Armalite AR-10 bolt questions.

[MBuechle]

I have an AR-10 bolt that I need to bush the firing pin on. The bolt seems at first examination to be fairly hard, too hard to machine with high speed steel. Anyone know if it's through hardened or case hardened.

The second question is, does any one know what Armalite did for the bolt in their .300 SAUM rifles. Obviously, open the bolt face, but what of the extractor? I want to build a 7MM SAUM upper, will do the barrel work myself. Any info would be appreciated. I realize that this is primarily a bolt action crowd, but their's a lot of experience here!

 

[mac86951]

I have an AR-10 bolt that I need to bush the firing pin on. The bolt seems at first examination to be fairly hard, too hard to machine with high speed steel. Anyone know if it's through hardened or case hardened.

The second question is, does any one know what Armalite did for the bolt in their .300 SAUM rifles. Obviously, open the bolt face, but what of the extractor? I want to build a 7MM SAUM upper, will do the barrel work myself. Any info would be appreciated. I realize that this is primarily a bolt action crowd, but their's a lot of experience here!

Should be thru hardened, but this is the one that I'm not 100% on. Magnum extractor is typically a modified 308. Try accuracy systems Inc in Byers CO. May have extractors. Get extra. And don't let pierced primers build up under extractor.

Finally get a +2 gas tube or custom length from white oak armament and drill gas port to match.

I've a 7SAUM lr-308 and love it. Plan on milling open the receiver ejection port too.

 

[Dusty Stevens]

Just get a new bolt and install it in the bolt carrier if youre actually blanking primers. Youll have to spot anneal that boltface to drill it they are made to be very very hard all the way thru. If you spot annealed it you may get too far and get the lugs so its best to just replace it

 

[SG4247]

True Armalite 308 bolts have different firing pin and hole dimensions and clearances - than all the other brands. You should not need to bush a genuine Armalite 308 bolt for high pressure. Other large frame ARs can often have primer problems that is solved with an Armalite 308 BCG assembly.

Also, the face depth of the bolt face is pretty thin, so even if you could drill the firing pin hole easily to plug and redrill it, there is not much material to work with.

 

[SG4247]

http://www.shootingtimes.com/long-guns/longgun_reviews_st_ar10ultra_062309/

Also note in the article the mention of a “special” barrel extension used.

 

[MBuechle]

Knew I'd get some good info here. The reason I wanted to bush is that 3 blown primers torched out my firing pin hole. The bolt is fitted to my .22-250 upper barrel extension with .001 - .0015" clearance. I have a new bolt but the lugs are .004 longer than the old one and won't fit that upper. The old bolt is from 2004, the new one is recent mfg. from Armalite. I guess I could chuck the new one up in the lathe and with a tool post grinder, adjust the lug length to get the clearance I need, or just live with the old one. Attempting to spot anneal seams like a ticket to a goat rodeo to me. I just get a little ridge around one side of the firing pin imprint and it's annoying but in 1500 or so rounds of .22-250, I've not blown any primers. The primers that did the damage were SA mil surp. .308 rounds.

I have read the Shooting times article, I believe the difference in extensions has to do with the feed ramp, which could be done on the Bridgeport. I used the BAT extension in the .22-250 and would do the same in any future upper builds. I concur on the +2 for gas port location, went +5 on the .22-250 and it shoots like a dream, very easy on brass. So that just leaves the extractor question un-answered. Guess I'll contact Accuracy systems.

 

[carlsbad]

If it were mine I'd try to do the bushing job with solid carbide tooling. There won't be an option to thread the bushing so I would press it in with a significant interference fit. I also would not be afraid to adjust the lug length with high quality carbide tooling.

Neither are jobs for beginners, both have some risk so smiths won't want to do them. Like I say, that's what I'd do if it were mine, make your own informed decision.
--Jerry

 

[SG4247]

The only tools that would cut the Armalite bolt or carrier for me was Carbide. I ruined 3 new HSS taps making threads in one hole. The tap did make the threads fine, but it was tricky. I was wishing I had a carbide tap at the time. Seems like the parts were RC 58 on the tester. Case hardness was deeper than I expected also, and would roll the threads on the HSS tap.

I have not ever attempted to cut the bolt lugs in the lathe. But they can be ground with little trouble on a Chevalier cylindrical grinder. I have even changed headspace by having the back of the bolt lugs ground slightly.