Very little IMO unless of course you need to mount a grenade launcher or depends on how light you want the rifle.
There are so many different contours it's difficult to know what you will get, A carbine "HBAR" is pretty heavy apx .850 under the guard and .730 from the GB to the muzzle. A carbine "gov." is thin under the guard and thicker apx .730 from the GB to the muzzle like a M4 without the grenade cutout which is backwards from how a barrel should be contoured. It should be larger in the rear and narrower out front like a bolt rifle barrel to retain rigidity and aid accuracy. The carbine "lightweight" is thin the whole length and the carbine M4 is thin under the guard and heavy out front with a grenade launcher cutout.
One thing to consider is some companies still use rifle type feed ramps on the barrel extension and some use M4 feed ramps, we used plain rifle ramps for years with no problems but, if the receiver you have has M4 ramps then the barrel also needs to have M4 ramps. If the barrel has the longer M4 ramps then you can always cut or grind to match the ramps of the receiver to the barrel extension.
Last edited by Constructor; 03-19-2010 at 12:38..