Would it harm anything to use a 20ga brush to clean the inside of the bolt carrier?

Even if it is ok, should you avoid cleaning the bolt side (where the rings make a seal) in order to prevent wear from cleaning?

If you're using a brass brush, you won't damage the carrier.

Good for you for taking the time to clean inside of the BCG! Over time a lot of carbon can build up inside of there. Using a brass brush won't hurt a thing. A clean and lubed BCG is a happy BCG!

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SIC SEMPER TYRANNIS

If you're talking about cleaning the bore where the bolt resides, there's this gem of a tool called the CAT M4 tool that'll pretty much take care of multiple needs. Including removing carbon from the bolt tail. Avoid oil based lubes on the bolt and the bolt bore.

If you're talking about the long open space to the rear of the firing pin, just wipe it out occasionally unless you're in a really challenging environment.

This is why I prefer a nitrided or one of the lubricious coatings for the BCG.

Cleaning with a brush is a much less violent operation than firing the gun.

It won't hurt a thing.

Thanks for the replies - I feel a bit better now about not harming the 'bolt bore'

I may give that CAT M4 tool a try once I wear my next brush out - it does look pretty nice, as well as the AR-15 Lug scraper tool.

I think I was thinking of it more like a piston / ring seal on an engine, but was overthinking it.

When I clean that area, I just wrap a patch with CLP around the small end of a nylon cleaning brush and rotate it a few times. Since I clean frequently, there's not a buildup problem.

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Registration? NEVER!

I have a variety of brushes purchased at a medical supply store. Lots of different sizes for cleaning medical equipment that also fit just about every round opening you'll find in firearms. You may also find sprays and powders for keeping hands dry. And "Mole Skin" for blisters when shooting 600 rounds in 2 days!