Variables of AR-10 accuracy

[Green Rabbit]

Hi all,

I'm new to this forum, not new to guns. I am a hobbyist builder, and I do own some basic tooling for my projects (lathe, mill, reamers etc). A while ago I decided to build most accurate AR 10 that I can do, and thought I'll share details so maybe some people will find it helpful.
I could have just bought a barrel from a reputable maker but I thoughtI wouldn't learn much.
So I decided to make one from scratch.. well, from a blank.
First attempt was not a success. I bought a cheap 16" blank made by Bergara, turned it into ar-10 barrel, .750 gas block at mid-length, chambered, put on a receiver. It shot ~1.5 moa on average, occasionally getting sub-moa groups. Not good enough. and I did not know how to troubleshoot it - too many variables, and some (like chamber, profile, etc) are already there and I cannot change them.

So on the second attempt, I decided to make step by step approach, testing accuracy between the steps - that way if accuracy is good in the beginning and then drops, I at least know which step is to blame.

The steps will be:
1) Barrel blank threaded and chambered, muzzle crowned, full 28" lenth, maximum thickness profile (1.200")
2) Blank cut to final length (20"), muzzle threaded 5/8x24, profiled (I'm thinking of rifle length gas system, .875 gas block, 1.000" between chamber end and tenon, .850 from gas block to the muzzle). I see no point testing accuracy after cutting to the length but before profiling - as shorter barrel of the same profile should not have any drop in accuracy
3) Drill gas port and install gas system, finally turning it from bolt action to semi-auto

My accuracy testing protocol will be 6 groups of 5 shots each from 100 yards, 175gr smk bullets, increasing Varget load from 42 to 44.5 grains, 0.5 gains step

The parts I have for this project are:
1) Shilen match blank 308, 1 in 10", 28" unturned
2) BAT machine 308 extension
3) 308 Winchester AR reamer from Pacific tool and gauge

Ultimately I'd like to achieve 0.5 moa with reloads

So far only the first step is completed - if there is interest, I will keep this thread updated as it progresses.
I do appreciate comments, feedback, suggestions (especially from folks with experience in similar projects)

Now it looks like a monster, it won't stay that way though



Assembled and ready for first accuracy testing

 

[Mike_in_FL]

I can't find the reamer on the PTG website. Dimensions? The shortcomings that I've seen in .308 AR barrels (only my opinion) is that they seem to use 7.62x51 headspace rather than a true .308. That seems contradictory to what builders did in the past with M1As. I can't see why anybody would buy a premium barrel and want to run some kind of cheap surplus through it, if that even exists any longer.

Does GAP use the PTG reamer named for them in the GAP-10 or just bolt actions? Interested in seeing your results.

 

[Green Rabbit]

About the reamer - PTG does not sell them anymore. I'll ask them to email me drawings.
I headspaced using 308 go gauge, the chamber so far is very tight (go+0.003 shim won't close).
Now PTG has 308 match GAP reamer, whatever that may mean. Maybe it's a better one
I don't plan to run any military brass through it - I only have Lapua and Fed.

Are there any specific dimensions that make 308 chamber well suitable for an AR? if my chamber is bad, I still can re-chamber, before I drilled the gas port location

 

[Mike_in_FL]

I would say to use true .308/ 1.630, headspace, try to hold dimensions to a minimum on the neck based on your brass, and base the throat for the 175 SMK at mag length with minimum jump. Although the SMKs probably jump better than anything I've seen.

I've heard of people building 1/4" or so AR15s. But they're not reliable for anything outside of a range and you're turning necks to do so. I don't know how many rifles you've barreled, if you can build an accurate bolt rifle keeping everything concentric, this ought to work.

 

[Green Rabbit]

I measured the reamer neck, it's .3459. That's close to SAAMI specs and about 0.002 more than a match chamber for a bolt rifle, but I guess it's justified considering how an AR slams rounds into the chamber.
175 smk has about 0.050 jump from the current seating depth before it touches the lands

 

[FCS]

You really don't want to have the bullets too close to lands in an AR, 0.050" jump is plenty close.
Make port only large enough to cycle reliably and lock back on empty magazine. I'd also go with a longer gas system do help deal with the larger gas volume available to cycle the BCG. Isn't a bolt rifle, is going to be a one bullet weight, limited powder choice set up but it is possible to get really great accuracy, drop dead reliable performance. You'll just have a pretty narrow range of ammo that will give you desired performance.

Chamber in a GAP-10 is not the same as chamber in GAP bolt rifle, at least not in the rifles I've measured.

 

[longshot2000]

I will follow the thread with interest. With that monster barrel, you will need a very heavy and reliable upper receiver and barrel nut. What are you using?

 

[Bakwa]

Following.

 

[Mike_in_FL]

I measured the reamer neck, it's .3459. That's close to SAAMI specs and about 0.002 more than a match chamber for a bolt rifle, but I guess it's justified considering how an AR slams rounds into the chamber.
175 smk has about 0.050 jump from the current seating depth before it touches the lands

I read that Dave Kiff specs his no turn neck dimensions at .342, so yes in a gasser this looks correct. I guess my point is you're starting off with a single shot rifle. IIRC, in Lapua you're talking .337/.338 with no clearance. You can't put more neck "in" once you cut it out. Would I ever consider that for a gun used in the field? Of course not. .050 is nothing for the SMK.

Take a look at this site, most of the info is like 15 years or older now but no sense in reinventing the wheel: http://www.radomski.us/njhp/ar10.htm

I wish they still sold remanufactured Hornady match .223 w/ 75 gr. for $300/1k, lol.

If you go to the page and click on the rifles you'll see the info on the builds, 007 - 009 are .308s. You'll also find some gas port dimensions if you try to go plus length on the location. Read the bottom of the initial page and it'll give you some interesting tidbits about velocity and barrel length, wish they'd have mentioned the caliber however. Amazing to read the gyrations these guys went through to get a match trigger back then.

And this random, completely irrelevant bit of information for this project I include for benefit of everyone building 6.5 guns, buying complete BCGs, and then buying high pressure bolts. This is advice from high power shooters building AR10s over 15 years ago

Bolt - don't use SR25 Primer piercings were reported by several shooters using SR25 bolts at modest pressures because of poor primer support on ignition. Problem went away with switch to Armalite bolt.

 

[Green Rabbit]

Here is first first test target, after zeroing in. 6 groups 5 shot each, about 0.7 moa on average, best group 0.41 moa, worst 1.25 moa. All with 175SMK, Varget powder in 0.5gr increments
Best group was with 44gr Varget. For over moa groups it could have been me (I didn't feel very comfortable about few of the shots due to muzzle brake blasts from fellow shooters around)

I'm not yet sure if I should be happy with that before moving to next step - before profiling and gas port drilling, I still can re-chamber. 0.41 moa group looks promising, but I will likely lose some of that with slimmer barrel profile and gas port drilled.

I also got reamer drawings from PTG - it's been made to shoot variety of ammo, including 7.62 NATO and 7.62x51 CIP. .3458 neck, .147 freebore, .3089 throat, 1-30 angle Long jump but fairly tight throat.

 

[quiksilverj]

interested

 

[jLorenzo]

Also the fit between the barrel extension and receiver is something to consider. Also try different torque settings on your barrel nut. Timing whatever muzzle device can help tune your barrel too. Add a washer, shoot, remove one shoot and see where the barrel likes the muzzle device.

 

[Green Rabbit]

A little bit of an update here.. I had a chance to acquire a match 308 reamer - PTG had them on sale. So as an experiment, I cut about an inch from the chamber side and recut the chumber using new reamer.
The previous results were: 0.74; 0.85; 1.25; 1.23; 0.41; 0.76 moa - average 0.87moa
With new chamber, and 1 inch shorter barrel, results got a bit better - 0.86; 1.05; 1.15; 0.42; 1.58 - average 0.79 moa (i didn't shoot 44.5 gr charge group). Also, 0.42 moa group looked more like a one hole 4 shot group (about 0.17 moa as I could measure) with one flyer about 3/8" from the group center, so pretty good.

There was no reason not to like this chamber, so I decided to settle with it and move with the rest of the plan: cut and profile the barrel.
What I did:
1) cut to 22 inch
2) used rifle +2" length gas system
3) Profiled to 1.19" around the chamber for 3", then down to 1" before the gas block, small step up to 1.05" at the gas block shoulder, 0.875" gas block, down to .850" to the muzzle, 5/8x24 end threads

No gas port, no muzzle brake yet.


The rifle looks cool indeed. However this is where good things end now - the accuracy started to suffer quite a bit.
Test group was (again, same protocol - 175smk, 42-44.5 gr Varget, increment 0.5gr): 1.22; 1.11; 1.64; 1.98, 1.24, 1.27. Average 1.41 moa.

Very noticeable accuracy drop, in fact no more submoa groups. I don't believe it can be explained by thinner barrel - first it got 4 inch shorter (should help to make is stiffer), and besides it is not a pencil profile by any mean.
I am going to redo this test again, before drilling gas port - verify extension and barrel nut torque, re-lap the muzzle (not sure I did a good job last time after crowning)

Other than bad crowning, any idea what could went wrong at this stage?

 

[Mike_in_FL]

I doubt simply profiling the barrel would induce any kind of stress you'd have to remove. I think you would have to have been using an indicator while profiling to know. The only thing I've seen is that fluting too aggressively can affect the barrel. But you did remove weight from a tuning fork and now it vibrates differently. Hard to say about the crown if you don't see any burrs but I've never seen anybody lap anything after crowning either.

I can't see any charges listed on the new target, what's the deal on the bottom left 3 shot? Maybe shorten the OAL and shoot that again. I think that .5 grain steps are too large and .since the node probably moved, you have to find it again.

I would use a chrono and follow other's lead by finding a single digit SD and varying seating depth a bit. If you're going to use that bipod, I'd move it out farther. I would just use sandbags at this point.

 

[Green Rabbit]

I think i did it right, but help me to understand how an indicator could help to profile barrel properly, other than center it around the bore axis when profiling? I definitely didn't bend it while turning to the point when subsequent cutter pass would cut one side more than the other. I could have get into a bit of chatter there and there while turning, but that was an indication for me to step back and make an easier pass or use a different cutter, so unlikely any harm was done by that.

those 3 bottom shots are just part of printed target, not actual bullet holes. I'll try to go to sandbags, although previous shots were taken with the same bipod. What the approximate FPS difference between powder charges that I should be looking for, to make sure I'm not missing nodes in between?

 

[Mike_in_FL]

Yes, fluting a barrel will produce what you described, a bend, will occur if you cut too much from a side and yes, that's what I meant by indicating. If you had an indicator in the muzzle while turning and profiling you'd know if anything happened. I would think if you do it now and see nothing out of the ordinary there's no problem. I don't think that's what changed.

I wouldn't care about velocity at all if that's what you meant by FPS. Google up Dan Newberry and Optimum Charge Weight. .You want low numbers on ES and SD. If you're really after accuracy you're going to have to step up your reloading.

I would move the bipod out to the end of the forend, in that position I know I'd be putting some vertical into the groups. Shooting a large frame AR isn't easy to me and using the bipod from the start would screw me up. I think it will shoot for you and thanks for documenting the build.

 

[physhphude]

This is very interesting. I feel like you should be getting better accuracy. That's about how my factory SR25 with factory ammo is shooting. I would think that an untouched barrel should do better.
Have you tried other variables? Different shooter, different (Factory) ammo, different scope? Are your mounts tight.
Also I have noticed that some of my guns don't shoot as well at 100 yards as they do at 200. Try that?

 

[Benito]

Holy smokes, that is the thickest barrel profile I have ever seen on any man-portable gun.

How heavy is your setup??

 

[Forgetful Coyote]

Any update? FWIW from what Ive read most button barrels are lapped after profiling to final contour because in some instances the bore will open up when contouring and/or otherwise end up with altered dimensions.. not saying that happened here, just figured Id point out button rifled barrel makers AFAIK all contour their barrels BEFORE final lapping...

 

[bfoosh006]

Actually.... I strongly suggest you rework up your reload.

You have changed the barrel enough to change the harmonics.

I'll bet you find you node again.

 

[Green Rabbit]

I apologize the lack of updates recently - has been busy with other projects, and the northwest weather was not most encouraging for making range trips. Anyway, this is how the rifle looks like now (I made a muzzle brake for it using pieces of cut barrel)

As for the accuracy troubleshooting - I found one thing which was wrong with it, which definitely had an impact. The match 308 reamer I got from PTG had a narrow throat - specs were calling for .3085 _+ 0.0004, and the actual reamer measured 0.3081. Technically within specs, however 175 SMB bullets are typically .3083 in the neck area. After chambering some live round and then removing them, I found that on few rounds, the bullets was seating deeper than before chambering - so that narrow neck was pushing bullets deeper. Definitely a disaster for accuracy.

Since that was a "clearance" reamer, PTG was not very helpful - they offered to take it back for the money paid, and buy a new one for full price. Instead, I decided to open up the neck using a different approach - I took few 175 SMK bullets, put them in empty cases with some locktite, seating them right at the depth then they were supposed to touch the lands assuming reamer would be proper size, put some 800 grit paste on the bullet, threaded back of the case and put threaded rod in it, and hand drill from the other side.. so you got the idea - using the bullet as polishing mandrel on the neck. Yeah I know that's a bit of unorthodox way, but at least rounds are chambering now properly.

my last trip to the range when I was testing it was not a success (some barely sub-moa groups), but I had other problems to work on - gas port was a bit to small (gun didnt cycle properly), and muzzle brake was coming loose. Now I fixed that, and planning to make another trip to the range to re-test. Hope to give you a full update next week

 

[Red_SC]

I prefer to start with a blank that I don't have to turn down behind the gas block, and only minimally in front of the gas block. That said, I've turned several down over the full length, and have never seen it affect accuracy. I just finished a 6mm Fat Rat for a friend with a Bartlein barrel turned down for the whole length. He chrono'd and shot a couple groups last Friday for a match Saturday, he said it shot 2" at 500 and 3" at 700. I wasn't there, I don't know anything about the number of rounds per group, but obviously reprofiling the barrel didn't hurt the accuracy.

A loose muzzle brake, however, could easily be the problem. If you continue to have issues, I'd test with and without the brake to see if it's affecting anything.

 

[Green Rabbit]

Holy smokes, that is the thickest barrel profile I have ever seen on any man-portable gun.

How heavy is your setup??

it's not too bad, about 13.6 lb with the scope, without magazine - considering barrel is now 22 inch, .875 at gas block. M110 is be about the same weight. If you are referring to the pictures in the beginning of the thread, it was before the profiling - 30" long, about 1.18" thick. That thing was probably 20 lb or so

 

[Green Rabbit]

Last week to the range was a success. I got most consistent results, even comparing to full bull non-contoured barrel. Here is what I did:
1) Opened up gas hole a little (from 0.093 to 0.096). Still not enough for rifle +2 inch gas system on 22 inch barrel, only cycles to about 30% of the rounds. I have a 0.098 reamer, but if this won't be enough, have to go to ebay for some larger ones - probably 0.101-0.104. Anyway, not concerned much about this, will get it working eventually, just don't want to overgas so taking it slow.
2) Tied up the muzzle brake. I don't use a crush washer, instead I just shortened it's threaded back to time properly (and cut too much, ended up adding 2 layers of 0.002" foil shims)
3) Wrapped the barrel extension with some stainless foil to eliminate play in the receiver - to the point when I had to gently tap it with a mallet to go in place.
4) Changed my accuracy testing protocol - went down from 0.5 to 0.3 grain increments of varget, and also seated bullets to nominal (2.800) length.
5) Went shooting to a different range - it was quieter (no air shakes from magnum calibers around), and most importantly - their tables are more stable (looking through the scope, I could see that target sight picture due to my heartbeat has decreased). I am somewhat confident that my aiming was within 0.25 moa or better most of the time.

Here are the 5 shot group sizes at 100 yards:

43.0 gr: 1.05"
43.3 gr: 0.68"
43.6 gr: 0.98"
43.9 gr: 0.65"
44.2 gr: 0.46"
44.5 gr: 1.26"

A very pronounced node around 44 grain of varget (surprisingly that didn't change from uncontoured longer barrel). I am very happy with the results - now I'm confident with a bit more load development I can make it consistently shooting better than 0.5 moa. Not bad for just the second AR barrel I made from a blank.
I'd like to specially thank all the people who provided their support and advice in this thread

Lessons learned:
1) Barrel contour before down to .875 at gas block, gas system, shortening barrel from 30 to 22 inches did not have any measurable accuracy decrease
2) It's impossible to change one thing at the time. Simple barrel removing for contouring requires re-torquing barrel extension and upper nut. Seems like these things have more impact on accuracy than barrel contour itself.
3) Match chambers are tricky. Too easy to make it "too tight" for the bullet, even with reamer from reputable vendor like PTG.

here is photo of the best group

 

[raptor99]

Sorry to hear about your troubles. When I built my AR10 I just used a factory chambered Shilen barrel with a stock contour and it is a HAMMER!!! I have shot some groups with it that people don't believe were shot with an AR....