is it alright to switch bolts and bolt carriers between uppers? I have had people tell me not to do it and people tell me that its alright.

My brother just purchased a well used pre-ban AR with a 20" upper on it, and he wants to put a M4 style upper in its place. Would it be alright for  him to use the bolt and carrier from the previous upper and put it in the M4 upper (ie. saving about 200 bucks)

I haven't looked at the bolt carrier to see what condition itis in, so I can't give any information about that.

Thanks in advance

Only a headspace guage can tell you for sure, but I wouldn't personally put a "well used" bolt in a new upper. Kinda like putting old tires on a new car--might work for a while but might blow out the first trip around the block.

I'd recommend he just sell the old upper and bolt etc and put the money into a complete new upper, bolt and carrier.

replace the barrel and change the bolt or keep the same barrel and replace the bolt.There sholdnt be a problem with changing carriers.