Very low ES and SD, but poor groups

[cbcanderson]

I built a 308 AR10 with a 20 inch 1:11 twist barrel from X-Caliber. Accuracy wasn't what I was hoping for with 168 grain reloads so I tried 175 Grain SMKs with Varget as my powder and Lapua brass (3rd reloadings). Powder charges varied from 43.9 to 45.0. 5-shot groups were still averaging 1.5 to 2.0 inches. The strange thing is that at the lowest and highest charge-weight I found two nice nodes -- my extreme spread was 9-10 FPS and my standard deviation was 5 FPS. Usually, reloads with that consistency shoot bug-holes groups for me. However, these groups were nothing to be happy about. I should also note that the scope base and rings are tight, scope is a Vortex Viper PST, and I can shoot a nice group (I have two rifles of mine that I can shoot consistent 0.3 to 0.5 inch groups).

Anybody else ever experience this (great ES and SD, but lousy groups? Any advice? Thanks!

 

[MK3XXX]

Have you messed with seating depth?

 

[The King]

When gas guns go wrong:

-Someone used to shooting a bolt gun and doing it well doesn't know about lock-time problems.
-The gas gun bolt velocity is too high. Damaging the bullets and/or causing setback.
-The barrel sucks. Even good companies produce a lemon, and said lemon is less common in the best makers but still there
-Headspace was close to the edges or flat out non-spec
-Barrel fit loosely in the upper before it was torqued down, causing rotational walking under firing stress
-Gas system touching rail under firing. Especially common when people put bipods or pic rails near the gas block

 

[Neurotic]

Three more:
- Barrel extension feed ramps damaging the boolit.
- Barrel nut undertorqued.
- Barrel crown or muzzle device damage.

Edit: gas port burr

 

[sam4886]

My first thought was seating depth too. All the above listed gas gun fuckery could also be causing problems, but I'd mess with seating depth first, its pretty easy to change.

 

[cbcanderson]

When gas guns go wrong:

-Someone used to shooting a bolt gun and doing it well doesn't know about lock-time problems.
-The gas gun bolt velocity is too high. Damaging the bullets and/or causing setback.
-The barrel sucks. Even good companies produce a lemon, and said lemon is less common in the best makers but still there
-Headspace was close to the edges or flat out non-spec
-Barrel fit loosely in the upper before it was torqued down, causing rotational walking under firing stress
-Gas system touching rail under firing. Especially common when people put bipods or pic rails near the gas block

Thanks for the advice. All really good points. I've tried to address most of these issues:
1) I used an adjustable gas block to try to get the right amount of gas to avoid an over-gassed situation (I hope that helps with lock-time) and the brass is coming out of the chamber in really good shape -- virtually no swipe marks and no pressure signs
2) The load I am using seems pretty much in the range of what gas-gun recommendations are for the cartridge/bullet combinations
3) Barrel may be a lemon -- I built a 6mm AR from the same company and it shoots 0.5-inch conistently in an AR15 platform
4) I do need to re-check distance to the lands and play around with seating depth -- that will be my next effort
5) Barrel fit pretty tight into the upper before I torqued it down, but no guaranteed that the barrel was bored in perfect alignment (I always wonder what about that)
6) I cut away a portion of the hand guard to ensure that the rail couldn't come in contact with the block

All that being said, even though I've been messing with gas gun builds for a decade, I feel like I still have a lot to learn. Thanks for sharing the advice!

 

[cbcanderson]

Three more:
- Barrel extension feed ramps damaging the boolit.
- Barrel nut undertorqued.
- Barrel crown or muzzle device damage.

Edit: gas port burr

All good points. I single hand-feed when I'm testing loads to make sure nothing is happening as the cartridge comes out of the mag. Barrel nut is on tight, but I do need to check the crown. I also wondered about a bur -- do you guys use a bore scope and is it obvious to see a burr on the gas port hole?

 

[cbcanderson]

My first thought was seating depth too. All the above listed gas gun fuckery could also be causing problems, but I'd mess with seating depth first, its pretty easy to change.

Thanks -- that is where I'm going next. I'll measure distance to lands and then mess with seating depth. Right now I'm seating the 175 SMKs at 2.810.

 

[The King]

Also, try some FGMM in it. if it don’t shoot that it ain’t gonna shoot imho.

 

[sea2summit]

Are the rifles you shoot 1/2 MOA with gas or bolt?

What muzzle device? AR platforms seem to be more sensitive to over torqued muzzle devices.

FWIW my .308s do a lot better with 4064 than varget.

 

[Fig]

Seating depth first. I've found more variation in gas guns than bolt guns. Start at ten thou off the lands, and set back in groups of five till the groups get tight. My guess is that this will fix it. If not then address the gas gun foibles, but for me seating depth is usually a critical aspect of tuning the ammunition. You're not done when you find a node.

 

[cbcanderson]

Are the rifles you shoot 1/2 MOA with gas or bolt?

What muzzle device? AR platforms seem to be more sensitive to over torqued muzzle devices.

FWIW my .308s do a lot better with 4064 than varget.

Two bolt guns, two gas guns (6mm AR, 6mm CM) in my stable that will do 0.5 MOA -- although the 6mm CM dips below and above that mark depending on the group. Muzzle device is a generic 3 baffle brake, with small ports on top.

 

[cbcanderson]

Seating depth first. I've found more variation in gas guns than bolt guns. Start at ten thou off the lands, and set back in groups of five till the groups get tight. My guess is that this will fix it. If not then address the gas gun foibles, but for me seating depth is usually a critical aspect of tuning the ammunition. You're not done when you find a node.

Thanks for the feedback. The next set of loads will be all about seating depth. I really value the advice, guys. I can't believe how much experience is available in this community of shooters. Your willingness to share hard-earned widsom is truly appreciated. I'll report back on how things go at the range. All the best!

 

[striped1]

Have you tried factory match ammo in a couple of different bullet weights and manufacturers to see if there is something it shoots well? If it shoots nothing well more likely barrel issue somewhere if it shoot something well it is more likely the handloads.

 

[cbcanderson]

Have you tried factory match ammo in a couple of different bullet weights and manufacturers to see if there is something it shoots well? If it shoots nothing well more likely barrel issue somewhere if it shoot something well it is more likely the handloads.

I haven't yet, but will see if I can track down some factory loads to test, including FGMM or Hornady Match. Thx!