Accuracy and Upper Receivers.

[Bigjake83]

I wonder how much effect our use of glue, and/or oversized extensions/tighter receivers has on the whole thing?

Jake?

Some are nearly an interference fit, and the ones that are not almost universally get glued.

Yes it is critical to have proper fitment with the barrel Extension and Receiver. If you don't want to spend the extra $60 for a oversized barrel extension, then use High Temp Red Loctite. If we are talking about a Precision Rig then don't Half Ass it! Polishe the Barrel extension to a mirror finish, true your Receiver Face (by a Professional) and Bed or Thermal Fit the Barrel, make sure to season the receiver theads and apply some Anti Seize, and lastly don't over torque the barrel nut, 50-65 FTLBs is ideal.

 

[natdscott]

...and lastly don't over torque the barrel nut, 50-65 FTLBs is ideal.

My opinion has always been that..for the price of a lot of barrel nuts, when you reach proper torque, just cut out anything that's in the way of the gas tube. I know that isn't always an option, but if it is, it makes sense.

They are a HUGE increase in cost over a mil-spec M4, but the "comp" extensions aren't $60.

The proprietary 20TPI from Holliger is $42: https://www.whiteoakarmament.com/competition-barrel-extension-m4.html I used one on my 1,000 yard upper. Works fine. Probably entirely unnecessary, but man it checks a box for the ticky, and I do think the finer threads are a good idea.

BAT Machine's is only $31, and is standard dimensions, albeit a little larger shank, depending on which you roder, 1.000" or 0.999". I cannot for the life of me figure out why a guy spec'ing out an accuracy AR would NOT call BAT and get one of these. Given the rest of BAT's work, the extensions are probably dimensionally perfect.

https://www.batmachine.com/batactions/ar-15/

I DON"T have a BAT extension to prove that to myself, but if you've never seen a lathe-built Remington 700 action screw, man their's are truly a work of art.

Pair a 0.999" BAT with a BCM tight receiver, and there wouldn't BE any bedding. I don't think you'd get a 1.000" in without turning some material off the shank, or nitrogen fitting.

-Nate

 

[whatsupdoc]

Unfortunately the Seekins test isn't going to happen because I'm now 0-2 in getting them to deliver me a usable receiver. My Valkyrie barrel has been bedded back into the VLTOR receiver and will be staying in it and the Seekins going back for a refund.

I've got several bolt guns and have had a ton more, almost all free floated in chassis or bedded stocks and I've never experienced any shift. The only time you're going to have any change on a bolt gun is if there's something wonky with the action to stock fitment. They're steel receivers with a lot of meat on them, they aren't going to flex enough under any normal or extreme use to see any change.

What is the issue with the Seekins upper?

 

[Potss]

Has the Seekins upper come back yet?

 

[padom]

My opinion has always been that..for the price of a lot of barrel nuts, when you reach proper torque, just cut out anything that's in the way of the gas tube. I know that isn't always an option, but if it is, it makes sense.

They are a HUGE increase in cost over a mil-spec M4, but the "comp" extensions aren't $60.

The proprietary 20TPI from Holliger is $42: https://www.whiteoakarmament.com/competition-barrel-extension-m4.html I used one on my 1,000 yard upper. Works fine. Probably entirely unnecessary, but man it checks a box for the ticky, and I do think the finer threads are a good idea.

BAT Machine's is only $31, and is standard dimensions, albeit a little larger shank, depending on which you roder, 1.000" or 0.999". I cannot for the life of me figure out why a guy spec'ing out an accuracy AR would NOT call BAT and get one of these. Given the rest of BAT's work, the extensions are probably dimensionally perfect.

https://www.batmachine.com/batactions/ar-15/

I DON"T have a BAT extension to prove that to myself, but if you've never seen a lathe-built Remington 700 action screw, man their's are truly a work of art.

Pair a 0.999" BAT with a BCM tight receiver, and there wouldn't BE any bedding. I don't think you'd get a 1.000" in without turning some material off the shank, or nitrogen fitting.

-Nate

I dont use BAT extensions anymore because of how sharp the back side of the lugs are. Just takes nasty chunks of brass out of the cases on extraction. Now you can take a Dremel 520-02 Impregnated wheel and polish the back sides (as suggested by JP) to smooth them out which I have done and works. Pain in the ass when the extension is already on the barrel and make sure you flush everything thoroughly.

I now use JP extensions only. They are already polished and coated and are smooth as butter. Yes, they are more expensive but the best extensions I have handled.

 

[padom]

I just put my 20" Keystone Kreiger 1:7 .223 Wylde in a MEGA SBU Billet upper and lower with a YHM 15" TJ Competition handguard. Talk about a tight fitting upper/lower and a beefy upper. Thats an understatement. I honestly though the upper/lower didnt fit together till I got it perfectly straight up and down and pressed it into the lower. Ive built a lot of AR's from all kinds of forged and billet receivers but this is by far the tightest set EVER.

On sale at JSE Surplus right now. Hard to beat this price for one beefy, tight fitting setup.


Regular MEGA SBU Upper

http://www.jsesurplus.com/AR-15SBUBilletUpperwithoutSideChargingHandleSlotJSEEXCLUSIVE.aspx

Side Charging MEGA SBU Upper

http://www.jsesurplus.com/AR-15SBUBilletUpperwithSideChargingHandleSlotM0300JSEEXCLUSIVE.aspx

MEGA Billet Lower

http://www.jsesurplus.com/megaarms-4-1-1.aspx

 

[natdscott]

I dont use BAT extensions anymore because of how sharp the back side of the lugs are. Just takes nasty chunks of brass out of the cases on extraction. Now you can take a Dremel 520-02 Impregnated wheel and polish the back sides (as suggested by JP) to smooth them out which I have done and works. Pain in the ass when the extension is already on the barrel and make sure you flush everything thoroughly.

I now use JP extensions only. They are already polished and coated and are smooth as butter. Yes, they are more expensive but the best extensions I have handled.

I do exactly what you quote.

Ordering John Holliger's 20TPI extension a few weeks before the barrel blank arrives, I, uh "work on it" to deburr and polish everything I see that needs it, with the obvious exception of load bearing surfaces, threads, etc.

It also gives me a chance to test fit in the upper, and to hand-fit the M4 extensions to the receiver to create one 'seamless' polished ramp.

Works.

My most recent one is on my 1,000 yard upper, and that brass needs all the integrity-help it can get!

-Nate

 

[Potss]

Excellent replies so far folks, thanks. Still hoping more people will actually get out there and do some testing, and maybe post pics. That would really lend some more credibility to these claims.

Quick side note, what are people's thoughts on lightweight uppers? Surely going in the opposite direction of the Vltor or DPMS (who make the walls thicker) would exacerbate the POI shift? For example, the 2a Armament Balios lite is quite light, and I wonder if it is more vulnerable to said POI shift.

 

[flyer]

If you make the upper too light, it will try to bend between the takedown pins and curl the Picatinny rail slightly.

It would probably be less of an issue than bending at the barrel nut but both will be factors and on an upper like the Aero M4E1 enhanced, it's probably the only thing that changes POI when loaded.

 

[redneckbmxer24]

Figured I’d post this here. SKD is doing a 24hr flash sale on the VLTOR MUR receivers for 20% off and then already have about the lowest prices on them.

Link here.

Add to cart for the sale price. The forward assist version is $143.97. The non forward assist should be a bit less.

 

[Mike_in_FL]

It seems like I've encountered this issue with a .300 BO recently. I could get no consistency and POI seemed to move around. There was nothing lose, no bad scope, yada yada. I ended up replacing the barrel with the same results and installed an AGB more in an effort to preserve brass.

What I did notice was that in using a Badger 22 moa base it extends out onto the rail over the barrel nut. The rail on this upper attaches like the MI do, you have two bolts on the bottom that clamp over the barrel nut. As it turns out when you install an AGB on a pistol length gas system and have a 13" rail you can't reach the torx on the front of an SLR.

So I wanted to remove the rail at the range to adjust the AGB. And it wouldn't simply slide off removing the two clamping bolts. I ended up hitting the top of the rail with a file to provide a little clearance and my groups have improved substantially. I can only guess that the downward pressure of the Badger base on the rail directly above the barrel nut was the culprit.

 

[buffalo hump]

A preface: I am a new member and have never replied to any of several forums that I have perused over the past several years but this thread while becoming somewhat old is as relevant going forward as ever given the evolving nature of the ar platform. So, I figured I would give it a go. I spent many years learning and developing my shooting and technical rifle knowledge in pursuit of the lowly ground hog beginning in my college days in the early 80's prior to invasion of canis latrans who saw to the demise of the furry favorite and in doing so elevated himself to the position of numero uno in the local rifleman's target aspirations. Back in the day we turned brass, weighed cases, bedded rifles, spent obscene amounts of nearly 100.00 for aftermarket triggers in order to score long distance hits on marmota monax. In other words we wasted a lot of time cutting bait and not fishing but it was fun. Now to the subject at hand, we learned accuracy technicalities and how to progress in that endeavor from benchresters who were on the cutting edge of the sport. Back then as now it was known that action stiffness was everything. It was determined that even the venerable remington 700 after being blueprinted was lacking in that parameter when competition got tough hence the receiver sleeve. This gave rise to an entire industry of over built and geometrically redesigned actions to improve the aforementioned weaknesses of barrel deflection, barrel harmonic disorders and ultimately translating to improved accuracy (read as match wins and bragging rights). These incremental successes were measured in 1000's on the target. Fast forward 10 years when someone got the idea to chop the handle off an ar, replace it with a weaver compatible pic rail and mount a scope alas a hunting rifle! And to beat all they shot phenomenally. My first colt hbar with military trigger would go 3/8" with handloads leaving us all mouths agape and scratching heads. So, we fast forward to today after light speed refinement of the platform and to a point in time where people are starting to look at this rifle in the same light a benchresters were the turn bolt in the 60's and 70's. Yes Virginia, action stiffness is everthing! The ar was a late 50's design if I recall which came to fruition in the early 60's. Given the inexperience of working with aluminum for such a purpose as a firearm and the limited (by todays cad standards) machining capabilities its ultimate success is astounding. But the platform was engineered to carry a lightweight barrel of 18 to 20 inches or so and its accuracy requirements for a battle rifle was laughable by todays standards. We are now hanging longer heavier barrels in addition to cantilevering floating handguards on an action whose design team never fathomed such things. And all of these exorbitant forces come to bare on the lowly trunion at the face of the receiver. To exacerbate the issue the handle has been removed which gave significant support in terms of resisting receiver flex. Don't agree with this, just ask any Detroit auto engineer the challenges of body flex after chopping off the top to make a convertible. As an owner of a mil supplier aka mil spec receiver unit by defunct sabre denfence I can attest to the poi shift gremlin first hand. The unit I am referring to wears a 24" heavy fluted match barrel capable of veeeerrrry good accuracy. But depending on how it is bagged (I never shoot for group size with a bipod) poi shift can be an issue. Any bench'er will tell you that a rifle which is split into two pieces in the middle, joined by hinge pins, most often rattles at the juncture between the two halves, and when trigger is pulled undergoes what one old shooter friend termed an "eternity of monkey motion before primer strike" is the worst of all evils to shoot small groups with. But lets stick with the receiver and trunion issue. To bring this diatribe or novella to an end may I say that being semi retired and self employed, being blessed with living in open country and no close neighbors I have the luxury of shooting every day if I choose. I have at my disposal many of the top brands, phenomenal optics (which brings another fly into the ointment of accuracy in how people now a days never question their OPTICs ability to shoot small groups but that is for another day) and ultimate add on accessories to eliminate human error. Of everything I have shot and studied there are three manufacturers that come to mind and who appear to be addressing this receiver weakness issue and move the platform into truely a 21st century ar and away from the 20th century mil-spec weaknesses. Oops I am running out of space..... to be continued

 

[flyer]

What a tease.

Also, paragraphs are your friend.

 

[buffalo hump]

this wrap up will be short and sweet, I laud the engineering inventiveness of seekins in their irmt 3 upper along with aero prec'n and their enhanced upper unit. In addition, pof and their critical examination of the weakness of the mil spec design and their engineering improvements to overcome those have greatly impressed me. I have several of the aforementioned and can whole heartedly attest to their success. In closing may I refer everone to a brilliant gentleman who goes by the name "varmint al". All should go to an peruse his eclectic website. He is a retired engineer from Sandia National Research Labs and a experienced shooter and coyote hunter. He has put together what amounts to a library of informative and entertaining information the likes of which will never be seen again and can only be likened to the Library of Alexandria of ancient lore only this dealing with rifles and shooting. It is here you will find countless data backed by scientific testing on action stiffness, barrel deflection as a result of length, mass, and diameter along with new data that really explains the gremlin of barrel harmonics. In college my biochem prof said "you can't be expected to remember everything so just know where to go to find it" (pre computer and google days). Thanks to Varmint Al for all your hard work you would make my physics and biochem professors proud. AAAhhh the effort to shoot small groups Carry on.

 

[buffalo hump]

flyer you are so right, my technical writing teacher is turning in his grave. Please excuse but this is my first attempt at popular casual prose after spending an entire career of ocd laced technical/scientific writing. And honestly, it feels good to violate the laws of of proper penmanship. Just hope this might help somewhat since we are all in this for fun. I promise to do better should I post in the future. God Bless.

 

[Almonz]

Seems like you'd have to be putting an incredible amount of force on a bipod to have any measurable POI shift. I don't buy it until I see hard evidence in more than one case.

 

[Back Spin]

It doesn’t take much pressure to change POI depending on the upper. On a couple of standard uppers, when I use sling support (not even super tight) I definitely see a low hit compared to the called shot (about 1.5 MOA low).

 

[Potss]

Thanks for the the introduction to Varmint Al buffalo hump.

Almonz, that is what this thread is all about. Trying to get some actual data on this effect. It is clear shifting happens under load, and effects both POI and accuracy. What isn't clear is to what extent, how much load is needed, and how much solutions like thick walled uppers/mono/polylithic uppers like Aero M4E1, Seekins, POF, and Vltor actually mitigate it. My own experience says the load doesn't need to be THAT significant on a normal upper. If I had the time and the budget I'd just buy them all and get to work, but I personally don't so think of this thread as crowd-sourcing data of which we've already had two good tests on the first page.

So open invitation to anyone who has an AR, shoot a control 5x5 and then put some load on the handguard and shoot another 5x5 and show the results, lets see what the data shows.

 

[vno.helix]

ok, here comes the dumbest question of all times from a non AR guy: Why does no-one build a vault like steel or stainless steel upper? Kinda like the "Accuracy International" of the AR's. I know, I know, extra weight, but who cares. If we hang a heavy tube on it and are precision shooting with it, what is a little extra weight if it comes with added stiffness and performance. An Aero precision M4E1 or M5E1 or Seekins IRMT like heavy steel/stainless upper for example. What I am I missing?

 

[Bigjake83]

ok, here comes the dumbest question of all times from a non AR guy: Why does no-one build a vault like steel or stainless steel upper? Kinda like the "Accuracy International" of the AR's. I know, I know, extra weight, but who cares. If we hang a heavy tube on it and are precision shooting with it, what is a little extra weight if it comes with added stiffness and performance. An Aero precision M4E1 or M5E1 or Seekins IRMT like heavy steel/stainless upper for example. What I am I missing?

If you use a Thick-Walled Upper Receiver made of 7075 it is just as strong as a steel metal upper could be. This is a fine example.

 

[lennyo3034]

ok, here comes the dumbest question of all times from a non AR guy: Why does no-one build a vault like steel or stainless steel upper? Kinda like the "Accuracy International" of the AR's. I know, I know, extra weight, but who cares. If we hang a heavy tube on it and are precision shooting with it, what is a little extra weight if it comes with added stiffness and performance. An Aero precision M4E1 or M5E1 or Seekins IRMT like heavy steel/stainless upper for example. What I am I missing?

The cost to machine steel vs Al would increase the price significantly. Also bolt action receivers are steel because the bolt lugs lock with lug abutments directly on the receiver. With a gas gun, the bolt lugs lock on the steel barrel extension.

 

[padom]

I've been using these MEGA SBU Thick wall uppers for all my precision builds lately. These things next to a forged or regular billet upper are way thicker and heavier. For precision rifles, these are my go to

 

[Potss]

@padom, I know you have a busy schedule, but would you be willing to do a side by side with these very thick walled uppers against more traditional uppers or even the Aero M4E1 system (which I know you've built on) to see if loading them with a bipod/sling/barricade changes POI on either system? That would be very interesting to see.

Same for @BigJake83 but with the Larue upper with that nifty no-touch-barrel-nut handgaurd. Would be very interesting to see if there is a difference in POI between them and a more traditional setup when loaded heavily.

 

[padom]

@padom, I know you have a busy schedule, but would you be willing to do a side by side with these very thick walled uppers against more traditional uppers or even the Aero M4E1 system (which I know you've built on) to see if loading them with a bipod/sling/barricade changes POI on either system? That would be very interesting to see.

Same for @BigJake83 but with the Larue upper with that nifty no-touch-barrel-nut handgaurd. Would be very interesting to see if there is a difference in POI between them and a more traditional setup when loaded heavily.

Could definitely do something along those lines in the next 30-60 days. Pretty backed up with reviews/testing right now. The 3 receivers I could use with the same, known half moa barrel (Rainier Ultramatch Gen1 Shilen 223 Wylde 20") would be a BCM MOD4 forged thermofit upper, Seekins IRMT-3 and the MEGA SBU thick wall.

 

[Potss]

That would be awesome if you could! To my knowledge nothing like this kind of testing has been done in around 10 years (since the release of the Vltor uppers), at least not publicly.

 

[MSTN]

1.) Has anyone else experienced POI shifts like the ones mentioned by Glen and others above?

2.) Does anyone have experience with the Seekins IMRT uppers, or Aero M4E1 uppers, or the Vltor MUR uppers which claim to mitigate said POI shift?

1. Most definitely! For decades, it has been a known fact.

2. The Seekins IMRT has great rigidity and therefore superlative accuracy potential. So does the Vltor VIS. The Vltor MUR is among the most rigid upper receivers built. The LaRue Stealth 2.0 is another really nice design for accuracy that few appreciate.

 

[bfoosh006]

Yes it is critical to have proper fitment with the barrel Extension and Receiver. If you don't want to spend the extra $60 for a oversized barrel extension, then use High Temp Red Loctite. If we are talking about a Precision Rig then don't Half Ass it! Polishe the Barrel extension to a mirror finish, true your Receiver Face (by a Professional) and Bed or Thermal Fit the Barrel, make sure to season the receiver theads and apply some Anti Seize, and lastly don't over torque the barrel nut, 50-65 FTLBs is ideal.

I am curious about the "Polishe the Barrel extension to a mirror finish" portion. Can you add info please .

 

[Bigjake83]

I am curious about the "Polishe the Barrel extension to a mirror finish" portion. Can you add info please .

Kind of self-explanatory, do you have more of a direct question? Some of the standard Barrel extensions like from BAT have been known to have sharp edges so to reduce or eliminate scratching or marring of the bullet upon chambering, I polish all my Barrel extensions to a mirror finish, JP and other high end rifle manufacturers do the same.

 

[bfoosh006]

Kind of self-explanatory, do you have more of a direct question? Some of the standard Barrel extensions like from BAT have been known to have sharp edges so to reduce or eliminate scratching or marring of the bullet upon chambering, I polish all my Barrel extensions to a mirror finish, JP and other high end rifle manufacturers do the same.

View attachment 7112204

My bad, I thought you were referring to the exterior of the barrel extension ! The inside makes much more sense. Thank you !

 

[MSTN]

AR-15 barrel extension feed ramps - even the best of them - have sharp edges, plain and simple. The surface of these feed ramps, while appearing smooth to the eye, can have the consistency of a very fine abrasive.

Several years ago, I did a brief test on the effect of feed ramps.

I had an order for a custom AR' with a top shelf 18" SS barrel.

First, I ran a test with unmodified feed ramps.

From a box of Black Hills Red Box 77 grain Sierra Match King ammo (OTM), I selected ten rounds with absolutely perfect bullet concentricity - 0.000" runout.

A known reliable Magpul 20-Round Pmag was used.

From a bench rest with bipod deployed, buttstock in the shoulder, five rounds were chambered into the rifle, starting with the bolt carrier group locked back, using the bolt release. Then, the cartridges were manually cycled out of the chamber. With no ejector, the rifle had to be tipped right to allow the round to fall out of the ejection port.

With the unmodified feed ramps:

A. All five rounds showed a badly deformed bullet tip. Under magnification, it looked like a very fine file had been run over them.

B. All five bullets showed two deep scratches in the bullet jacket as the bullet went up and over the top of the respective feed ramp. That will happen to any bullet design. We've all noticed these on rounds that have been cycled through an AR' chamber.

C. All five bullets had been knocked out of concentricity by 0.002" - 0.003".

A, B, and C combined can have a debilitating effect on the cartridge's precision potential.

The rifle was disassembled and the feed ramps were contoured and polished After rifle reassembly, the same test was performed.

With the contoured and polished feed ramps, there were:

A. No deformed bullet tips.

B. No scratches on the bullet jackets.

C. The bullet concentricity was unaffected.

In other words, the precision potential of the cartridge was unaffected by the trip from the magazine to the barrel's chamber.

My conclusion is subtle contouring and then polishing of the barrel extension's feed ramps provides an excellent return on the time spent.

 

[bill alexander]

The receiver can be regarded as an open bottom U section with a slot in the side and an end plate on the end the barrel cantilevers on. Think of this like a piece of plastic guttering and you get an idea how it deforms. Barrel and hand guard are attached at the same point on the end wall.

Flex is elastic. Consider the modulus of steel and aluminum. They are similar and will flex in a similar manner. Section is the governing criteria in controlling the deviation of the barrel from the top rail sight line. Thicker side walls are the key.

The old A2 receivers with the carry handle we considerably stiffer than the current A5 flat top The handle contributed considerably. Also the front sight would follow the barrel flex.

 

[RifleBlog]

The receiver can be regarded as an open bottom U section with a slot in the side and an end plate on the end the barrel cantilevers on. Think of this like a piece of plastic guttering and you get an idea how it deforms. Barrel and hand guard are attached at the same point on the end wall.

Flex is elastic. Consider the modulus of steel and aluminum. They are similar and will flex in a similar manner. Section is the governing criteria in controlling the deviation of the barrel from the top rail sight line. Thicker side walls are the key.

The old A2 receivers with the carry handle we considerably stiffer than the current A5 flat top The handle contributed considerably. Also the front sight would follow the barrel flex.

I’m excited that you’re posting here and looking forward to learning more.

 

[Shawn1492]

Really wish I saw this thread last year. My BCM upper allows for great groups but POI shifts with loading the bipod or resting on a barricade or bag.
Can some help with advice on how to explain to my wife why I need a VLTOR upper?

 

[Back Spin]

Really wish I saw this thread last year. My BCM upper allows for great groups but POI shifts with loading the bipod or resting on a barricade or bag.
Can some help with advice on how to explain to my wife why I need a VLTOR upper?

What length/profile barrel ? Which handguard?

I have a 5.56 16” light weight Lothar Walther barrel with a BCM KMR handguard that doesn’t exhibit such POI shift between bipod and barricade bags. Upper receiver is a mil-spec receiver from DSG Arms.

My .224 Valkyrie 20” heavy profile barrel with HERA Arms handguard (built by Craddock) does have an offset between bipod support and bags. The upper receiver was supplied by Craddock.

I was curious if the barrel profile had something to do with an upper’s resistance to POI shift.

 

[Shawn1492]

What length/profile barrel ? Which handguard?

I have a 5.56 16” light weight Lothar Walther barrel with a BCM KMR handguard that doesn’t exhibit such POI shift between bipod and barricade bags. Upper receiver is a mil-spec receiver from DSG Arms.

My .224 Valkyrie 20” heavy profile barrel with HERA Arms handguard (built by Craddock) does have an offset between bipod support and bags. The upper receiver was supplied by Craddock.

I was curious if the barrel profile had something to do with an upper’s resistance to POI shift.

22 inches long and .8 at the muzzle 224V from Craddock. The barrel is heavy and long. Lots of leverage to flex an upper, and because it shoots such tight groups I’m able to notice the shift.

 

[deersniper]

What do you think of the Vltor poly upper?

 

[LeadZeke]

So any preferences between the Seekins and Larue uppers? I guess the Larue is more purely "free floated" but not quite as thick?

 

[newguy2k3v2]

Also the front sight would follow the barrel flex.

That's an excellent point. They probably do get the same flex but it just isn't seen since the the front sight moves with the barrel.

 

[nick338]

A little behind on this thread but still hopefully relevant to a current build I'm involved in. The thicker walled upper receiver makes a lot of sense but what are you guys thinking on something like a Lantac Enhanced BCG? This is for a 6.5 Grendel build and I have come across several sources that make mention of larger group sizes from rounds that feed from opposite sides of the magazine due to some BCG's that are not as tight fitting in the upper receiver, and a supported BCG like the Lantac fixing this because it is a tighter fit and does not wobble.

 

[deersniper]

I use JP BCGs pretty extensively, but don’t know 100% if fit makes a big difference. Which is rare for me, since I make test fixtures and crap whenever I want to know know.

From a machinists point of view, the best would be a steel upper with a freeze/heat press fit. Next would be a forged upper with extensive reinforcement (the Vltor is pretty much this). 3rd would be a billet upper with enhancements made using stress analysis software (San Tan Tactical).

I think the main reason I don’t know if bolt fit matters is mostly due to excellent results observing the other variable stacks. Once you shoot sub-half with a gasser a good bit, I stop chasing.

I have also found that using bolts with additional buffering yields excellent results and as such it’s my primary decision point over other factors, all else equal.

So JP VmOS in big guns and Surefire in the little ones.

The Surefire even got me to my holy grail AR. An 18” gun that shot Hornady 556 Superformance with a can with excellent reliability AND consistent accuracy.

The bcg is supposed to rest agains the extension totally flat - and if it does it’s upper fit is less critical IMHO.

What suppressors did you use that hurt accuracy?

 

[nick338]

I use JP BCGs pretty extensively, but don’t know 100% if fit makes a big difference. Which is rare for me, since I make test fixtures and crap whenever I want to know know.

From a machinists point of view, the best would be a steel upper with a freeze/heat press fit. Next would be a forged upper with extensive reinforcement (the Vltor is pretty much this). 3rd would be a billet upper with enhancements made using stress analysis software (San Tan Tactical).

I think the main reason I don’t know if bolt fit matters is mostly due to excellent results observing the other variable stacks. Once you shoot sub-half with a gasser a good bit, I stop chasing.

I have also found that using bolts with additional buffering yields excellent results and as such it’s my primary decision point over other factors, all else equal.

So JP VmOS in big guns and Surefire in the little ones.

The Surefire even got me to my holy grail AR. An 18” gun that shot Hornady 556 Superformance with a can with excellent reliability AND consistent accuracy.

The bcg is supposed to rest agains the extension totally flat - and if it does it’s upper fit is less critical IMHO.

Great thanks for the info. I have a Vltor and Wilson Combat billet upper which is also supposed to be more rigid so I'm trying to decide which one to build it on along with BCG.

 

[Earnhardt]

Hey, The King...
Great post man!

 

[nick338]

Just to update I received a Vltor upper and weighed it along with the Wilson Combat billet upper.

Vltor-9.75 oz with forward assist
Wilson-10.05 oz no forward assist

The Wilson looks like a tank and appears to be much beefier in every area so I will build this on the Wilson.

 

[RyanScott]

For comparison a USGI upper is about 8.6 or 8.8 ounces and drop two ounces if you eliminate the forward assist.

 

[nick338]

For comparison a USGI upper is about 8.6 or 8.8 ounces and drop two ounces if you eliminate the forward assist.

Nice thanks for posting that.

 

[svxwilson]

Are the Wilson Combat Billet uppers thermofit receivers?
Update, Called Wilson to get the answer, they are NOT thermofit.

 

[Nivium]

Are the Wilson Combat Billet uppers thermofit receivers?
Update, Called Wilson to get the answer, they are NOT thermofit.

Have worked with Wilson Combat uppers. Definitely not thermofit, in fact, I would say relatively loose fitting. However, you can tell that the bolt carrier raceway has tighter tolerances to reduce the negative accuracy impact of a sloppy BCG

 

[Rocketvapor]

The cheapo BCA side charger weighs 10.2oz (without the little rear plug).
Not the thickest out there. Seems pretty sturdy.

 

[buffalo hump]

Well it has been awhile, glad to see bill A. posting since he is extremely knowledgeable on the issue. Several additional things to keep in mind; doubt you will ever see a steel receiver since there will be no market for it ($). Would make an interesting test vehicle for this discussion but I think it would only confirm what is already known. Also, I learned early on that there is a quantum leap in rigidity or toughness between 6061 and 7075 aluminum. I under NO circumstances even consider a 6061 receiver (learned from easton gamegetters vs xx75's) you archers will catch that.
My original post did not mention mega arms because they stopped manufacturing that receiver but it was a good one. Another tidbit: keep in mind where the bipod is attached (distance forward) will affect the ability to influence flex ie: length of fulcrum. A great example on receiver flex can be learned with smith's m&p 15-22, plastic rec'r where you can shift poi just by exerting force between the forehand and your rear hand. We love the little gun but when we shoot we keep that in mind. Have tested a larue since and they are awsome seem to work along the same lines as a mega. Just not plentiful in the marketplace and seem hard to come by.
I need Bill A. to clarify this since I am too long removed from college physics but when comparing materials strength to one another I believe they had to be of equal mass not size or volume. So aluminum is very efficient for mass but is a long way from a steel rec'r in terms of strength and resistance to flex. If two brands of rec'rs are of similar mass then where that mass is concentrated in the design will greatly affect its success in mitigating the POI shift issue we are dealing with here. I would cast my vote for designs that remove the direct bearing force from the barrel nut/trunion area, ie: larue, mega, seekins to name a few that I am familiar with. My apologies but it is early in the morning before my coffee has had time to kick in so this may appear as ramblings for which I apologize. Carry on and stay safe!