Range Report Lower than anticipated velocities?

[kuhnskaos]

Good morning to all, I am seeking additional input for a situation that I continue to observe. I have a DPMS 18" 1 in 8 twist barrel and an ABS 16" 1 in 7 twist barrel, both chambered 5.56. My chrony indicates that the 18" regularly shoots about 300 fps slower than I would anticipate from book velocity data, I would anticipate only about 150 fps based on a reduction of 25 fps per inch of barrel. In turn the 16" regularly sends the same rounds down range 100-150 fps faster than the 18" Any thoughts on where I amy be loosing the additional pressures, thereby velocity? Brass does not appear to be dirty so I assume the chamber is tight enough, but I have noticed that carbon builds up around the gas block, would a leak there possibly result in this type of problem? I would think that by the time the bullet has passed the gas block it would have achieved full velocity and a gas leak at the block would typically affect cycling more than muzzle velocity.

 

[Rhys]

Re: Lower than anticipated velocities?

Just differences between 2 barrels. You just appen to have some that may be on the extreme ends of the spread.