There are a lot of factors that play into shift of point of impact when we mount a suppressor on our favorite rifle. Some of it is based on knowns, some of it is not.
The knowns are barrel harmonics, weight of the suppressor, type of attachment method, and construction technique.
The unknowns are ….. well….. unknown.
What we know is that the shift SHOULD be low with the suppressor attached. This is due to the weight of the suppressor working with gravity to pull the muzzle of the barrel down a little and the suppressor will also provide some harmonic dampening as well. Sometimes is will be high though. This unusual shift phenomenon has to do with the way the gas is redirected inside the suppressor as well as possibly disturbing the barrel harmonics.
The gas inside the suppressor is moving several times faster than the projectile. The slightest change in the direction of the gas will cause the gas to push the bullet in one direction of another. Muzzle pressure on a 308 winchester is approximately 8000 PSI. This is more than enough to move the bullet off of it’s normal axis of flight. Think about it, if a small 5mph wind can push the bullet off to the point of needing to adjust for it, what do you think of a “wind” that is moving in excess of Mach 9 would do to that bullet?
Suppressors that redirect the gas across the bullet path will usually produce more shift than suppressors that have concentric baffle designs that do not direct the bullet path. Sometimes though, there are designs that do not have a consistent shift, sometimes due to construction techniques, but most of the time, it is because of the weapon and mounting system. Some people forget that the suppressor and the weapon comprise a system that works harmoniously and it should be viewed that way.
Shift of 4 to 6 inches at 100 yards is fairly common in most suppressor designs and should not alarm you, as long as it is repeatable. Your data book will be your friend at this point…..or simply zero your rifle system with the suppressor mounted and don’t shoot it without the suppressor.