Trouble with a newly build 308 AR10

[LVLAaron]

Built a new AR10.
* Aero Precision Receiver
* Toolcraft Bolt/BCG
* Elf trigger
* Ballistic Advantage 18" .308 Heavy Rifle barrel (1:10 twist)
* VG6 Gamma brake
* SWFA 10x SS Scope


I've doubled checked that everything is tight and nothing is loose. I know the scope is good, I took it off a .223 I'd shoot bugholes all day with.

The first rounds through the gun were 20 Hornady Black 168's

Sticking with Hornady 308 brass and CCI-BR2 primers.

Next I tested a bunch of other Hornady 168 BTHP's and wasn't in love. Ran out of those, picked up some sierra match king 168's and that brings us to today.

12 groups in .3 grain increments of IMR 4166


Rough load data from Quickload:

Cartridge          : .308 Win. (SAAMI)
Bullet             : .308, 168, Sierra HPBT MatchK 2200 G7Litz
Useable Case Capaci: 48.726 grain H2O = 3.164 cm³
Cartridge O.A.L. L6: 2.800 inch = 71.12 mm
Barrel Length      : 18.0 inch = 457.2 mm
Powder             : IMR 4166 Enduron

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.671% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms
                                
-06.7   97    41.70   2332    2029   46606   9268     89.8    1.078
-06.0   98    42.00   2349    2059   47579   9349     90.1    1.067
-05.4   98    42.30   2366    2089   48572   9429     90.5    1.057
-04.7   99    42.60   2383    2119   49587   9509     90.8    1.046
-04.0  100    42.90   2400    2149   50623   9588     91.1    1.036
-03.4  100    43.20   2418    2180   51682   9666     91.4    1.026
-02.7  101    43.50   2435    2211   52764   9743     91.8    1.016  ! Near Maximum !
-02.0  102    43.80   2452    2242   53869   9819     92.1    1.007  ! Near Maximum !
-01.3  103    44.10   2469    2274   54999   9895     92.4    0.997  ! Near Maximum !
-00.7  103    44.40   2486    2305   56154   9969     92.7    0.988  ! Near Maximum !
+00.0  104    44.70   2503    2337   57334  10043     93.0    0.978  ! Near Maximum !
+00.7  105    45.00   2520    2369   58540  10116     93.3    0.969  ! Near Maximum !
+01.3  105    45.30   2537    2402   59774  10188     93.6    0.960  ! Near Maximum !
+02.0  106    45.60   2554    2434   61035  10259     93.8    0.951  ! Near Maximum !
+02.7  107    45.90   2572    2467   62324  10329     94.1    0.941  !DANGEROUS LOAD-DO NOT USE!
+03.4  107    46.20   2589    2500   63644  10397     94.4    0.933  !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 10% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 10% relative to nominal value:
+Ba    104    44.70   2627    2575   66762  10297     97.8    0.913  !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 10% relative to nominal value:
-Ba    104    44.70   2350    2061   48186   9439     85.6    1.061

Chrono (labradar) data from the trip

4166 / Grains
Stats - Average 2405.55 Group 1  - 41.4
Stats - Highest 2450.22
Stats - Lowest 2367.33
Stats - Ext. Spread 82.89
Stats - Std. Dev 31.78

Stats - Average 2419.86 Group 2 - 41.7
Stats - Highest 2433.56
Stats - Lowest 2406.06
Stats - Ext. Spread 27.5
Stats - Std. Dev 11.85

Stats - Average 2428.67 Group 3 - 42
Stats - Highest 2449.49
Stats - Lowest 2374.84
Stats - Ext. Spread 74.65
Stats - Std. Dev 31.47

Stats - Average 2437.98 Group 4 - 42.3
Stats - Highest 2471.77
Stats - Lowest 2408.07
Stats - Ext. Spread 63.7
Stats - Std. Dev 25.8

Stats - Average 2462.62 Group 5 - 42.6
Stats - Highest 2474.12
Stats - Lowest 2442.51
Stats - Ext. Spread 31.61
Stats - Std. Dev 12.17

Stats - Average 2481.8 Group 6 - 42.9
Stats - Highest 2512.48
Stats - Lowest 2452.42
Stats - Ext. Spread 60.06
Stats - Std. Dev 21.61

Stats - Average 2509.11 Group 7 - 43.2
Stats - Highest 2518.03
Stats - Lowest 2501.35
Stats - Ext. Spread 16.69
Stats - Std. Dev 6.71

Stats - Average 2508.6 Group 8 - 43.5
Stats - Highest 2519.9
Stats - Lowest 2487.17
Stats - Ext. Spread 32.73
Stats - Std. Dev 15.22

Stats - Average 2518.48 Group 9 - 43.8
Stats - Highest 2548.3
Stats - Lowest 2491.72
Stats - Ext. Spread 56.58
Stats - Std. Dev 23.17

Stats - Average 2546.29 Group 10 - 44.1
Stats - Highest 2573.08
Stats - Lowest 2518.43
Stats - Ext. Spread 54.65
Stats - Std. Dev 20.93

Stats - Average 2376.08 Group 11 - 44.4
Stats - Highest 2391.89
Stats - Lowest 2365.77
Stats - Ext. Spread 26.12
Stats - Std. Dev 10.58

Stats - Average 2403.9 Group 12 - 44.7
Stats - Highest 2422.26
Stats - Lowest 2382.89
Stats - Ext. Spread 39.38
Stats - Std. Dev 16.87

I'm struggling to keep 2 inch groups @ 100 yards. I'm tired from the holiday and a little frustrated, so I could use some help from the hive mind.

 

[dminn1]

You might check for binding between the gas tube and carrier key.

 

[SG4247]

Might shoot federal 168 gold metal match ammo in it.

Nearly all the 308 ARs (and bolt guns) like this ammo.

I usually shoot that stuff to get the rifle broke in, then start my load development - which is usually the 168 Amax and Varget.

 

[243Mendoza]

Try Varget or H4895.
The velocities for your loads look low for a 168 bullets.

 

[jepp2]

How much torque on your barrel nut? First thing I would do is take off the brake, and put on a thread protector and try that.

 

[Gr8Horses2003]

Go over to http://www.usrifleteams.com/forums/nationalmatchcompetition. Ask "Panzer". I have several large frame AR's that I have built, and he is the man.

Your MV's are way slow. 4155 may be too slow (is!) for that length. You should try 42 gr of IMR 4064 and see what you get. You may wind up moving more toward Varget-ish burn rate. SG4247 has a great idea...start w/ a known standard then work around to get what you're after.
Harry

 

[DirtySteve]

If you do not have an adjustable gas block, install one. Then tune it to were it is just kicking the brass out and not flinging them into oblivion. AR10's are always over gassed and causes accuracy problems. It shrank the groups of my wife's AR10 from well over and inch to 1/2 to 3/4"... And I agree with Jepp2, Do not over tighten the barrel nut. Put in on and go hand tight, then tighten to the next hole for the gas tube.

Edited.. Get a gas block that can be adjusted with the hand guard on.

 

[Depart123]

I'll second the gas block suggestions. What I am seeing is that your groups don't have any common pattern. It looks completely random. Dispersion that is attributal to barrel harmonics will show a definite pattern as the pressures progress. This looks like bolt bounce, which is caused by inconsistent bolt lock up due to over gassing.

 

[LVLAaron]

Try Varget or H4895.
The velocities for your loads look low for a 168 bullets.

Playing around with Quickload makes me think ~2500 is about the max for a 18 inch barrel. IMR4166 is supposed to be "about the same as Varget" and is right above IMR4064 (not available in any of my stores). My first loads I tried using 8208XBR because I have incredible results with it in other calibers and it's faster than 4166.

 

[LVLAaron]

Go over to http://www.usrifleteams.com/forums/nationalmatchcompetition. Ask "Panzer". I have several large frame AR's that I have built, and he is the man.

Your MV's are way slow. 4155 may be too slow (is!) for that length. You should try 42 gr of IMR 4064 and see what you get. You may wind up moving more toward Varget-ish burn rate. SG4247 has a great idea...start w/ a known standard then work around to get what you're after.
Harry

See my previous comment about burn rate. Was using 8208 (faster) with the same (poor) results. 4166 should be nearly identical to Varget or 4064

 

[LVLAaron]

How much torque on your barrel nut? First thing I would do is take off the brake, and put on a thread protector and try that.

Nut is indexed and uses shims. It's torqued "pretty good" One more or less shim and it was way too far off. The nut on this guy didn't want to turn much past hand tight. Not like my AR15's that seem to have quite a bit of give after hand tight.

 

[LVLAaron]

Sounds like a gas block is a unanimous opinion here. Will get one and try again.

 

[Depart123]

Sounds like a gas block is a unanimous opinion here. Will get one and try again.

It was also suggested to look for binding between the gas tube and carrier key. Binding in that area sure wouldn't be doing you any good.

Sierra 168's are probably the most forgiving 30 cal bullets I can think of. If those don't shoot, then there is problem that probably won't be fixed by further load developement. Another question, is this a barrel with standard rifling or polygonal rifling? Polygonal rifling can be a little inconsistent. You get one that shoots bug holes and the next one just won't seem to shoot at all.

 

[LVLAaron]

It was also suggested to look for binding between the gas tube and carrier key. Binding in that area sure wouldn't be doing you any good.

Sierra 168's are probably the most forgiving 30 cal bullets I can think of. If those don't shoot, then there is problem that probably won't be fixed by further load developement. Another question, is this a barrel with standard rifling or polygonal rifling? Polygonal rifling can be a little inconsistent. You get one that shoots bug holes and the next one just won't seem to shoot at all.

Bolt carrier is smooth as butter and there is no visible sign of binding/rubbing on these parts. I've put 250+ rounds through this so far.

Not sure on the rifling. It is this barrel
http://ballisticadvantage.com/18-inch-308-rifle-ss-premium-barrel.html

 

[Depart123]

Bolt carrier is smooth as butter and there is no visible sign of binding/rubbing on these parts. I've put 250+ rounds through this so far.

Not sure on the rifling. It is this barrel
http://ballisticadvantage.com/18-inch-308-rifle-ss-premium-barrel.html

I couldn't find it mentioned anywhere on the website either. It would be nice if specs like rifling type and gas port diameters were listed for all to see prior to buying.

 

[EWaugh]

Sounds like a gas block is a unanimous opinion here. Will get one and try again.

What Brand/Model of adjustable gas block are you shopping for? Just curious. You may be able to save me some leg work as I'm looking for one as well for a new AR308 build. Also a couple years ago I basically had the same problem on another AR308. I found no factory loads that worked. Tried just about everything that I could get my hands on. Nothing would group. Finally had to handload some 168's with Varget. That got me within 1moa. I may go back and put an adjustable block on that rifle to see if the groups tighten up any. Thanks for the post.

 

[LVLAaron]

I ordered this: https://www.midwayusa.com/product/3...ble-bleed-off-clamp-on-gas-block-ar-15-lr-308

Didn't do a ton of research but it can be adjusted while the handguard is still on which is important to me, and it's the one that Faxon ships/sells with it's barrels.

 

[EWaugh]

I ordered this: https://www.midwayusa.com/product/3...ble-bleed-off-clamp-on-gas-block-ar-15-lr-308

Didn't do a ton of research but it can be adjusted while the handguard is still on which is important to me, and it's the one that Faxon ships/sells with it's barrels.

Yep I just seen that one on Brownells. Looks to have good reviews. Hope that it cures the problem.

 

[LVLAaron]

I have a hard time believing it will make much of a difference. Worst case I put it on a 300 blackout rifle that desperately needs one.


Tempted to order a Faxon heavy barrel while I'm at it. :/

 

[jlmurphy]

I built a DPMS about 10 years ago and it shot about like yours. I guessed that the varying pressure from the mag spring was moving the BC and tilting the bolt. I brazed brass buttons on four spots at the front and the rear of the BC and turned it down on the lathe, reducing the clearance from .030" to .004". I shoots 1/2 MOA all day, I took it out to check zero, 3 shots into 3/8 MOA, including first cold bore shot.