AR 10 Timing and overpressure

[normandinpatriot]

Hello,
I am new to the DPMS pattern LR10 rifle. Not new to the ar platform in general and have shot for years. I have some questions about "typical" damage to brass after it is fired and ejected. but first lets start with a description of my build.

CMT Matched Billet Upper and Lower(dpms pattern)
Lantac Enhanced BCG bolt swapped out to JP high pressure bolt w/ JP firing pin
Ballistic Advantage 1:10 18" barrel (factory guaranteed -1moa) Mid length gas system
Heavy Buffers 5.5oz buffer weight and their recommended buffer spring
SureFire Muzzle Brake/adapter

Issue... After initially breaking in the barrel (only 40rd break in) using 20 168gn Hornady and 20 175gn Sig elite performance. These 2 different rds had some shell damage on the neck. Appeared to from smashing into the locking lugs on the barrel extension, but were otherwise pristine. The next batch of ammo showed signs of excessive pressure. I found online some Lake City M118LR. This ammo had ejector marks on the base, flattened,cratered, and popped primers. I had also purchased some Lapua 167gn as I do want to reload these cases for long range shooting. the Lapua ammo also had flattened, mildly cratered primers and some very mild ejector marks. At this point my Lantac BCG still had the lantac bolt and I was running a standard weight 308 carbine buffer and spring. After some research, and suggestions from shooters on another forum I purchased a heavier buffer and the JP High Pressure bolt assembly. These to things seemed to help, somewhat... the M118 still had some heavily cratered primers and the Lapua was almost as pristine as could be expected. I did just get an adjustable gas block but have yet had a chance to get out and test.

My real question is, can I expect to get rid of these signs with the gas block? or should I be looking for some other issue?

[ray1970]

I assume the headspace checked out good?

[Not_A_Llama]

Preface: Some say the plural of ?anecdote? is not ?data?.

The high pressure bolt with small firing pin hole is the answer to the otherwise disconcerting primer pressure signs you?ll see. I accept that a small degree of ejector swipe is expected.

The adjustable gas block is required on large frame ARs, in my opinion. Troubleshooting is not super straightforward, though, and overgassed can look like undergassed. Play with it a bit if you?re not getting desired operation off the bat.

Edit: AGB can help with ejector swipe. It will do nothing for primer signs.

[Madusa]

Adjustable gas block can reduce the force and speed the action opens. It doesn’t adjust chamber pressure.

[Little Dutch]

It sounds like a headspace problem. I don't think an adjustable gas block will resolve what you're describing.

[normandinpatriot]

Headspace is inspec but on the upper end of the tolerance. I had it checked by a reputable local shop. I contacted Ballistic Advantage to try and get some info about the chamber/ go no gauges/ bolt combo they use to test it at the factory and was NOT given great customer service. They basically said that their products are in-spec and if it wont shoot sub MOA to send it back for a refund or new barrel..

I am curios about why with the first 40 rounds i fired I had 0 signs of over pressure but on this different ammo(particulary the M118LR) it shows as overpressure...

Those who said it sounds like a headspace problem, I would done right agree but these same rounds shot out of another LR308 have the same symptoms. My friends rifle is a factory DPMS LR10 with less than 1000rds through it... It makes me think it may just be the ammo is loaded to hot...

[normandinpatriot]

Preface: Some say the plural of ?anecdote? is not ?data?.

The high pressure bolt with small firing pin hole is the answer to the otherwise disconcerting primer pressure signs you?ll see. I accept that a small degree of ejector swipe is expected.

The adjustable gas block is required on large frame ARs, in my opinion. Troubleshooting is not super straightforward, though, and overgassed can look like undergassed. Play with it a bit if you?re not getting desired operation off the bat.

Edit: AGB can help with ejector swipe. It will do nothing for primer signs.

Not_A_Llama, can you give me a little bit more info? How does overgassed look like undergassed? and vice-versa? I put an AGB(Odin Works) on it and started at about 10 clicks out from closed and the bolt didnt move. I ended up at 18clicks to get the bolt to lock back when shooting one round. I only tried 2mags with multiple rounds in them and both times the bolt didnt lock back...I did adjust the gas to the full open position(20) between the two mags. The condition/ overpressure sign on the shells seemed to be reduced to mild flattening of the primers on the Lapua ammo and mild cratering of the M118 with one popped primer. No ejector swipe to speak of.

[Not_A_Llama]

Not_A_Llama, can you give me a little bit more info? How does overgassed look like undergassed? and vice-versa? I put an AGB(Odin Works) on it and started at about 10 clicks out from closed and the bolt didnt move. I ended up at 18clicks to get the bolt to lock back when shooting one round. I only tried 2mags with multiple rounds in them and both times the bolt didnt lock back...I did adjust the gas to the full open position(20) between the two mags. The condition/ overpressure sign on the shells seemed to be reduced to mild flattening of the primers on the Lapua ammo and mild cratering of the M118 with one popped primer. No ejector swipe to speak of.

Sorry about the delay in response - the forum didn't show your post as new to me.

In my experience: Getting too much gas into the system can sometimes cause failure to lock back on a mag with a single round. Exacerbating the issue, some AGBs have steps that are too big, and you can step from undergassed to overgassed very quickly. I've found that using a continuous infinitely adjustable gas block is better than a detent-based system in large frame ARs, because they're much more gas-sensitive than small frames.

[Delfuego]

Better to add weight to the carrier and/or recoil system. I had an over gassed AR260 (most AR10's are) and it solved the issue with no moving parts. No more marks on brass and they all landed in the same spot. Gas port sizes are not standard and many are too large. Adjustable gas block can solve it, but can be a pain and unreliable depending on the make/model.

[normandinpatriot]

Better to add weight to the carrier and/or recoil system. I had an over gassed AR260 (most AR10's are) and it solved the issue with no moving parts. No more marks on brass and they all landed in the same spot. Gas port sizes are not standard and many are too large. Adjustable gas block can solve it, but can be a pain and unreliable depending on the make/model.

I have a heavy than standard buffer/ spring combo init already(5.5oz) from heavybuffers.com. The gas block i installed has made a difference. I finally got a chronograph, and was able to get some velocities on the ammo that has caused me concern... seems HOT coming out of an 18" barrel... averaging just over 2600fps. I havent been able to find the Sig or Hornady ammo i used for my barrel break-in. to test their velocities...

Did several 5 shot strings of the Lapua 167gn otm and didnt allow for full cool down in between the first and second string... velocity jumped up by an average of 50fps... 3 strings with full cool down in between averaged just over 2600fps.
The lake city M118LR i only shot 2 strings with full cooldown in between and that had an average of about 2630fps. still seems hot for a 18" barrel...
Thoughts?