Originally Posted by
Coal Dragger
My preference would be the Colt, although the FN is probably a good carbine. I base this off of my issued M16's while I was in the USMC, and although I never had a Colt M16A4 (don't think Colt made any) I had several M16A2's, and one FN M16A4. Overall I preferred the quality and fit and finish of the Colt's over the one FN that I had. All of them were reliable and well made, but I give the edge to Colt. In particular the Colt GI triggers were noticeably better that the FN GI trigger that I had.
The Bushmaster is very unlikely to have some key features that make a rifle reliable. You'll probably find the gas carrier key isn't adequately staked (if at all), and the buffer tube castle nut probably also not staked on the Bushmaster. I have a friend who had a Bushmaster that gave him fits with unreliable feeding and extraction issues. Skip that rifle.
Look at the other two closely and open them up to look at the bolt carrier groups if the store will let you, look for well staked bolts on the gas carrier key. Look at the castle nut that secures the buffer tube, make sure it is staked properly. Pay attention to the quality of machining, look for rough tool marks etc. Dry fire both if possible to see which has the better trigger. If both are totally equal as far as you can tell, then buy the one you like best. Probably can't go wrong with either.
So long as both the Colt and FN are assembled correctly, the only major difference between the two is the barrel manufacturing method. The Colt is button rifled and chrome lined, and the FN will be cold hammer forged and chrome lined. Both methods can produce an excellent barrel, although the hammer forged unit will give a little bit longer service life although both will have consumed many thousands of $$$ worth of ammo before they need replacement. If you can afford to shoot either one out, you can afford to replace the barrel.