There are a lot of different bullets and acronyms out there.  

What is the difference?  Is one better or worse to shoot than the others?  Using a Bushmaster M4A3 if that makes any difference.  

For my carbine, I have switched to reloading FMJ's for the typical 100-yard range shoots.

The BT's are sexy (and reload easier), but for the average 100-yard (or less) rifle-range the FMJ's seem to be the way to go based on the studies I've read.

The problem is that both FMJ and FMJBT bullets have open bases.  The base of the bullet has a lot to do with how consistently it will be pushed out of the muzzle.  On the other hand, well made FMJ bullets can be very accurate, while not-so-well-made FMJ bullets can shoot like crap.  For example, look at commercial-packed Winchester 55 gr FMJs and Hornady 55 gr FMJs, particularly their bases.  Note how smooth and even the Hornady bullets are, especially compared to the Winchester bullets.

Bullet profile is only one factor, but one that you often use as a starting point in building a load or deciding what load to buy.  It's a good idea to learn everything you can about a bullet before you go on to choose it.