Bolt Carrier Machining?

[Dave Berryhill]

Quick update - I received a Brownells M16 bolt carrier (don't know who makes them for Brownells) and turned down a portion of the rear, leaving a shoulder for the buffer spring to seat against. It looks similar to the NEA bolt in the photo without the plastic tip. I decided to try turning it first because that still left the option of chopping if off if I totally FUBAR'd it. While it was pretty hard, a good coated carbide insert handled it. The bigger problem was the slot that runs lengthwise down the bolt. This gave an interrupted cut that busted the tip off my insert twice.

NEA.jpg

Taking light cuts (about .010 to .015) I was able to whittle it down and keep the cutter intact with a good surface finish and gave it back to my customer. If I had to do a bunch of them, I'm sure there is a tougher carbide insert that could handle the interrupted cut that would be better suited for the job. All in all, it was a fun challenge.

I asked my customer to keep me updated on his testing.

[MarkG]

Quick update - I received a Brownells M16 bolt carrier (don't know who makes them for Brownells) and turned down a portion of the rear, leaving a shoulder for the buffer spring to seat against. It looks similar to the NEA bolt in the photo without the plastic tip. I decided to try turning it first because that still left the option of chopping if off if I totally FUBAR'd it. While it was pretty hard, a good coated carbide insert handled it. The bigger problem was the slot that runs lengthwise down the bolt. This gave an interrupted cut that busted the tip off my insert twice.

NEA.jpg

Taking light cuts (about .010 to .015) I was able to whittle it down and keep the cutter intact with a good surface finish and gave it back to my customer. If I had to do a bunch of them, I'm sure there is a tougher carbide insert that could handle the interrupted cut that would be better suited for the job. All in all, it was a fun challenge.

I asked my customer to keep me updated on his testing.

Did you refinish it? I'd imagine it will rust very quickly.

Sent from my XT1056 using Tapatalk

[Dave Berryhill]

Did you refinish it? I'd imagine it will rust very quickly.

No, it's just a customer's experiment at this point. A little oil will take care of that for now.

[mtdawg169]

No, it's just a customer's experiment at this point. A little oil will take care of that for now.

Dave, can you expand on this a little? I'm not even sure what the owner is trying to accomplish. Is he trying to replicate a part to work with that stock and receiver extension or is he trying to redesign the carrier / buffer?

[Dave Berryhill]

He's trying to make a shorty/PDW 300BLK using something like a 4" pistol buffer tube.

[justin_247]

I'm very curious. Is he going to be using a stronger spring being that he is eliminating the buffer assembly and reducing the mass of the carrier? The gas pressure at the port is going to be very high and he will definitely need to account for that [now missing] weight somehow.

[Dave Berryhill]

The project belongs to a customer. I was asked to do the machine work on the bolt carrier and I offered a few suggestions to him but I'm not building the gun.

[kapusta]

My concern would be that if something goes kablooey, some bottom feeding attorney is going to be chasing after anyone who had anything to do with this "experiment". I know, I hate all the liability crap too, but sometimes I think it's best to be a spectator. The cons seem to outweigh the pros on this job.

[Phreakish]

I realize the work has already been completed, but for those that stumble across this in the future:

A "mil-spec" carrier is through hardened

The carrier is typically made from 8620, is heat treated and carburized. There are areas which receive hard chrome, the rest is phosphate after heat treat. The core hardness equates to around RHC 35 if I remember correctly. Tough material, but once you get through the .020" or so of case, it should machine readily.

[Dave Berryhill]

... Tough material, but once you get through the .020" or so of case, it should machine readily.

Yes, I would say that my experience with this carrier matched your description. The interrupted cut was a bigger challenge than cutting though the hard, carburized layer.