listen.. i have a bulgarian AK-74 and im looking to convert it from 5.45x39mm to 5.56mm...
here are some things to keep in mind.. 5.56mm typically offers around 1400ft/lbs of muzzle energy.. by comparison a 7.62x39 offers around 1,800ft/lbs at the muzzle.. the 5.45 is lucky to make 1,000 making it a very low powered cartridge in comparison
ammunition is cheap now, sure, but many of the countries that did have 5.45mm rifles have already switched to 5.56 so surplus is going to get harder to find, and when it does youll be at a point where itll be very expensive to shoot and ammo will be hard to find, and factory ammo when they start making it wont even have the tumble effect that makes up for the cartridges lack of power
then there are magazine issues... the 5.45, like other AK rounds are quite tapered and ar-15 magazines are not, in fact the top third is perfectly straight, so feeding issues wont be uncommon
my recommendations for a build in this caliber would involve getting an AK-74 parts kit and if youre looking to build an AR15 in an alternative caliber then to choose something designed for the action, like 6.5 grendel, 6.8mm SPC, or even the 6x45mm wildcare (necked up 223)