DPMS 308 Issues

[AlterEgo]

Gents,

I've got a DPMS 308 that I don't think is quite right yet. In the interest of disclosure, the rifle is as follows:
-1/10 24" SS
-Standard DPMS tube FF HG (and the barrel nut is now properly torqued)
-CMMG lower, RRA 2 stage non adjustable trigger/hammer, A2 buttstock w/extender
-Bushnell 4200 FFP 3-12 in an extended GGG mount
-JP adjustable gas block

Had it out some time ago and was shooting ~150 gr Federal something through it (FMJ) so I know that's probably optimal for the twist rate, HOWEVER the rifle was putting down pretty crappy groups in semi (from bags). I tightened down the setscrew fully on the JP block to shut the gas off and the groups tightened significantly, to say ~1.5".

When in semi I seemed to be chasing the thing around a bit, and it seems that the gas adjustment was moving around.
Have any of you guys had problems similar to this?
Within a window of a normally functioning rifle, how sensitive is grouping to the gas adjustment on the block? What increment of adjustment seems meaningful?
While the JP block does seem to adjust gas well enough, I would describe the arrangement of holding that adjustment as piss poor to non-existent. What do you guys do to alleviate that?

 

[RobertM]

Re: DPMS 308 Issues

Good post from SH here that I saved for further study. Would be bulletproof if you have the tools to do it:

AR adjustable gas block

 

[AlterEgo]

Re: DPMS 308 Issues

Yeah, I've seen that. Might do it, the downside is if one gets too enthusiastic on the retaining setscrew you can booger the threads, which would probably booger the block when the adjusting setscrew is removed. Obviously that method will lock down the adjustment though. Was thinking about spot facing the existing hole and adjusting length of 6-32 screw (or whatever it is)...

 

[*greenhorn*]

Re: DPMS 308 Issues

Yea the gas block has moved.

Put in a box take it to a gun smith or mark where the set screws on the gas block screw in to barrel make an indention so the sets have a bit on barrel and red lock tight them.

 

[Zebra644]

Re: DPMS 308 Issues

The 150 grain is NOT the optimal load for a 1:10 twist on the 24 barrel. The 150 grainers are just break in fodder. That bullet weight and velocity is not meant for a 1:10 twist, You need use 168 and 175 in Federal Gold Metal Match or Black Hills you will see the pattern tighten up. The gas block issue, my 2 cents is why run and adj. gas block if you dont have too. Are you running a supressed weapon sustem or having over gassing issues? If possible call DPMS and order the gas block for that barrel. I would then buy you a 20 rnd P-Mag for the .308 to ensure no feeding problems and then go shoot. My experience is with this system that yo wont see the true capibility of this guns accruacy until about 300-350 rnds. Buy then you should have shot it enough to learn the platform and the barrel has enough copper fowling in it. You have a good set up dont get frustrated with it.

 

[KYpatriot]

Re: DPMS 308 Issues

I agree with zebra...it definitely sounds like you have a problem with the gas block, but the 150 grain rounds aren't helping.

I'm guessing that once you have a consistent setting on the block that you will see some significant improvement in the groups with 168s or 175SMKs, 178AMAX or something along those lines.

As usual, don't forget to torque the rings/scope mount. You could have a shifting zero problem here.

 

[AlterEgo]

Re: DPMS 308 Issues

Understood about the 150grainers, knew that going in. HOWEVER, it did shoot them fairly well w/o gas. I would not have expected the difference between gassed and not gassed if the twist rate was the dominant factor.....but understood, 168's or preferably heavier. Rings/scope is tight, been caught out by that before.

This thing seemed pretty overgassed from the get-go. It is reliable and much calmer to shoot with the adjustment screw ~3 turns out from bottomed on the gas block.

 

[Zebra644]

Re: DPMS 308 Issues

Dont take this as me sounding rude. All the gas does is cycle the bolt. Obviously you are moving a much larger and heavier BCG than it little cousin in 223 / 5.56. Were you using the stock gas block before you put on the adjustible gas block. Were you having issues with the brass being deformed other than by being ejected or primers poping out? Or were you using the recoil as a sign of over pressure? What made you think there was an issue or the did the gun come built this way? Also the twist rate and the bullet velocity and weight are the difference between shooting fairly well and a .5 moa group using the correct bullet for that barrel and twist. As I stated before I have a DPMS LR-308 with a Wilson Combat TTU single stage trigger. The barrel on these are aftermarket barrles not in house DPMS. Mine will shoot .5 moa as long as I do my job yours is also capible. The reiliablity you mentioned was the gun having feeding issues? If so that most likely would come from the mags themselves. Invest the 20 buck in a Magpull 20 rounder and that will solve you issue there.

 

[TenZero]

Re: DPMS 308 Issues

Your issue is the cheap ammo.

Mine shoots 1.5-2.5" with 150gr Federal FMJ. It shoots 5 shots sub MOA with both FGMM 168s and 175s.

I have the same gun, but stock trigger and gas block.

 

[bunsen27]

Re: DPMS 308 Issues

I run the JP adjustable block on my 3gun AR's and LR-308 and have had no accuracy issues. If anything it has improved accuracy by reducing any overgas issues. The ammo selection wasn't optimal as noted. Shutting the gas off makes the rifle similar to shoot as a bolt gun. AR's tend to take a little bit more of follow through than bolt guns. Get some FGM 168's and verify everything is tourqed to spec then see where you are at.

 

[GoBig]

Re: DPMS 308 Issues

Yes...try 168 to 175/178 grain of either black hills or Hornady Tap and report back to us.

 

[AlterEgo]

Re: DPMS 308 Issues

Well went to the range yesterday and got a hold of some FGMM 168's. Have a cold right now and felt like crap so shot like crap, actually I wouldn't call it shooting firearms... perhaps rather just discharging them or straight up turning money into noise.

Anyhow did get 1 or 2 good groups with the 24" upper. Also shot a mutt upper of a Cobb barrel, DPMS upper, and Armalite BCG group. Never got it on paper, LOL and my FAL slung rounds all over the place. Oh well. The AK I made did work though

 

[Zebra644]

Re: DPMS 308 Issues

Did the ammo change take care of your problems?

 

[Tx_Flyboy]

Re: DPMS 308 Issues

Alterego,

I too bought an adjustable gas block in anticipation of adding a suppressor to my rifle in the future. After several range outing with handloads here is what i have found;
1. The setscrews on most adjustable gasblocks such as the JP gas block is a sorry design. They become loose the more you back them out, may fall off the gas block entirely.
2. Adjustable gas block affect your group size and accuracy nodes. I noticed a shift in accuracy nodes (0.6 grains worth) with the setscrew backed off significantly and with the setscrew all the way in.
3. The position of the setscrew also affects your POI.
4. If you loose the setcrew (mine fell off the gasblock), you will have cycling issues.
5. The only gasblocks worth buying are designs with a throw lever that stay in a fixed position such as the noveske switchblock...if nothing else, at least you know the exact position of the lever, and what your accuracy nodes oughtta be.

I have gone back to to an A2 front sight post. I dont have to worry about the gas block moving, or loosing a setscrew or anything like that.

Lots of luck.

 

[cz777]

Re: DPMS 308 Issues

if somebody would make or mod samson gas block ar-15 [but for the 308] with 4 set screws and dimple the barrel with blue loctight ...