slid a long precision ground slug through the barrel. This checks both land I.D. and if any warp is present
• measured the gas port size with pin gauges (I cant remember specific size of this one, but it's in my class notes). Edit: It is .074 mid length gas
• scoped the barrel looking at Crown, gas port, rifling, machining/tool marks, chamber - throat alignment/transition, & aligned gas block over port with scope.
• head space check with go, no go, & if necessary field (it wasn't)
• feed ramps were mocked up in upper and then blended to M4 notches in upper
• barrel extension teeth were deburred
• edit: forgot one, there was a gauge (don't know proper name) that had hash marks & you slid it in the muzzle end to check if the bore swelled at the muzzle threads. It was good.

The crazy thing is that this barrel passed all those checks with flying colors. I built a 300BO pistol in the same class with a used Balistic advantage barrel that gauged loose on land I.D. Based on the inspection I thought the 308 would be a shooter, but in practice the 300BO pistol shoots better & thats with a Red dot.

This is all part of the Hobby. I enjoy building my own rifles & learning along the way. It's a matter of perspective...frustrating vs Challenging, I try to chose the later. It will feel good when I finally get this thing working.

To be honest part of the reason for posting this is so others down the road will have it for reference. What was tried & worked vs what didn't, how to trouble shoot, etc. I've seen far to many posts like this start where there is never a follow through. My plan is to stick with this post till I get it sorted.

Suggestions are welcome & so is debate, but please give rational for either.

 

[357Max]

Good choice on using the SLR Rifleworks gas block. Have you adjusted it down to where it barely locks to the rear on the last round? The bolt wear could be a result of the bolt trying to unlock too early.

Yes SLR are a great design, it's set on the edge for 168FGMM 1 click up from intermittent fail to lock back.
I'm not a fan of the dual ejector in these Toolcraft HP bolts. It still wrecks the brass necks and throws brass 8' with gas turned down. I just bought a Rubber City HP single ejector bolt from Billiam1211 this morning. So it will be used going forward (not in this barrel though).

Edit: Updated down thread.

 

[mudpig]

Ok.....you lapped receiver face.....bedded barrel extension into receiver. How was the barrel extension to barrel thread fit???? If those threads are loose, doesn't matter if you torqued nut to 150fp, the barrel is still going to have some wiggle.

 

[357Max]

Thread fit was good, not sloppy at all. I had already dry fit it (mocked it up) several times before the build. The class did not have machinist equipment so I had to have my ducks in order beforehand.
Below is a picture of my set up in a rotary table to mill barrel nut step I.D. +.040 bigger to pass over barrel extension & same setup shortened barrel nut .060 receiver side as it would have bottomed against receiver before tightening against extension step.

Extension - receiver fit was not bad, about .001 slip fit hence the Loctite 620. there was not enough room to add .001 stainless shim stock.

The lack of mill spec on these large frames means checking everything is mandatory.

Barrel nut was installed with light grease. Nut was torqued/loosened (at reduced torque) 3 times before final torque.

 

[mudpig]

Wow.....covering all the normal places to look.
You said the crown was good to go. Did you slug the barrel for major diameter (groove diameter). Just grasping at straws now

 

[357Max]

Wow.....covering all the normal places to look.
You said the crown was good to go. Did you slug the barrel for major diameter (groove diameter). Just grasping at straws now

Aside from visual with bore scope no. The barrel gauge was not rifled so it rides on lands and this barrel was dead nuts.

 

[357Max]

Update & this will take more then 1 post.

Toolcraft replaced the bolt so I thought I'd give the Bergara barrel 1 last go. The new bolt is stamped MPI vs the old one that was not.
Toolcraft included a new firing pin & sent me back the old bolt for spare parts. I decided to trim the ejector springs in an attempt to not beat the shit out of my brass. After trimming the springs from the old bolt I Found out Toolcraft changed the ejector bore & spring size on the new bolt (Fuck!) Rinse repeat with springs from new bolt. (shortened springs .050)

While wrenching on this Turd I pulled the rail & sure enough I see a rub mark where the bottom of gas block appears to just be touching the spine in the bottom of the CMR rail. Out comes the Die grinder to clearance this & back together it goes fingers crossed.
And let me not forget to take the damn muzzle brake off, next range trip with thread protector only. Hell what's left to try? Nothing I can think of.

So I took it to range and forgot the DPMS steel 10 round mag I'd previously worked over (polished and tweaked lips). Great now I'm single loading mag fed to avoid the bolt override issue (Fucking Turd is getting fixed come hell or high water). Leaning heavily towards "Rotten Bitch" for it's new name.

Range report:

First the good = Brass is now hitting the deflector square and no longer denting the hell out of the neck. The brass stripe shown below was not there before. It use to just wack the neck shoulder junction on the outer edge of the deflector with these dual super fucker ejectors. Were good on that now

Now the bad: Best 5 shot group was about 2-1/4" with FGMM 168's
Rotten Bitch 308 is getting a new barrel. I'll have to eat Crow if I just suck that bad with a large gasser Stay tuned.
Proof Carbon 18" 1-11 twist rifle gas with matching JP high pressure bolt inbound.

 

[357Max]

Parts changing time.

Before disassembling I noticed the damn Centurion rail had walked forward again. Had about .60 gap to receiver. Pretty frustrating since I'd Red loctited the guard screws & pushed the torque on them high as I dared to prevent this.
Placed a call to Centurion and the nice lady said where did you by the rail (was it Rainier Arms), Yep sure was. She said it must have been the batch with bad barrel nuts. They shipped me a new nut, wrench, & guard screws free of charge. Great service!
New barrel nut has the recessed ring properly located to firmly lock the rail against the receiver when the middle screw is installed. The redesign also uses a better wrench design.

This new nut also fit over the barrel extension of the new proof barrel It still hit the receiver before clamping the extension. This may be the upper that's a little out of spec so back to the mill to take .040 off.

After looking at the new brass pattern on the deflector I decided to try making an improvement while I had it apart. Used the old barrel clamped in vise to position the upper in mill and had a go at the deflector. Aligned the notch with the brass stripe that'd been left. Finished it off with a chemical acid etch that turns aluminum Black.

Mocked up new barrel to check feed ramp alignment with M4 notches in upper. They needed some TLC with the die grinder. Sorry no after picture of this but they align tits now.

Almost ready just needed to double check head space and in the process noticed 2 issues.

• The bottom ramp of the Toolcraft bolt carrier looks like it was machined by a Monkey on a Friday. I'm not going to try correcting this since it's nitrited & I don't think this should effect function.

• The JP 1 pc gas ring is a complete piece of shit! I swapped it for the 3 pc rings from the old spare parts Toolcraft bolt. Now it will not only not fall out, but will actually support the weight of the carrier. I no the latter isn't mandatory, but it gives me piece of mind.
  1. JP ring is to small. Measured .005-.006 undersized from 3 other sets of rings I had to compare to. JP advertises them as low friction, yeah as in the fucking bolt will fall out of the carrier when turned upside down. IDGAF about friction, they need to seal & these wouldn't have.
  2. There are 2 mechanisms that cause rings to seal. First is that gas gets behind the ring through the gap and expands the ring to the bore. Second is the ring flexes or rocks in the bore as the bolt moves resulting in the edge of the ring sealing. 1 pc rings even if the right size will do neither of these. There's a good reason 1 pc multi spiral rings aren't used in internal combustion engines (hint).

What the hell, switching out the Larue trigger for a Geissele SSA-E. After having some trigger time with both I much prefer the Geissele. Larue is a good trigger but it will be going in a less precision oriented 350L pistol.

You may notice in the above picture that the fire control pocket looks a little off center to the left. It is, and that is because I machined the lower in a mill vise, not a 80% jig. I indicated off the front pivot pin location & that is how far off center it was. It's matters more that the front to back pivot & take down pins + fire control pocket align to keep everything in a straight line without binding. Had I not done this the hammer would have been off center. Keep that in mind for your 80% jigs. Firing pin wear pattern on hammer of this rifle is dead center.

The new barrel is rifle length gas and .875 so time to fit up the new SLR adj block. Gas port on this proof barrel is .090 BTW. Looks like the port location in the barrel is set to allow for a stock type hand guard so the block needs to be spaced out a little. Since I have a lathe I just made a brass .028 spacer.

LR 308 DPMS Gen 1 Take 2
All parts were new.

• American made tactical DPMS 80% lower
• Davidson Defense LR-308 Billet Upper Receiver .308 Ar (DPMS High Profile Style) This one matches lower and has forward assist in correct location.
• Barrel - Bergara 18" 308 Win. 1/10 SS mid length gas with .750 GB, 5/8-24 thread Proof Carbon 18" 11 twist rifle gas .875 GB 5/8 x 24 thread
• VG6 Gamma 300 BLK 2 port muzzle break with shim kit Heathen .30 cal 3 port self time brake.
• SLR Sentry 7 .750 adjustable gas block 4 screw clamp on. SLR Sentry 8 .875 AGB set screw style.
• melonited mid length gas tube from SLR Aero rifle length melonited tube.
• Centurion CMR 12" hand guard . Barrel nut did not fit over barrel extension shoulder (opened nut ID .040) Barrel nut also bottomed on upper before tightening barrel (shortened barrel nut .060) Makes me wonder about Barrel/extension. Shouldn't have had to modify barrel nut from Centurion? New barrel nut cut .040 to clear receiver.
• Std upper completion parts (dust cover & forward assist)
• Aero lower parts kit less FCG
• Trigger is Larue MBT-2s 2.5lb 1st and 2lb second stage Geissele SSA-E 2 stage 3.5lbs
• BCG is Toolcraft nitrite with small firing pin & double ejector JP HP small firing pin bolt, swapped gas ring to 3 pc.
• Heavy Buffers XH carbine buffer 8.5oz
• ArmaLite #EA1095 rifle length buffer spring
• Magpul UBR Gen 2 stock with A5 length tube
• Noveske branded Geissele super charging handle
• Wilson Combat/BCM mod 3 grip
• trigger shims.com for front take down pin - lower shimming
• DPMS 10 round steel mags, Magpul 10 rnd mag, Magpul 20 rnd mag, & Magpul 25 rnd mag. Note: there are slight differences between the magpuls.
• Athlon Ares BTR 2.5-15 x 50 in Eotech rings Leupold MK 5 3.6-18 TMR in Leupold mount.

Rotten Bitch is Done, well almost. There's still the reliability to tackle.

First range outing. The Rotten Bitch was destroying 1 out of every 3 rounds so I had to load mags single round mag fed.
1st pic is first 5 rounds out of barrel FGMM 168 MK 100yards for zero shot on left is first, far right is second (oops I went 3 mil right instead of 1.5), center is next 3 after 1.5 mil back left.

Next 5 rounds out of the barrel .1 mil up @100yards

No more 2.5 - 4 moa bullshit. Rotten Bitch shoots now.

Stay tuned for reliability fix for this Whore.
Hears a picture of a $10 Whore right here.

 

[sinister]

Nice trouble-shooting. You covered quite the range of potential problems to isolate your grievances. To start, if the Proof doesn't outshoot the Bergara there's definitely something wrong.

Arm-chair quarterbacking, I'd swap out the Armalite spring for a Tubb flat wire.

Did you epoxy the front takedown pin shims to the lower ears so they don't allow lateral slop?

I've never seen anyone get optimum precision from ANY rifle using an Atlas bipod for proofing -- you'll have both pan and tilt variance. Can you bag both front and rear?

Strong work, man, and nice looking rifle.

 

[Bolo]

Damn nice writeup...
Can't wait to read what you find in the reliability arena.
Personally, I'm betting on the "Stevie Wonder"-style carrier.

 

[silentwoods]

This ^^^^^
Excellent work and information.
Thanks for taking the time to post it all.

 

[hellishot]

That was an enjoyable read. I learn quite a bit throughout OP’s journey.

 

[hlee]

What is it doing to thrash your loaded rounds?

 

[Jumper]

I'm late to the party but read the whole thread. On the reliability issue I'm leaning towards it being overgassed. The bolt carrier is cycling too fast for the magazine spring to lift the next cartridge into position. Also a .090 gas port ( I think that what I read) is huge. And brass launching 8ft,... overgassed. I'd shut the gas completely off and open it incrementally with a 1 round magazine inserted until it locks back on empty.

Just my 2 cents. Thanks for documenting your process. This is a good thread.

Edited to add: this is assuming the magazines are working properly; proper springs and followers etc.

 

[Keel Haul]

I can’t comment on any of the other issues you’ve been having but this caught my eye-

“The reliability issues have been bolt override where the case is ejected & next round is partially chambered & smashed against feed ramps with bolt ontop of case (like a short stroke, but it's not).”

I had a DPMS 223 factory lower that was doing this very same thing and it drove me nuts.Every second or third shot, it would hang up just as you described and no matter what parts I swapped in and out, it kept happening. I had enough swapped parts in the end to build a new rifle, which I did on a different receiver and it performed flawlessly. Stumped I had two different smiths check it out and were of no help. After telling a friend of my issues he checked it out and compared it to his rifle and in about three minutes he says “your bolt catch is too low.”

So he takes a mag and inserts it into his lower and marks it with a pencil where it’s sitting in his rifle, he then takes the mag and puts it in my DPMS lower and you can see it’s sitting low in the receiver. I get a new receiver, not from DPMS and take out all the parts from the DPMS and move them over. Flawless performance.

This may or may not be your feeding issue but it’s worth looking into.

 

[hlee]

I can’t comment on any of the other issues you’ve been having but this caught my eye-

“The reliability issues have been bolt override where the case is ejected & next round is partially chambered & smashed against feed ramps with bolt ontop of case (like a short stroke, but it's not).”

I had a DPMS 223 factory lower that was doing this very same thing and it drove me nuts.Every second or third shot, it would hang up just as you described and no matter what parts I swapped in and out, it kept happening. I had enough swapped parts in the end to build a new rifle, which I did on a different receiver and it performed flawlessly. Stumped I had two different smiths check it out and were of no help. After telling a friend of my issues he checked it out and compared it to his rifle and in about three minutes he says “your bolt catch is too low.”

So he takes a mag and inserts it into his lower and marks it with a pencil where it’s sitting in his rifle, he then takes the mag and puts it in my DPMS lower and you can see it’s sitting low in the receiver. I get a new receiver, not from DPMS and take out all the parts from the DPMS and move them over. Flawless performance.

This may or may not be your feeding issue but it’s worth looking into.

I kind of thought the same thing, and then looked at the build list.

80% lower...

 

[mudpig]

I can’t comment on any of the other issues you’ve been having but this caught my eye-

“The reliability issues have been bolt override where the case is ejected & next round is partially chambered & smashed against feed ramps with bolt ontop of case (like a short stroke, but it's not).”

I had a DPMS 223 factory lower that was doing this very same thing and it drove me nuts.Every second or third shot, it would hang up just as you described and no matter what parts I swapped in and out, it kept happening. I had enough swapped parts in the end to build a new rifle, which I did on a different receiver and it performed flawlessly. Stumped I had two different smiths check it out and were of no help. After telling a friend of my issues he checked it out and compared it to his rifle and in about three minutes he says “your bolt catch is too low.”

So he takes a mag and inserts it into his lower and marks it with a pencil where it’s sitting in his rifle, he then takes the mag and puts it in my DPMS lower and you can see it’s sitting low in the receiver. I get a new receiver, not from DPMS and take out all the parts from the DPMS and move them over. Flawless performance.

This may or may not be your feeding issue but it’s worth looking into.

I have seen this very issue with a few AR10 platform 80% lowers. The mag catch itself can be on the small side and the inlet in the receiver for the mag catch can be too low from the deck to hold the mag high enough to reliably feed rounds from the mag. Higher quality parts kits will contain full size mag catches and the receiver inlet and be milled slightly higher, then epoxy bedding the lower portion of the inlet in order to raise the location of the mag catch.

 

[sinister]

Looks like the AmericanMade lower comes with mag catch slot already machined-out. Tolerance stack may or may not apply to the Aero mag catch in OP's lower parts kit.

 

[NCHillbilly]

I have followed this thread intently, as I have been collecting parts for my first "ground-up" build. I wanted to do this not because I think I can out-build the major mfg's, but because I want the challenge and satisfaction (hopefully) of being able to say I built it. My question is this: Has anyone had any experience using Noreen receivers? I was really excited about this project until I read this whole thread. Now, not so sure. I feel like I have purchased decent quality pieces, But sounds like the OP bought decent stuff, for the most part, also. Fingers crossed.

 

[357Max]

Nice trouble-shooting. You covered quite the range of potential problems to isolate your grievances. To start, if the Proof doesn't outshoot the Bergara there's definitely something wrong.

Arm-chair quarterbacking, I'd swap out the Armalite spring for a Tubb flat wire.

Did you epoxy the front takedown pin shims to the lower ears so they don't allow lateral slop?

I've never seen anyone get optimum precision from ANY rifle using an Atlas bipod for proofing -- you'll have both pan and tilt variance. Can you bag both front and rear?

Strong work, man, and nice looking rifle.

The odd thing is that with the changes to rifle gas @ same barrel length etc, reliability got worse. Rifle gas Should've delayed unlocking even with std Armalite spring. That particular spring was reccomended from Slash aka Heavy Buffers.
I did not epoxy the pivot pin shim. .002 is all that would fit and it's very snug. Purchased them here BTW https://triggershims.com/
The upper - lower fit is very tight. Zero rattle.
As for bagging front and rear. I've never tried & honestly the goal for me isn't to shoot a 1 hole group, but just to have confidence the rifle is more capable then I. Then I can focus on fundamentals & look for a good training opportunity.

I'm late to the party but read the whole thread. On the reliability issue I'm leaning towards it being overgassed. The bolt carrier is cycling too fast for the magazine spring to lift the next cartridge into position. Also a .090 gas port ( I think that what I read) is huge. And brass launching 8ft,... overgassed. I'd shut the gas completely off and open it incrementally with a 1 round magazine inserted until it locks back on empty.

Just my 2 cents. Thanks for documenting your process. This is a good thread.

Edited to add: this is assuming the magazines are working properly; proper springs and followers etc.

Good thoughts. In this case though I think Proof has the port about right. When switching from the mid gas .074 port to the rifle gas @ same barrel length you loose dwell time & to make up for it the port has to be bigger. 18" rifle gas port size should be .089 - .092 from everything I could find.

I can’t comment on any of the other issues you’ve been having but this caught my eye-

“The reliability issues have been bolt override where the case is ejected & next round is partially chambered & smashed against feed ramps with bolt ontop of case (like a short stroke, but it's not).”

I had a DPMS 223 factory lower that was doing this very same thing and it drove me nuts.Every second or third shot, it would hang up just as you described and no matter what parts I swapped in and out, it kept happening. I had enough swapped parts in the end to build a new rifle, which I did on a different receiver and it performed flawlessly. Stumped I had two different smiths check it out and were of no help. After telling a friend of my issues he checked it out and compared it to his rifle and in about three minutes he says “your bolt catch is too low.”

So he takes a mag and inserts it into his lower and marks it with a pencil where it’s sitting in his rifle, he then takes the mag and puts it in my DPMS lower and you can see it’s sitting low in the receiver. I get a new receiver, not from DPMS and take out all the parts from the DPMS and move them over. Flawless performance.

This may or may not be your feeding issue but it’s worth looking into.

@Keel Haul - You win the prize and the prize is a brand new super low friction JP gas ring
Read on in the next post for trouble shooting process and solution.

 

[357Max]

I have followed this thread intently, as I have been collecting parts for my first "ground-up" build. I wanted to do this not because I think I can out-build the major mfg's, but because I want the challenge and satisfaction (hopefully) of being able to say I built it. My question is this: Has anyone had any experience using Noreen receivers? I was really excited about this project until I read this whole thread. Now, not so sure. I feel like I have purchased decent quality pieces, But sounds like the OP bought decent stuff, for the most part, also. Fingers crossed.

@NCHillbilly - Sorry but no experience with Noreen receivers. Hopefully this thread may help you trouble shooting if needed. I look forward to a thread on your build.
I ruled out the Noreen because I wanted hard Black Anodized & I don't think Noreen offered it.
If/when I build another 80% large frame it will be on JL Billet receivers for sure! I built one of the JL billet 80% small frame ambi lowers & machining/fit was excellent. JL Billet is also the only Ambi 80% I'm aware of.

 

[NCHillbilly]

I certainly have learned a lot, following this thread, and I absolutely will post once it's finished and I've had a chance to shake it down. Fortunately, I don't get discouraged very easy, so if it doesn't run right, I'll fix it. I've built a few ar lowers, so this is just the next logical step. Not looking to make a living doing these things, but would like to build a few just to say I did. Regarding the anodizing, I just finished a 224 valkyrie that I duracoated the receivers, handgaurd, stock, and scope mount. It looks great,and if it is as durable as advertised, I'm gonna like using that stuff. It's kinda in my wheelhouse anyway, as I'm a body and paint guy.

 

[357Max]

Before I get into the reliability trouble shooting I think it'd be worthwhile to discuss a few things related to 80% receivers for these large frames.
Building one of these is a challenge plain and simple! If you don't have access to the proper equipment & skill set I don't recommend starting with a large frame 80%. It is very important to have a good fundamental understanding of the large frame operating systems & how they interact before attempting one.

I've never seen all the possible large frame 80% quirks itemized in one place & keep in mind this is just for DPMS gen 1 & just as it relates to lowers.
Here goes:

• There is no mill spec!!!!
• The original Armalite AR10 was not an adaptation or scaled up version of an AR 15, the AR 10 was actually designed first.
• There are multiple receiver patterns that don't play well with each other. Example Armalite vs DPMS etc. This includes receivers of the same pattern that don't play well. The lower 80% receiver used for this build (American made tactical) is a DPMS gen 1 & it does not fit well with an Aero upper for example. I selected the Davidson defense LR 308 upper because it matched the lower in width across fire control pocket & it had the forward assist in the correct location to actually function (many do not).
• Prior to machining I scoured the internet for a DPMS blueprint. What I found was 3 different versions for DPMS gen 1 (Swell)!
• There are 3 different locations for the fire control group (trigger pins & safety selector) measured from the front pivot pin center line.
• There are at least 2 different locations for the bolt catch location.
• Although pre machined, I found 3 different locations for the mag catch measured from the deck of lower (all supposedly for the same mags).

Prior to machining this 80% I discovered the above. I'd purchased 2 x 80% billet AR 15 lowers from American made tactical (AMT) and sent them both back because they were out of spec. Rear take down pocket was to wide (never would have been able to get slop out). AMT forged 80% AR 15 lowers are GTG.
So how do you check spec when there is none?? How do you know your fancy 80% jig has shit located in the right place??
Answer to both is you don't.
I chose to abandon the idea of using the 80% jig. Instead I mocked up the upper with my BCG and took dimensions to figure out where the trigger pins needed to be to have the hammer square to the firing pin when struck. I then compared the required location to the existing safety detent hole pre drilled to see if it would work (it did).
To machine it I chucked the lower in a mill vise that was indicated to the table on the X axis. The AMT DPMS lower does not have the rear pocket pre-milled so I indicated from the front pivot pin center line in both the X & Y axis. I then discovered the AMT lower was actually machined a little off center (bastards), but determined it was not enough to cause any problems if I aligned everything. What was off center was the front pivot pocket, mag well, buffer detent hole, & buffer tube threading. At least they were all aligned with each other so I machined the fire control pocket & rear take down pocket in that same plane.

To sum up the above, had I thrown this thing in the 80% jig it would have been a wreck.
If your not prepared to go to this level of checking then I'd recommend purchasing a matched upper/lower & purchase the jig from the same company selling the lowers. At least that way you may have some recourse if it's FUBAR.

 

[357Max]

Alright onto reliability trouble shooting. @Keel Haul already nailed it, but I'm still going to post the long version to help others with trouble shooting.

The issue again is that after ejecting the bolt partially picks up next round and jams it against the extension (bolt head over case rim).
Typically the issue described is usually caused by one of 2 issues:

• short stroking where the bolt doesn't go back far enough to grab the back of the next round and kind of drags it out of mag.
  • Caused by being under gassed or overly heavy bolt carrier/buffer/spring combo.
    • under gassed caused by one or combination of; miss aligned gas block, undersized port in barrel, improper length gas tube, improperly adjusted gas block, bad gas rings on bolt, loose gas key on bolt carrier, out of spec bolt carrier gas run bore or bolt tail bore.
      • Diagnostic check for this is to confirm bolt lock back on empty mag after last round. If yes then confirm head room, meaning your not already wide open on gas block to just barely achieve lock back. I was at 8 clicks out with 15 being wide open. If heavy BCG/buffer combo try lightening buffer.
• The other typical culprit is to much bolt speed where the bolt returns to grab next round before the mag spring has it fully in place.
  • Caused by being over gassed, over sprung, or light weight bolt carrier/buffer combo. Worn out mag spring is also a possible cause for this.
    • Diagnostics for this include; Check buffer stopper to see if it's impacting back of buffer tube, confirm at least .080 over travel of bolt carrier beyond bolt stop, turn down gas, remove suppressor, add buffer weight, change mags to new or known good mag.

At this point many parts have been changed and precision is no longer an issue, however the reliability got even worse with the parts changes.
What could cause it to get worse?


Basic checks first:

• SLR adjusted to achieve bolt lock on last round. Still malfunctioned.
  • possible issue excess mag spring pressure on bolt when full mag loaded causing bolt drag.
    • attempted fix open SLR block for full gas. Still malfunctioned, but oddly seamed a little better. (Under gassed ruled out!)
    • try different mags loaded light 3 on 10, 5 on 20, check if feeding issue from one side of mag only. (Nope!)
    • Possible heavy BCG/Buffer combined with rifle gas made things worse?
      • Removed 2 x Tungsten weights from XH buffer & replaced with Aluminum spacer. Buffer weight now 6.05 oz from 8.5. Results...Fail!
      • Added JP OSR 308 buffer spring + 7% pressure & OD ground. Results......fail!
• Buffer stopper has never hit back of tube from the looks of it. Due to heavy BCG/buffer combined with AGB turned down I ruled out excessive bolt speed.

My first thoughts were that the original buffer system was specked out for a mid length gas system & the switch to rifle length may have made things worse.
So I tried lightening the buffer from 8.5 to 6.05 oz to no effect. Threw the JP + 7% spring in there since I had it, also to no effect.
Hunting around the google I saw a lot of similar issues with DPMS large frames where guys had basically tried every mag they could find and maybe settled on one mag and or ammo that seemed to function pretty well.

This was unacceptable to me as this is meant to be a battle ready rifle that will run any mag, any ammo, anytime!
The next logical thing to focus on as Keel Haul pointed out was the mag catch. I dug out my folder from the lower machining and the mag catch location was within the spec of the 3 different locations, but on the lower end. I found a .045 variation for the mag catch location across 3 different blueprints WTF.
Even though it was "within spec" I honed in on this as the culprit & compared bolt - mag mesh of my small frame AR to this DPMS.

As you can see in the pictures below the small frame 6.8SPC bolt lugs sit well below the mag feed lips and the DPMS did not that "Rotten Bitch"

Hard to believe nobody makes anything to fix this?

Found a hunk of 7075 T-6 in the garage, time to get busy!

I'll bet money this is a more common problem then people realize, so it's hard to believe no one is making stepped mag catches. I'd have gladly paid for one & a simple file would be all that's required to modify the lower.

I chose to machine the new mag catch to step up .045 putting it about .010 above the high side of spec. I find it curious that when installed the Magpul step at back of mag now is perfectly level with the deck of lower. This would be an easy check to diagnose this problem.

Bolt now gets a good purchase on back of case head.

If you don't look to close it'd be easy to miss the modification.

So now all I need is the shittiest ammo I can find!
Aguila 7.62x51 150grn Hecho En Mexico............. Terrible reviews for FTF, horrible accuracy, filthy etc perfect pile of shit to test with right there!

40 rounds through 4 different mags including a 10 round rapid fire at steel @300yrds zero issues with gun. Ejection pattern in a nice pile @ 4:30 & no dents in brass Fuck yeah!!!
The Aguila lived up to it's horrible reviews. Brass soft as butter, filthy, horribly inaccurate, and 1 pierced primer!
Just look at the fucked up case heads of the Aguila compared to FGMM case heads. The ejector swipes on the Aguila raised a bur (not a scratch on FGMM) & what the hell are the primers (Brass cups?). The center Aguila is the pierced primer. That crap isn't even re-loadable in my opinion.
Another fun fact is the case heads of the Aguila measure .4675 vs FGMM @ .470 so the Aguila would make a low mag catch issue worse.

It's a "Sexy Bitch" now!
Time to get back on a diet of the good stuff. Case of Federal GMM 175 BTHP inbound for next range trip.

Before.

After.

Might run back through this thread and add some more info, checks, pics etc.

.224 Valkyrie build on deck. Cheers!

 

[NCHillbilly]

Well damn, don't know if I want to take this on or not. I'm not a machinist, so I may have a pile of parts for sale.

 

[NCHillbilly]

FWIW, the upper and lower are.both the same brand, and forged. I'm gonna give it a shot. I may have to revisit this thread, hopefully it won't put up too much of a fight. Thank you so much for documenting your journey. I'm certain others will find it helpful.

 

[Keel Haul]

@357Max I am glad my bad lower experience helped you out here. I am blown away how simple the fix was when you made the stepped mag catch. Absolutely brilliant fix.

 

[357Max]

Quick update. Gun is now at 70 trouble free rounds & it likes FGMM 175's.
I may put the tungsten back in the buffer to bring it back up to 8.5oz. Never really found a good explanation for tuning buffer weight. I'm thinking it may help a little to spot shots. I'm sure there is a trade off between slightly softer recoil vs heavier reciprocating mass.
Any of the AR experts @padom have an opinion on this? I'm thinking it may help a little to spot shots at higher magnification.

I may upgrade to an Atlas Cal & I wonder if I'm leaving anything on the table with the SSA-E? For now I'm sticking with it, and will just work on learning to drive it better.

100 yards zero check with 175's

500 yards & Think this plate is 8" could be 10" & not much wind 2- 3ish from 10 o'clock. held left 1/3 Scope @ 12x with 3.8 mil elev.

 

[sr_carver]

might be a headspace issue. I like to buy the bolt and the barrel together so they can assure me its headspaced correctly.
pic is a 6.5 cm on an aero frame kreiger barrel 1-8". Still looking for <.5 MOA and I think it'll get there. If your gun wont shoot around 1 MOA there must be a reason