Today I went out in the desert to test some loads and some sight adjustments I made to a G17 gen 4 that is stock except for some springs and a connector.

I had been moving the rear sight around back and forth trying to get it centered at 20-22 yards off a rest. The first group was with factory 9mm 115g ammo that had 5 shot groups at 2-3" about 2" left of center but spot on for elevation. Dang it I just moved the sights to get the groups to move left after previously having the groups spot on for elevation but 2-3" to the right of the bull.

Next up was some cast lead 115g reloads that are a wee bit hotter based on felt recoil than the factory stuff and this G17 will handle lead bullets without leading the barrel. This 5 shot group also was in the 2-3" size and centered on the bull !!! Repeat and same results with both kinds of ammo.

I knew to expect that I could get some vertical difference in the position of the groups due to the nature of the ammo used with certainly some velocity change. Almost all groups were in the 2-3" group size and shot off a good solid bench. It was new to me to see how these groups were 2-4" in variance from one another but the same elevation on the target.

Anyone else see groups move left to right with the same elevation with different ammo?

I get it with my G20. I assume that it is due to my lack of glock form, or technique. I shoot my G22 pretty fair. ...but nothing like a 1911. If I use a pistol rest my results improve.

My groups were shot off a rest, using the same gun and the same guy behind the trigger.............me.

From looking at the group size I would say that is speaks well of the accuracy of the pistol and those groups are not that bad for the shooter.....again me.

The only variable in the equation was the ammo and most likely the ammo operated at different velocities which is still a puzzle where one would expect to see a vertical difference between the groups and not a horizontal spread. Still puzzling.

Could it be a variation in your grip between groups? I've seen very experienced hand gunners scratch their heads for the same reasons.

We try not to change the sights on a fixed sighted gun until we've found the load that we like the best, then we compensate if necessary by moving the sights. After the sights have been drifted for our chosen load, we leave them alone. Even if we are shooting another load that won't hit perfectly to point of aim, we leave them alone. It's our chosen load that we worry about.

Left and right are normally a sign of changing grip mechanics. Cast lead reduces friction in the bore, but you maintained that the cast lead load was hotter. This could result in some leading, which could effect time in bore. Time in bore isn't only responsible for elevation changes, but if you consider torque to the platform by a bullet encountering left or right rifling, you can encounter windage changes too, if your grip isn't totally consistent.

Weaker grips cause this. Or, grips too high, too low, too much trigger, too much thumb...etc. It goes on and on.