I've already built pistols in .223 Wylde,  300 BLK (PWS barrels),  9mm (KAK) (dedicated Colt mag PSA lower), and a 7.62x39mm (BCA). All of them are 10.5" barrels and paired with ALG 10" rails EMR V-1 (My favorite, although discontinued, I have a few left), KAK slimline flash cans and Shockwave blades.  

I've been looking, but can't find a 10.5" barrel in .22LR.  Would it just be easier to buy a cheap .223 barrel, and get a .22LR conversion?  I'd love a .357 SIG or .45 ACP, but I don't like Glock mags, and I already have a pile of stripped lowers.

Any other calibers I should be looking at, that aren't uber expensive in 10.5" realm?  I reload and so I have that going for me.  What calibers am I missing out on?  Right now the only other one I can think of, which I already have a rifle in, is 6.8 SPC.  What other calibers work well in the 10.5" range?

Thanks!

Yes you must add a X39 to your collection of pistols, otherwise you'll be up at night thinking about it until you do...
at least that was my reasoning for adding one to the AR pistol family.

Now I need one of those too

This was built using the BCA 10.5 7.6x39  it is great shooter.
https://goo.gl/photos/FWAupRedH7yV9jXn9
This pistol is using the the CMMG  9 inch 22lr  barrel and bolt.
https://goo.gl/photos/94Aet2bgUV2ag1VP7

Thanks for the influence!  I ended up getting the barrel and all of the parts for it in, just have to assemble!  Might swap the gray for a black handguard on my 9mm build, haven't decided just yet.  Still thinking I might do a .22LR too.  Unfortunately, ALG is discontinuing their V1 rails, so supply is drying up pretty quick.  I might have to snag a couple more, as I'm thinking a 6.8SPC build is coming down the line too.

Gahhhh, this is too addicting!

Ok, so that one is in the books, just waiting for the rail covers!  What caliber next?  6.8, .22LR (hard to find, can I just use a .223 barrel and conversion kit?)  What about big calibers?  Want to stick with standard AR lower and 10.5" barrels.

Ready, GO!