All rifle-caliber pistols are extremely loud. In addition to being very noisy, my AK pistols produce a noticeable concussion that can be felt even when standing behind the shooter. These phenomona are the result of the relatively large amount of propellant not being able to completely burn off in the shorter barrel of the "pistol". The result is an explosive effect at the muzzle. Big flash as well.
Keykutter is correct about not being able to legally alter a rifle lower or use it as something other than a rifle without permission from ATF. On ARs, it's the lower that's the regulated part (on AKs it's the receiver). You can use and alter any other parts as you see fit provided they don't violate any existing laws (i.e. full-auto).
I think rifle-caliber pistols are great for home defense (and a lot of fun at the range too). The hi-power round, large capacity (30 rounds), and easy to control platform makes for a formidable close-quarter weapon.
(Insert random tough-guy quote here)
"See my gun?? Aren't you impressed?" - Anonymous sheepdog
Guns - the alternative to running for your life.