I'm putting together an AR-22 upper for Steel Challenge. I'm going with the Taccom ULW barrel and handguard. Still figuring out the bolt. Is the CMMG stainless still the best one, or has someone come out with a better one?

The three CMMG .22 bolts with which I'm familiar all run great. Righttobear.com has clones (I've read made by the same company, but who knows) sometimes for a few bucks less. Make sure you find the kit with 3 mags, still in stock here and there for $199.99.

I think CMMG/RTB is about the only game available now.

We use CMMG bolts with upgrades from https://borebuddy.com/product-category/22lr/

Very reliable with CCI AR Tactial or Mini Mags.
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Which upgrades did you find the most worthwhile?

There's a YT vid, from Iraqivet88 I believe where they put over 2,000 rounds, full auto, using many 25 round mags and 50 round drums back to back with assistants reloading, thru a CMMG upper w/suppressed 4.5" barrel. Worth checking out. Their objective was to break it--they couldn't. Did have to clean it at about 1,500 or so, but the first 1,000 were pretty much flawless. Worth watching.

The one which feels the best is the adjustable buffer weight -- it's hard to describe but it makes the cycling action feel more solid to me and I can tell when I have it in versus when I don't (I take it out so I can cycle standard velocity ammo).

I also recommend the extractor and more powerful extractor springs. It does a good job avoiding the risk of the extractor skipping over the rim of the cartridge and causing a double feed, which is really the worst to clear.

And by that point you might as well get one of their complete kits.

I have no affiliation with them, I just use and like their products.

Thank you! I probably should have mentioned I have an existing 22lr AR SBR running a CMMG bolt and the 4.5" barrel, so I'm pretty familiar with the system. Those mods do seem interesting.