I thought I knew a lot about guns until I started reading this forum. I am replacing the barrel on my lightweight Match Target Colt. I purchased a 14.5 Bushmaster with the Phantom. I have replaced the barrel before but never had the headspace checked. Is this someting I need to have a gunsmith check? My rifle has about 400-500 rounds through it. Will it be ok with the original bolt? One more question, I was thinking of purchasing a new upper for my other AR. Will I have any problems mounting a Bushmaster upper to a Colt lower?