Does anyone make a 10mm adapter for the AR15?
I am asking for a friend that lives behind enemy lines in Californiastan... he con not buy 10mm PCC pistols as they are not on the dOJ list of approved pistols. He has to build a long gun PCC.

I want to say mags are too wide for a standard magwell. I have a glock large frame lower from New Frontier Armory with a 40smiff n western barrel that I reamed to 10mm. There aren't nearly as many mag choices for 10mm, especially stendo klips.

Stern Defense makes an adapter for 45acp using Smith and Wesson mags.  I would think it would work with Smith 10mm mags.

This may or may not be of any use:
Decades ago I had an Olympic Arms 10mm upper, in fact i had two of them, but I digress; the magazines supplied at that time were Uzi .45acp magazines adapted to fit the AR mag well using plated and blocks spot welded together. These were a fucker to load.
They also used 9mm and .41AE magazines and all would work with the 10mm irrespective of caliber they were designed for.
A year or two before Oly folded up they had samples of a custom made, high quality Polymer magazines that looked a lot like the CMMG .22LR magazines in 10mm as well as other calibers: I never saw one for sale and have no idea if they ever went into production.
10mm isn't any longer or fatter than .45acp and while 9mm magazines often easily accommodate .40 S&W, the longer length MAY be an issues when adapting surplus or other magazines.
10mm is no pussy cartridge and out of a carbine barrel IS quite effective and snubbed as it may be the hi-Point 10mm carbine works and works well, I'd still rather have a MRS chambered for it with good magazines, but that seems to elude manufacturers.

I 3d printed some adapters  and followers for 9mm and 45acp to use a pmag to feed from. It could probably be adapted to 40mm pretty easy. It won't be quite as double stacked so you will probably lose a few rounds capacity.

I really needed them injection molded, but the ones I printed last a pretty good while.

It's a thought I guess, if you wanna try it, and a standard lower would be a requirement.