So, the apps47 link this morning shows this: https://www.brownells.com/rifle-parts/receiver-parts/receivers/lower-receivers/brn-180-stripped-lower-receiver-forged-prod130438.aspx?avad=avant&aid=35987&cm_mmc=affiliate-_-Itwine-_-Avantlink-_-Custom+Link&utm_medium=affiliate&utm_source=Avantlink&utm_content=NA&utm_campaign=Itwine

Am I crazy for wanting to make a 10-12" pistol length .300 BLK out of it?

Problem is, Brownell's looks a little stingy about selling the upper receiver without it being attached to a barrel.

Like, they're not doing it. At all.

I've got their 18" upper, but it is currently on a standard lower.   Still need to get a good adapter to get the folder working.

Just a thought: 

There are a number of states with laws, still on the books, that date to the state level "Uniform Firearms Act" movement of the mid-1920s.  In those juristictions, state laws have a different definition of "pistols" than generally accepted today.  Those states that followed the UFA movement usually define a "pistol" as any firearm with a barrel less than 12 inches in length.  As a result, in those juristictions, handguns with 12 inch or longer barrels are exempt from "pistol" transport / concealed carry restrictions and are treated legally the same as rifles.  These laws impact loaded carry in vehicles, including those found in hunting related laws/regulations.

Just something to be aware of as you choose the barrel length of your braced pistol!

could throw a JAKL upper on it when they drop

that's my plan.