DPMS LR-260 Consistantcy

[zombies-R-real]

Have a question concerning repeatable accuracy with my LR-260. I've been working up a load for 140 A-Max using H4350. Now this may sound funny but it is something I noticed. I am using Nosler brass and it seams that the accuracy is dropping off as the brass has been resized around the third time. I do crimp with a Lee FCD so the neck tension is the same. The only mod on the rifle is a Gieselle DMR trigger. I think the rifle might be over gassed a little and will be installing a JP adjustable gas block to slow the action down a bit. The extractor is marking up the brass pretty good. Seeing I'm new to AR's, not sure if that's normal.

Any info and recomendations would be a big help.

 

[bowslngr]

Re: DPMS LR-260 Consistantcy

Yes, it is fairly common to get ejector swipes in the DPMS 260s, and yes, it is a symptom of an a pressure problem. However, it is not technically an 'overgassed' situation. The problem isn't too much port pressure, but rather too much chamber pressure at the time the bolt is trying to unlock.
An adjustable gas block may help, but it treats the symptom rather than the cause. I think most guys have good luck with H4350 & 140 Amaxs as long as they don't load too hot.
Your crimp is also adding pressure, and honestly I would do away with it if I were you. It is not needed, and it will increase variability in neck tension if not done perfectly consistent every time -- something that is nearly impossible to do.

 

[Greg Langelius *]

Re: DPMS LR-260 Consistantcy

I tried the FCD for a time, did not find any additional accuracy there.

Where necks are concerned, brass hardening has the largest effect on accuracy. Whatever works the brass least seems to be the better approach. I'd remove the FCD and start over with new brass. See if that helps.

I don't shoot the .260 in a semi. I have read repeatedly about difficulties with the .260, 140-ish bullets, and pressures in AR based semis. Unless I were primarily tasking the rifle for maximum distances, I'd try something in the 120gr-ish range and try to keep the pressures down below the max loads. I know the .260 can drive 120's to Kyd. But I also think it helps to have a long barrel, less twist, and a firearm that's not finicky about pressures; preferably a bolt gun.

Greg

 

[bowslngr]

Re: DPMS LR-260 Consistantcy

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Greg Langelius *</div><div class="ubbcode-body">I tried the FCD for a time, did not find any additional accuracy there.

Where necks are concerned, brass hardening has the largest effect on accuracy. Whatever works the brass least seems to be the better approach. I'd remove the FCD and start over with new brass. See if that helps.

I don't shoot the .260 in a semi. I have read repeatedly about difficulties with the .260, 140-ish bullets, and pressures in AR based semis. Unless I were primarily tasking the rifle for maximum distances, I'd try something in the 120gr-ish range and try to keep the pressures down below the max loads. I know the .260 can drive 120's to Kyd. But I also think it helps to have a long barrel, less twist, and a firearm that's not finicky about pressures; preferably a bolt gun.

Greg </div></div>

Greg,
Believe it or not, you run into the port/chamber pressure problem more frequently with the 120s than the 140s. In most cases, in oder to get around it you have to run fast for bullet weight powders that wind up limiting velocity greatly.

 

[Greg Langelius *]

Re: DPMS LR-260 Consistantcy

OK; good to know; thanks for the upddate.

Greg

 

[CowboyBart]

Re: DPMS LR-260 Consistantcy

I am using a surplus powder (IMR 7383) and have had good accuracy results. I have had some failure to eject issues using the starting load and will be bumping up the charge a bit in an effort to correct this.
I don't know how long the bullet is that you are shooting, but at an 8.5" twist you should be OK (unless you are shooting in Death Valley or somewhere else well below sea level).
I do not use the FCD and have loaded my plinking ammo in once fired 243 brass. My accuracy loads went into new 260 brass, but now will be going into once fired 260 brass.
You could try annealing and see if that makes a difference.

 

[zombies-R-real]

Re: DPMS LR-260 Consistantcy

Thanks guys. Bowslngr, I am not really pushing the load that much. 39.7 gr of H4350 is well below max load. I have worked a little bit with the BL-C2 powder witch is faster just not as consistant as the 4350. I think that maybe an adjustable gas block adjusted down a little would delay the bolt enough that the chamber pressure would be less at the point of the bolt unlocking and extraction. I did notice that with a faster burning powder, the bolt doesn't seam to cycle as hard. Do the 308's have the same problem in the AR platform?

 

[bowslngr]

Re: DPMS LR-260 Consistantcy

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: ZombieStacker</div><div class="ubbcode-body">Thanks guys. Bowslngr, I am not really pushing the load that much. 39.7 gr of H4350 is well below max load. I have worked a little bit with the BL-C2 powder witch is faster just not as consistant as the 4350. I think that maybe an adjustable gas block adjusted down a little would delay the bolt enough that the chamber pressure would be less at the point of the bolt unlocking and extraction. I did notice that with a faster burning powder, the bolt doesn't seam to cycle as hard. Do the 308's have the same problem in the AR platform? </div></div>

At 39.7 grains you shouldn't be seeing any ill effects. Very odd. If you have the coin, an adjustable gas block may be a good idea. Even if it doesn't solve this problem you can use it to help better tune the rifle to other loads.

On the 308s: No -- they do not have this problem typically. Different shaped pressure curves due to the larger bore. If DPMS or some other manufacturer were to go back to the drawing board on the 260 AR I would bet they would come up with a configuration that moves the gas port farther down the barrel a bit.