I have a Bushmaster with a 16" chrome plated HBAR.  Lately i've seen alot of ar's with the M4 contour barrel which, as far as I know is a light coutour barrel.  I was always told that a heavy barrel is going to be more accurate than a light one.  So why are these M4 style barrels so popular?  Which are really more accurate?

Steve

The M4s are popular because they're a half-pound lighter than the HBAR.  (And because of the "it's what the Army uses" cool factor.)  They're approx. .63" instead of .9" under the handguards, which is a fair amount less metal.  From the sight base forward they're the same except for the notch.  

The HBAR will probably hold onto its accuracy a little longer as it gets hot, but for slow fire at carbine distances I doubt you'd notice the difference.