Touching on the 5.56 issue, It would certainly be silly to replace a high powered rifle round with a pistol cartridge. Although, I have heard many stories of the 5.56 not doing the trick when it comes to putting down the enemy.. sort of like those 9mm stories. I personally am a bigger fan of 7.62x39, if they could make a rifle to shoot it accurately.
I don't need to make a custom carbine/pistol. I could spend my money on other guns
. I'll just settle with my CZ52 for my 7.62x25 needs.