I have some local guys that build ARs and are now building some prototype Gen 2 AR10s. Very nice. They have an issue with a couple of their prototypes. Every casing has marks imprinted on it from the dual ejectors. I was called in because they think it is a headspace issue and want me to fix it. There is no deformation of the primer. Other higher level competition barrels have no issue using same type barrel extension and on the same platform so barrel is only variable. Appears to be about .008 more space in the ones creating the marks. I don’t have the barrels yet but expect them to be in my possession in the next 2 days.
Need some help on this and if you think headspace is the problem, then how should I resolve it? I figure it will involve a lathe but would like to hear from the pros. KB
I have been assuming the 0.08″ is the excessive headspace, or head clearance if you talk to certain purists. That’s quite a bit for a new high-power rifle. Nevertheless I agree with Jeff that excessive headspace usually doesn’t cause ejector indentation of the brass head. Rather it is a red-flag indicator of over pressure. –TL
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OK so I want to understand the problem/issue. The base of the case is marked on the back where it is blown into the breech face correct? This could be from headspace but usually from pressure like others mentioned. Too much pressure and a combination of headspace could cause it. Too much pressure could be from too tight a neck area, too short a throat, tighter chamber overall, too much powder etc. Ken
Question Follow Up
Just so you understand my interest in helping these guys, they are good people, honest and fair. They have helped me learn to properly Cerakote and allowed me to even use their facility and materials for free. We have set up a mutually beneficial relationship, I send new higher end AR purchasers to them and they send gunsmith business to me. All are prior service in the military (spec ops) and have considerable depth of knowledge in use and building of the firearms they sell. This one just has them stumped.
Yes, it is making compression marks on the casing in the shape of the ejectors. Every casing. Also, to make it clear these guys have the barrels and the extensions made separately and mate them up. They have built a considerable number of custom ARs, both 15s and 10s. They are now working a project to develop a Gen 2 AR10. The issue is on their prototypes with a particular type of barrel. They are currently experimenting with 2 types of barrels.
They did some rudimentary measuring and found they had roughly .008 more headspace in the problem barrels than the other type they use. They are getting no compression marks on the other type of barrels that have less headspace. Using same ammo. So their assumption was headspace, I don’t know enough yet to completely agree. I loaned them my go/no go and haven’t heard back yet. So, I am asking the pros. I will discuss pressure with them and get more data points. I was shooting for simple first, but alas.
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Are you using 7.62×51 ammo or commercial 308 Win ? Huge pressure difference , 308 being 12000+ psi higher.
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Sounds like you have located the problem. Headspace. There is a gap between the base of the case and the breech face. The constant tension ejector pushes the case forward fully in the chamber then when the round is fired the case is driven rearward against the breech face by the expanding gases. Even if the ejector didn’t push the case forward the firing pin would when it strikes the primer. Ken
