Help on AR10 Headspace Please!
Seems there was a recent post about how somebody was having extraction problems on an early, NIB AR10. He checked headspace and found it excessive. This got me thinking and I did the same check on mine. Sure enough, it closed on a Forster NO-GO gauge!
Did I do something wrong or is the AR10 headspace correctly calculated in a different manner?
I stripped the bolt of extractor and ejector. I held the upper vertically then dropped the gauge in the chamber. I took the stripped bolt by itself and placed it over the rear of the gauge. By twisting the bolt, it turned and locked with minimal effort.
While the rifle is older (about 5 years) and I bought it used, it does not show enough wear to be shot out. It did have a "C" series bolt which I exchanged and this could be the problem.
The rifle does eject fine, but being that loose concerns me. Has anybody else checked their headspace in this manner? (This is an AR10A4)
I've forwarded your post to ArmaLite as they are better qualified to respond.
A Forrester NO GO is 1.634", AR-10 specs are 1.633-1.636", so most AR-10s are going to lock on a Forrester NO GO.
Take a look at armalite.com, Technical Note 11.......that note has been updated and will, hopefully, be helpful.
The original barrel on my AR10A4 closed on a 1.634" gauge. Using my Stoney Point case measuring tool to compare fired cases to the 1.634" gauge, I concluded that the headspace is around 1.637". My fired cases measured exactly .002" longer than the 1.634" gauge and I allowed .001" more for springback to come up with 1.637".
This much headspace is not going to be a problem unless you handload. If you are going to handload for it get a set of Redding competition shellholders. For target shooting, use the shellholder that sizes the case just enough for the bolt to close. This will give you minimum headspace and prevent case head seperations. But know this, tight headspace is not a good idea for a fighting rifle, whether it's LE, self defense, or whatever. Ammo that might be used in a fight should not be assembled on fired brass anyway in my opinion. Use new stuff for that.
If you use a standard shellholder to resize you will have too much headspace and if you repeatedly fire a case under these conditions you will experience a case head seperation at some point, usually around the fourth cycle.
After reading this thread, including the Armalite Technical Note 11, I see no way to check an AR10 for headspace. Armalite gives their specs, then at the end says you need a custom guage.
But when I look at their catalog it looks like they only offer a 'standard' Forster guage in 308?
Thanks for the replies. I found the tech note and I now understand that Armalite runs the headspace a bit loose by design. That's their call, I guess.
The only rifle for which I have a military technical manual is the FAL. It suggests headspacing the weapon as close as 1.630" as possible and condemming the barrel at 1.638". i.e. normal SAAMI "go" and "field reject" points. Obviously, this doesn't mean anything in a discussion about AR10s, but it's the only point of reference that I have for a well established military weapon.
My other question would be, "at what point does Armalite condemm the barrel?" Their specs don't leave much room for the headspace to grow, and this is not mentioned in the tech note.
Another concern is the fired brass. It has a shiny ring around it near the base, which is indicative of large headspace, as this is where the brass is stretching and thinning. The same ammo fired through the FAL does not do this, so clearly the AR10 chamber is larger.
I dunno, I don't think I have a problem from Armalite's perspective, but the whole setup looks strange from my experience.
FN specs for a brand new FAL were 1.6315 thru 1.638, and they have a max spec of 1.640 for a FN factory overhaul. Don't know what field reject is for a FAL or a AR-10, but for the M14 it is 1.6445.
This is an interesting and too confusing topic, which is why we published tech note 11.
Who'da guessed that you can have interferrence allowed by cartridge and chamber specs.
Normal NATO and SAAMI tolerances allow .010 between min and max. 1.630 to 1.640. Each rifle design tends to work best at a range within this loose spec.
Field reject and Overseas Embarkation gages don't sort for safety: they're extra narrow ranges that predict that a rifle will last long enough for combat before wearing out. The military doesn't like to send out rifles that are about due for a new barrel.
We can tell you that we've never coded a barrel out for headspace growth. On the AR-10 and M14 you'll see a tiny bit of growth as phosphated surfaces wear in, but then headspace won't change much for 10s of thousands of rounds. The FAL receiver yields over time and headspace grows faster. That's corrected by replaceable, numbered locking shoulders (the shoulder the bolt locks down int.)
Barrels code out as they wear, and accuracy degrades. Throat erosion is the first thing noticed, but accuracy is the real gage.
We suspect that the ring you see on the cartridge is due to the cartridge turning during unlocking. The FAL bolt doesn't turn; it tips up and the cartridge won't get that ring.
Hope this helps.