AR pistol build

[HandGunner]

I have a friend with a completed AR pistol build and man do I want one. So now Im planing to start a build but would like to know what are the lagalities. Yes my father has a license for pistol purchase and should be ligit for AR buying. My fried's is a ar-10 NOT ar-15 lower with a 15 1/2" barrel in a 243 wsm he says the ar15 lower isnt big enough for the higher power cartriges. Uses it for deer hunting, what I would like to know are the max overall length of the gun and if their is anything special I will need to do prior before starting this build, how would I get a lower registered as a pistol? how would I get an upper with a barrel length of 15-15 1/2 inches? and how so in my own chambering?

another thing is should I go with a 243 wsm? we have a .308 in a hangun 15" barrel and lose about 500fps he said he went with 243 wsm since it loses very little fps and still has great accuracy as the cartrige was made for shorter barrel rifles. what about a 270 wsm?

really which caliber has enough knockdown power but loses little to no fps while still holding the same accuracy out of a shorter barrel. Either I'll have to custom order a barrel with a specified twist rate or use a regular rifle length barrel cut down to 15 or 15 1/2 inches but the twist rate remains the same

he has contacted and asked questions so his pistol is ligit even with a 15 1/2 barrel the 1/2 comes from the muzzle break, I see mine in the same way as his but maybe without the muzzle break. I wont be the owner of the gun my father is its really me building it for him since im the once who can research and figure out all the ar rifle building stuff but he would be machining the reciever if it requires a unfinished reciever really its a project for the both of us I just have the bigger task since me poppa is old school and does not know how to research and use the internet

ideas are open I need help planning on how to do so on all of this