Bullet weight moves group center?

B

Bart Noir

New member
Yesterday I fired off some 147 Blazer FMJ rounds from a certain 9mm pistol. And the 14-round group had a center that was maybe 2 inches left of the aiming point, at the 15-yard distance.

Then, same gun at same distance with same cross-wind but using 115 grain Blazer FMJ rounds. The 14-round group was dead-on in windage.

(You will notice that I am carefully not stating the actual size of the group :D )

So is there any reason that bullet weight, and bullet velocity, will change the group to the left? Or was I just getting warmed up and shooting straighter?

Bart Noir
 
You expect bullet weight to affect the vertical dispersion of a group, but not the lateral. The light bullet (at higher velocity) will group lower than the heavier bullet, assuming the heavier bullet is at a lower velocity.

But, as you point out, there are times when no logical explanation is seen in variation of groups.

Yeah, you could have warmed up in your shooting, held your tongue differently, or .......................

Bob Wright
 
The gun starts to recoil while the bullet is still in the barrel. Often slower heavier bullets will hit higher than lighter faster bullets because the recoil causes more muzzle rise more before the bullet exits the barrel.

Your wrist will often try to rotate as the gun recoils. It is usually not enough for most of us to notice except on really hard kicking magnum revolvers. Different loads cause the gun to recoil differently. This will cause bullet impact to change. Horizontal changes are less common, but not unheard of.
 
Windage changes do sometimes happen to me with different bullet weights. It has even happened with different manufacturers in the same weight!
 
I get the same effect, heavier hits higher and somewhat more to the left. Recoil from the heavier bullet is stouter, and the gun shoots away from the palm of the hand...I"m a rightie so it moves left for me. Recoil is the culprit here in my mind....try shooting the same bullet weight but at higher and lower velocities...and I mean several hundred feet per second so you can see the difference. Best Regards, Rod
 
This is not unusual. Height is more noticeable in pistols, but recoil also seldon meets exact resistance to recoil exactly straight upwards, so it usually pulls slightly more to one side as well.

I find that patterns moving left to right with velocity are more noticeable in rifles as recoil tends to swing it as much to the side as it does upwards.
 
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