a high profile murder trial in Florida persuaded me to upgrade my compact firepower.....I'm a game player but also a realist, and assumed it might be advantageous to have a relatively compact firearm that extends practical range to 150m or so... my solution was a 10 1/2" AR "pistol" in 5.56.... a friend offered me a Mk18 upper, so looking at the ATF rulings from their technical division at the time, decided that would be the platform (disclaimer.... make sure you look up the proper ATF requirements and don't rely on me for your information)... the way I read the law, (1) as to not make a short barreled rifle (SBR) was that first it had to be a rifle, so purchased a new receiver and had the ATF4473 marked as "other" (2) the ruling that any receiver extension( buffer tube) could be used, but that a stock could not be readily available... I shoot nose to charging handle so a carbine buffer tube could be used that would get me close, but still a little too close (3) a rubber crutch tip type end cap was allowed, and this added a little anti slip for my purpose.... I chose a commercial "pistol" buffer tube and W2 buffer as to not have the adjustable holes associated a carbine stock (if I had used an adjustable carbine buffer tube, I had planned on adding a sling swivel to prevent the addition of a stock)... (4) with the Sig Brace ruling, I replaced the shorter pistol tube with a slightly longer tube from KAK, and with the addition of a SigBrace puts my nose to charging handle... a linear comp helps push the blast away from the shooter... it accomplishes what I wanted without the expense and time required for a revocable living trust and tax stamp for a SBR.... Les