Over-pressure only on 2nd and later rounds?

Moderators:gds, bakerjw, renegade

Post Reply
BlogSarge
Silent But Deadly
Posts:1311
Joined:Mon Oct 21, 2013 11:10 pm
Over-pressure only on 2nd and later rounds?

Post by BlogSarge » Thu Apr 02, 2020 5:53 am

An acquaintance posed me this question.

He has a bolt gun and AR10, both in 308 win. He has some handloads from a friend.

In his bolt gun they fire fine and group respectfully. In his AR10 the first round fires well, and the brass shows no over-pressure signs. The next rounds out of the magazine fire and these pieces of brass have primer cratering and mis-shapen extractor marks on their rim. He did not specify if the AR10 groups are good or bad.

He has tried the loads in both guns with and without his Omega 30. The results are the same.

I suggested he shoot some factory rounds and see if the same thing happens.

I have not seen the brass myself, nor shot the guns, so I can’t give first hand observation.


Any ideas? I told him it looks to me like the friend’s hand loads are a bit hot. But I do not understand why the first round out of the AR10 does not show the signs.

BulletFlight for Android
golfindia
Silent But Deadly
Posts:1380
Joined:Thu Jun 20, 2013 1:16 pm

Re: Over-pressure only on 2nd and later rounds?

Post by golfindia » Thu Apr 02, 2020 7:07 am

Not sure about the first round part, but my bet is that it's overgassed. Pretty common in the large platform ARs.

The rims get chewed up from violent extraction.

User avatar
Dr.Phil
Silent But Deadly
Posts:1654
Joined:Tue Oct 05, 2010 4:50 pm

Re: Over-pressure only on 2nd and later rounds?

Post by Dr.Phil » Thu Apr 02, 2020 7:11 am

Deleted because bullet setback is clearly the cause.

Slight crimp would resolve the issue.

If you ever meet my Wife, please tell her that I can & do admit when I'm wrong.
(After I delete evidence of it from the internet...)
Last edited by Dr.Phil on Sat Apr 04, 2020 11:11 am, edited 1 time in total.
"Don't tell fish stories where the people know you; but particularly,
don't tell them where they know the fish."
--Mark Twain

User avatar
bangbangping
Silent But Deadly
Posts:2622
Joined:Sun Sep 15, 2013 6:34 pm
Location:Texas Gulf Coast

Re: Over-pressure only on 2nd and later rounds?

Post by bangbangping » Thu Apr 02, 2020 9:54 am

A possibility is that the bullet is moving forward to touch the lands when cycling after firing. Hard to guess without knowing anything about anything.

TRshootem
Silent But Deadly
Posts:236
Joined:Fri Dec 26, 2014 1:13 am
Location:Montana

Re: Over-pressure only on 2nd and later rounds?

Post by TRshootem » Thu Apr 02, 2020 2:30 pm

...hand loads from a friend....

I'd be pulling some bullets, weighing the powder charge, hope he has the load data for specific powder used. I have only one friend I trust reloads from...me. All else get checked as above, it's easy to be safe, sorry can hurt. What performs as intended in my bolt gun can make trash brass in AR10's. These days, regardless of build, the gas port is measured and recorded, adjustable gas blocks then can be installed if needed.

User avatar
plant.one
Silent But Deadly
Posts:6788
Joined:Tue Jun 23, 2009 8:31 pm
Location:Oakland County, MI

Re: Over-pressure only on 2nd and later rounds?

Post by plant.one » Thu Apr 02, 2020 2:43 pm

bangbangping wrote:
Thu Apr 02, 2020 9:54 am
A possibility is that the bullet is moving forward to touch the lands when cycling after firing. Hard to guess without knowing anything about anything.
even chances of that or possibly getting pushed back into the case too. first cycle is not nearly as violent of a cycling as doing so under full gas operation during fire - espeically if the system is overgassed , like most 308's already are.

i bet that buffer is getting slammed into the bottom of the receiver extension, doubly so if the loads are running at the high edge of things.


again - as you and others mention - without more info its just a WAG at this point.
Reloading info shared is based on experiences w/ my guns. Be safe and work up your loads from published data. Web data may not be accurate/safe.
This disclaimer will self destruct in 10 seconds.

BlogSarge
Silent But Deadly
Posts:1311
Joined:Mon Oct 21, 2013 11:10 pm

Re: Over-pressure only on 2nd and later rounds?

Post by BlogSarge » Thu Apr 02, 2020 3:35 pm

Hi All,

Thanks for sharing your knowledge and thoughts. I thought it through as best I could but ended up coming here for better thoughts! I’ll pass the info on to my friend.

I told him when he asked the question that the best thing he could do was to shoot factory loads. I think I’ll reinforce that to him again.

Cheers to all, and stay healthy.

Post Reply