What is the difference in accuracy between say a 16" HB and a M4 barrel or between a 20" HB and a 20" A2 barrel?

accuarcy is not determined by barrel length.  velocity and the ability of the bullet to optimize impact is determined by barrel lenght.  the optimum barrel length lethality wise for the 5.56mm weapon system is about 19".

accuarcy unfortunately is determined by the barrel itself.  some bbls same lenght shoot better than others.  muzzle crown and compensation affect accuarcy.

what are you planning to do with your rifle?  that will determine bbl lenght.  also hbars are more accurate under heavy fire instances - but... there always is a but.... they are heavier.

steve

I was not planning on using it for competition.  I have a bushmaster shorty 16" heavy barrel and I am thinking about going to something lighter like the M4 or M4 Disapator.  I was wondering if there would be a accuracy penalty switching to the lighter barrel.

Thanks

nah, a good lightweight will shoot just as well as a good hbar.  

steve