had a mild bout of covid a month and half ago, so had time on my hands... I was looking at the actual objective measurements of the proposed 4999 compliance sheet for braces... I have no crystal ball on any subjective criteria, but I put together a spare AR pistol lower that meets their objective measurements... my 10.5 5.56 with a Strike Industries 5.5" buffer tube, short buffer, and flat wire spring meets the minimum length for the buffer tube and provides an acceptable length of pull and overall length... problem was that being that short coupled I could not get a nose to charging handle support position without the charging handle digging into my cheek... I was originally going to modify the charging handle to remove the left side and retain the latch, or convert to a right side latch, but I've always wanted to play with a side charger, so picked up a 10.5" Bear Creek right side charging upper... had to remove some of the checkering of the charging handle hole plug, but it works (not very comfortably, but works) and meets all the 4999 measurements... currently my go-to KAK 8.5" tube and rubber end cap are on the carbine, but if the ATF retains the same measurements I have an AR pistol that works... regards Les