How Does One Tune for Accuracy in an AR?

[sentry1]

So I've got a 5.56 load worked up that gives me a 1 MOA-ish group in my 16" AR. I'm loading up some .308 rounds to find a good recipe to use in my MATEN as well.

The MATEN has a Mega Arms match barrel,1:10 twist, 20", rifle length gas system. Both ARs are fed with LC brass that has had primer pockets reamed for uniformity, and I do a 0.003" bump when FL sizing, I've also got Geissele triggers.

So, all things outside the chamber kept constant, what can I experiment with to try for more accuracy with my handloads? Assuming I'm constrained by magazine length for OAL.

Next, let's say I'm fine with single loading for slow-fire accuracy, will pushing the round to the lands help? What's a reliable way to get the measurement to the lands without spending more money on specialized tools I'll never use again?

 

[918v]

U can tune with powder charge weight and adjust gas to compensate for the difference in pressure if your gun has an adjustable gas system. If not, get one.

 

[sic65stang]

An adjustable gas system will help a bit. I only load my AR's (223 and 308) to mag length. I may be able to squeeze a little bit more out of them by messing with length and single feed, but then why would I want an AR? Finding the right powder and bullet combo is what your after. Good luck

 

[Boogaloo]

About all you can do is mess with powders, primers and bullets in an AR once you have taken out the other equipment related variables. Adjustable gas blocks do help a bit, particularly in mitigating over gas scenarios where the bolt is slamming backwards and bottoming out the buffer etc. The other thing is eliminating any slop between the upper and lower with an Accu-Wedge if your interface is sloppy.

For AR10's I really like the JP low mass bolt carriers and silent captured buffer/spring combo's. Anything you can do to get the mass down seems to aid in accuracy, and with those components an adjustable gas block is mandatory. Once you do all of that, you really need to find the load you will run almost exclusively in that system or you will be constantly fucking with the gas block as you change loads.

Try magnum primers and slower powder, regular primers and fast(er) powder, compressed, non compressed and every combo you can think of until you find the one that rings the system just right and stick with it.

 

[sentry1]

For the MATEN, I got a Seekins adjustable gas block, but it's completely wrong for the system. The Seekins system is just a clusterfuck on my MATEN, the brass tension screw is rear-facing, which means you can't access it with the thicker .308 barrel profile. The adjustment screw on the side requires I take my handguard off to tweak it.

As soon as a front adjustable gas block like the Wilson Combat model is back in stock, I'll be switching to that.

 

[ksthomas]

Sentry1,

The magic answer to making ARs shoot is really pretty simple; a good, high quality match-grade barrel, proper installation with no binding or contact where it shouldn't (carrier key/gas tube interface), a floated barrel (big issue, no AR really likes to shoot without being floated), and a top notch trigger, which it sounds like you already have.

You also don't really need to worry all that much about how many thou you are off the lands. Trust me, it'll change substantially as soon as the bolt goes forward. What you're dealing with in any AR is the world's largest and most expensive kinetic bullet puller. Most match bullets will pull a bit when they reach that sudden dead-end at the shoulder, and will often "soft seat" the bullets out a bit. If you're close to the leade, the ogive will likely be touching once you chamber the round. Not a problem, and it'll shoot just fine.

 

[427Cobra]

Do 5 shot groups with 6 in the mag, I think it has something to do with a round pushing up on the BCG, I notice last round fliers.

 

[sentry1]

427Cobra: Interesting, never thought about that variable. I'm going to try that for longer ranges, I doubt it'll make a noticeable difference at 100 yds though.

On external factors, I've noticed that I shoot the AR15 poorly in the prone with a scope. The scope height required for an AR with the low profile of the Magpul CTR stock makes for an awkward position for me, I'm resting my jawbone on the stock instead of the bottom ridge of my cheekbone, and that's much less stable.

This isn't an issue on my MATEN, which has a Battleline SAPR on it. I don't see another SAPR in the budget, so I think I may try building up the AR15 stock with some adhesive foam.

 

[Boogaloo]

JP makes a nice adjustable gas block too, but the adjustment screw is side facing. It is very robust however, and uses 8 screws (4 on each side of the block) for attachment. JP also recommends green loctite between the gas block and the barrel interface for sealing and retention purposes.

Here are a few pics of the most recent AR10's I built. Both of these are for a customer of mine, both are built with Mega matched upper/lowers with sequential serial numbers. Both have JP SuperMatch 20" barrels in 10 twist, JP's blended brakes, JP low mass bolt carriers and silent captured springs, Geissele Match 2 stage triggers with lightened Geissele springs (1.5 lbs total for both stages). PRI Carbon fiber handguards (free float of course), and JP heatsinks on both barrels.. These things are hammers, and were easily the most accurate AR's I have ever built or shot. Top notch stuff. The customer was very happy during test and tune.

 

[insight3b]

This really helped my groups tighten and eased up the abuse on my brass:
AR-15 Carrier Weight System - AR 15 PRODUCTS by Superior Shooting Systems

For just a simple drop in part it works extremely well

 

[81Z4ME]

Slow burning powders (IMR4350 was magic in my M1A) and 180 grain match bullets weren't good for the M1A but shouldn't be an issue in the AR15, especially if you have an adjustable gas block.

Every gun is different. Experiment with powders, charge weights and bullets till you find your rifle's "magic".

 

[nso123]

Do 5 shot groups with 6 in the mag, I think it has something to do with a round pushing up on the BCG, I notice last round fliers.

This is interesting. I have not had any problems with a last round flier, but it won't hurt to try.

 

[nso123]

This really helped my groups tighten and eased up the abuse on my brass:
AR-15 Carrier Weight System - AR 15 PRODUCTS by Superior Shooting Systems

For just a simple drop in part it works extremely well

What else was involved in your rifle? This intrigues me.

 

[P7id10t]

An interesting question turns into gun porn. That's my kind of topic !!!
Boogaloo, awesome customs!

I load for mag length (2.815 in a DPMS LR308, 18"), find slower powders work much better with heavier bullets. Non adjustable gas block, do not notice any fliers. I chrono 2800-2950 using 150gr and 4064 or 2230, but they shoot like scattershot. N150 or RL17 under 180s and they are ~2400 fps, and < 1MOA, IMO not competition worthy but guarantee it's dead at 100, 200 or 300 yards.

JP makes a nice adjustable gas block too, but the adjustment screw is side facing. It is very robust however, and uses 8 screws (4 on each side of the block) for attachment. JP also recommends green loctite between the gas block and the barrel interface for sealing and retention purposes.

Here are a few pics of the most recent AR10's I built. Both of these are for a customer of mine, both are built with Mega matched upper/lowers with sequential serial numbers. Both have JP SuperMatch 20" barrels in 10 twist, JP's blended brakes, JP low mass bolt carriers and silent captured springs, Geissele Match 2 stage triggers with lightened Geissele springs (1.5 lbs total for both stages). PRI Carbon fiber handguards (free float of course), and JP heatsinks on both barrels.. These things are hammers, and were easily the most accurate AR's I have ever built or shot. Top notch stuff. The customer was very happy during test and tune.

 

[rsplante]

Sentry1,

You also don't really need to worry all that much about how many thou you are off the lands. Trust me, it'll change substantially as soon as the bolt goes forward. What you're dealing with in any AR is the world's largest and most expensive kinetic bullet puller. Most match bullets will pull a bit when they reach that sudden dead-end at the shoulder, and will often "soft seat" the bullets out a bit. If you're close to the leade, the ogive will likely be touching once you chamber the round. Not a problem, and it'll shoot just fine.

Thanks Kevin, you just saved me a bunch of time tomorrow. I've been agonizing about where to start a ladder test for seating depth, and as soon as I read your post, I knew without a doubt that you were correct. Why didn't it occur to me before? If I can completely remove a bullet with one to two hammer blows, why wouldn't the bolt carrier slamming home cause it to move some? Just the opposite of setback.

 

[rsplante]

Thanks Kevin, you just saved me a bunch of time tomorrow. I've been agonizing about where to start a ladder test for seating depth, and as soon as I read your post, I knew without a doubt that you were correct. Why didn't it occur to me before? If I can completely remove a bullet with one to two hammer blows, why wouldn't the bolt carrier slamming home cause it to move some? Just the opposite of setback.

Afraid I have some bad news Kevin. I REALLY REALLY wanted you to be right too. I wanted to Prove you were right. So I went and loaded up a round, .224 77gr SMK, LC11 once fired by me, 25gr Varget. I measured it three times with a caliper and Hornady L-N-L bullet comparitor and anvil. I loaded it in my LaRue 5.56 OBR and slammed it home. I ejected the round and measured it again three times. No measurable movement. I slammed it home 4 more times and it had moved a total of 0.001". I slammed it home 5 more times and it moved another 0.001". Ten times and it moved a total of 0.002". I don't think my neck tension is overly tight. I use a Redding Competition Bushing Neck Die with a .245 bushing (.001-.002" compression) (skimmed neck with 21st Century Shooting neck lathe). I've treated the bullets with David Tubb's Boron Nitride, so they may be slicker than normal. Oh well, it Really sounded good.

 

[rsplante]

I've noticed that I shoot the AR15 poorly in the prone with a scope. The scope height required for an AR with the low profile of the Magpul CTR stock makes for an awkward position for me, .

Try the Magpul PRS stock. I shoot prone with it and love it. I do seem to shoot better off a bench though, which doesn't really make sense. Maybe I'm full of it...

 

[[email protected]]

I'm going to start reloading for my precision firearms 6.5 grendel, and have a type S die that in going to put .004"-.005" neck tension and I highly doubt that bullet is going to move. The reason I say this is because with .004" neck tension I was able to steady the bullet in a .338 WM browning bar.

 

[rsplante]

Afraid I have some bad news Kevin. I REALLY REALLY wanted you to be right too. I wanted to Prove you were right. So I went and loaded up a round, .224 77gr SMK, LC11 once fired by me, 25gr Varget. I measured it three times with a caliper and Hornady L-N-L bullet comparitor and anvil. I loaded it in my LaRue 5.56 OBR and slammed it home. I ejected the round and measured it again three times. No measurable movement. I slammed it home 4 more times and it had moved a total of 0.001". I slammed it home 5 more times and it moved another 0.001". Ten times and it moved a total of 0.002". I don't think my neck tension is overly tight. I use a Redding Competition Bushing Neck Die with a .245 bushing (.001-.002" compression) (skimmed neck with 21st Century Shooting neck lathe). I've treated the bullets with David Tubb's Boron Nitride, so they may be slicker than normal. Oh well, it Really sounded good.

Mea Culpa, Mea Culpa, Mea Mega Culpa. I had my head in a dark and stinky place. That'll teach me to try to do precision work after midnight. Kevin, you were right of course. I went the wrong direction and was jamming my bullets, of course they didn't grow in length.

When I finally pulled my head out, it happened just like Kevin said it would, maybe not as fast, but it still happened. The first time I slammed the bolt home, the bullet pulled out .004", the second time on the same bullet, it extended another .001", followed by .002", .0015" and then .0015"

BIG SAFETY NOTE!!!! I should have mentioned!!! I conducted this test with NO PRIMERS INSTALLED. Never forget that an AR-15 is susceptable to slam fire. I had my rifle pointed in a safe direction, and had no primer in the round. It would have been even more safe without the powder in the round, but I wanted to include what effect 25gr of Varget hitting the rear of the bullet would have.

 

[BC 6.8]

I'm going to start reloading for my precision firearms 6.5 grendel, and have a type S die that in going to put .004"-.005" neck tension and I highly doubt that bullet is going to move. The reason I say this is because with .004" neck tension I was able to steady the bullet in a .338 WM browning bar.

0.004-0.005" seems like quite a bit of neck tension. My non-bushing .223 and 6.8 dies leave about 0.0015" tension with the expander in the die. I haven't had a problem with bullet setback or changing COAL.

 

[former naval person]

Try it again with measuring the follow up round after the first round fires and the second one is fed. The dynamics are different then too.

 

[BC 6.8]

Mea Culpa, Mea Culpa, Mea Mega Culpa. I had my head in a dark and stinky place. That'll teach me to try to do precision work after midnight. Kevin, you were right of course. I went the wrong direction and was jamming my bullets, of course they didn't grow in length.

When I finally pulled my head out, it happened just like Kevin said it would, maybe not as fast, but it still happened. The first time I slammed the bolt home, the bullet pulled out .004", the second time on the same bullet, it extended another .001", followed by .002", .0015" and then .0015"

BIG SAFETY NOTE!!!! I should have mentioned!!! I conducted this test with NO PRIMERS INSTALLED. Never forget that an AR-15 is susceptable to slam fire. I had my rifle pointed in a safe direction, and had no primer in the round. It would have been even more safe without the powder in the round, but I wanted to include what effect 25gr of Varget hitting the rear of the bullet would have.

What COAL/OAL were you using when the bullet was hitting the lands (rifling)?

 

[arklatexgeo]

JP makes a nice adjustable gas block too, but the adjustment screw is side facing. It is very robust however, and uses 8 screws (4 on each side of the block) for attachment. JP also recommends green loctite between the gas block and the barrel interface for sealing and retention purposes.

Here are a few pics of the most recent AR10's I built. Both of these are for a customer of mine, both are built with Mega matched upper/lowers with sequential serial numbers. Both have JP SuperMatch 20" barrels in 10 twist, JP's blended brakes, JP low mass bolt carriers and silent captured springs, Geissele Match 2 stage triggers with lightened Geissele springs (1.5 lbs total for both stages). PRI Carbon fiber handguards (free float of course), and JP heatsinks on both barrels.. These things are hammers, and were easily the most accurate AR's I have ever built or shot. Top notch stuff. The customer was very happy during test and tune.

Gun porn indeed!!!
I have the Magpul PRS stock on the 300 AAC Blackout I just finished building. I absolutely love it. I just started tweaking rounds for the rifle this past weekend and I have to say that the stock helped in terms of cheek weld and eye positioning (obviously) and weight/balance.
I guess I'll add a little to the gun porn if that's okay...(no hijack intended)
Pic taken this past weekend while setting up to chrono the rounds I had just loaded up.

 

[BCP]

I've never had that happen but then I seat deep for ARs which may help. 2.24" etc.