AR-15 bolt hardening

[Trent]

Are AR-15 bolts surface hardened or hardened through & through?

I have a Professional Ordnance Carbon 15 pistol which broke a bolt in half back in 2005ish. The barrel extension has rounded locking lugs, instead of squared. The bolt looks like a standard AR15 bolt, dimensionally, except the locking lugs are rounded off. (Why they did this, I have no frigging idea).

I was thinking about taking a standard AR15 bolt and using a precision bench grinder, rounding off the lugs slightly so it would work in the Carbon 15 pistol.

(The original parts are unavailable: Bushmaster bought out Professional Ordinance and their Carbon 15 line, and subsequently changed the Carbon 15 to take standard AR parts. The rounded receiver extensions and bolts were discontinued and are no longer available. I'm looking at quite a lot of money to buy a new barrel and bolt, much more than just the cost of an AR-15 bolt...)

If the AR-15 bolts are just surface hardened I'm setting myself up for an eventual disaster, if they're hardened through and through, I should be able to perform the modifications without trouble.

Thanks.

 

[taliv]

knight's SR15 uses rounded bolt lugs, but their spare parts probably cost as much as the whole carbon 15

 

[Trent]

Yeah I was hoping to get away with a $40 stripped bolt for this little project (once supplies are available again).

 

[jakk280rem]

As a last ditch effort contact Bushmaster. They may warrant the bolt. Stranger things have happened. On the upside, bolts and barrels seem to be available, its the carriers that are hard to come by.

 

[Trent]

I already talked to Bushmaster (right after it happened, in fact).

Even though Professional Ordnance was bought out, they said they WOULD warranty the bolt. But I'd have to pay for a new barrel + labor for install since they no longer had those in supply! Estimate they gave was over $200.

(This "bolt breaking in half" is a COMMON problem on early Carbon 15 pistols.)

 

[briansmithwins]

AR bolts are surface hardened by shot peening. Even with that, AR bolts are over stressed and prone to breakage.

I prefer to keep a known good spare handy.

BSW

 

[Trent]

Hmm.. crap. That wasn't the answer I was hoping for.

Yeah, they are fundamentally flawed by design, this bolt broke at the weak spot where the big cam pin goes through. The problem is GREATLY amplified on AR pistols, with pistol length (sub carbine length) gas systems. The first 300 or so rounds were a mild handload, using nickle plated brass to ease extraction and help it run smooth with the mild load. I switched to FC 5.56 factory ammo after running out of handloads one day at the range, but the port pressure of the 5.56 was too much for the little bugger. It failed after 10 rounds of that stuff.

To be honest, I don't know if the handloads weakened the bolt, if the bolt was faulty (metal looked almost pourous!), or if the lake city was just to much for it.

Short gas system AR's are much more prone to wear and tear if the powder / bullet weight isn't giving the right port pressure.

 

[matrem]

Yep. That hole for the cam pin is awful big for that diameter shaft.
I look at spare bolts the same way I look at a spare tire.

 

[taliv]

(note the folks at kac agree and their SR15 bolt has smaller cam pin hole and smaller cam pin too of course, along with the rounded lugs and a couple other features.)

it's hard to say, but odds are strong that it had a defect... an internal crack. that is what the MP test is supposed to identify. and that this bolt would have been rejected. but some companies *cough* cut this corner in order to save money.

 

[Trent]

Well, they wanted a CHEAP AR-15 platform when they built the Carbon-15 line.

I should have expected it not to be very durable, but I was 21 and loved the huge fireballs the thing threw out the end.

Nowadays, I'm a little less impressed (I've moved on to bigger and better things like DShK's), but I'd like to get this repaired because I have a 15 year old boy who likes to make huge fireballs.



EDIT: They are truly impressive to fire at night, BTW. If you like totally destroying your ability to see anything afterwards.

 

[Trent]

I guess I'm worrying over nothing; I got to thinking about this, and the bearing surface (the important part that is under stress when firing) is the BACK of the locking lugs, not the sides.... The parts I'm rounding off just slides through... sure, it might wear a little over time, but if it does, so what? Buy another $40 stripped bolt and do it again.

It'll probably break in half anyway before I wear it out.

 

[madcratebuilder]

It may be cheaper in the long run just to replace the barrel. I see used pistol barrels all the time, cheap.

The AR bolt gets a heat treat after it is shot peened. Then it should get a HPT and MPI. MPI well detect cracks that are on the surface. The HPT can cause cracks that MPI can not detect. I'm not a fan of destructive testing.

 

[joustin]

A decent gunsmith should be able to install a new receiver extension with standard lugs I would think.

Sent from my DROID RAZR using Tapatalk 2

 

[Trent]

How are barrel extensions even attached to the barrel? Is it a hydraulic press fit?

 

[matrem]

They thread on.

 

[Trent]

Seriously? That's it? No pins, or anything?

I've never installed a barrel extension, I assume you need go/no go gauges to set headspacing? what locks the barrel extension in place?

 

[matrem]

They have an index pin at the top.
Other than that, they just screw in.

 

[briansmithwins]

You're buying a world of hurt if you're thinking of replacing the barrel extension.

It's not a replaceable part. The way AR barrels are made is the extension is installed and, the chamber reamed, and then the gas port hole drilled. There is no way to get a extension to index correctly once the chamber is already reamed and the gas hole drilled. If you had a 55 gallon rum full of barrel extensions you might get lucky and find one that happened to torque on and have everything line up and have headspace in spec, but I wouldn't bet on it.

It's going to be much cheaper to replace the barrel assembly and bolt and chalk this one up to a lesson in why not to buy proprietary parts that can't be replaced.

BSW

 

[Trent]

brian - well I guess that rules out that idea.