Question about uneven wear on bolt's lug faces

[SDW]

Received my lapping tool today. It's the AR-Stoner version, BTW. The In-the-white piece. Won't fit in the receiver. At all. Not even a quarter inch. The way the tool is machined, it has a slight taper on the end, no doubt to help inserting it. But after riding up that a few mm, it gets stuck.

I don't know if it's worth the hassle of sending back. I don't have a lathe though, so turning it down wouldn't be super easy. I might be able to rig something with a drill, and clamp a center punch or something in my vice to go into the pilot hole on the other end, for a "spit roast" effect, and then just go at it with some Emery cloth until it slip into the receiver. One thing I obviously want to avoid is enlarging the bore on the upper receiver.

FWIW, it tried fitting it into my Grendel's upper and a spare Daniel Defense upper I have laying around. No go in either.

[Old Bob]

I have the same Stoner lap tool & it fits in some uppers but not in others. Some uppers have tighter than mil-spec bores. A lapping tool manufactured for mil-spec uppers isn't gonna work. Turning it down is an option I suppose. If mine doesn't fit, I bypass the lapping step.

[SDW]

Understood. I searched online for other people finding this kind of tool too tight. Not hardly any complaints though, which I find strange. Not a single negative product review either saying that it would not fit. Has me thinking, I don't imagine any of these tools working well then in a BCM Upper, with its interference fit on the barrel extension.

That technique I thought of, clamping a spikey pick tool in the vice to use as a pilot, and spinning the lapper with my drill actually works extremely well. It gives me a free hand to apply Emory cloth to the lapper as it spins. I've made a little bit of progress, but it's not quite fitting smoothly yet. And I have to run out to the HW store to buy more Fine and Super-fine/Polishing grade cloth to continue the job.

[SDW]

So... I got that tool turned down to the correct size for a snug fit in the upper receiver, which after some reflection I now recall to be an Aero Precision upper receiver. Overall I think this job was worth the effort. After about 15-30 seconds of spinning it in the drill with 200-grit paste, I inspected the Upper's face and saw that a section of bright metal had appeared, centered at about the 4 oclock position. So more spinning and checking made that bright metal surface grow to half and then finally about 80% of the circumference. I figured that'd do the job. Didn't want to go nuts with it.

I have a couple of other uppers, 5.56s, that I want to hit with this tool at some point.

[Texcl2]

So one thing to remember is with a cartridge in the chamber the bolt won’t touch the barrel in most cases (it shouldn’t) but with no cartridge it will touch the barrel extension.