I just got back a Glock 19 from ATEI that I had milled for a Trijicon RMR. So I'm at the range zeroing my new RMR and trying it out. I ran into something I don't quite understand I was hoping that you might give me some insight.
I'm shooting that Glock 19 off a pistol rest. Its a good pistol rest and not just a range bag or something. I'm shooting...
124 gr. Federal American Eagle FMJ. Its rated at 1150 FPS from their website.
With the target at 25 yards, I adjust that RMR until the group is centered then confirm. From the rest, it prints a good 10 round group centered on the bullseye and about as tight as I'd expect that ammo to be capable of with me shooting the Glock from that rest.
The next thing I do is I want to run some of my preferred defensive ammo through it and see how close that zero is with defensive ammo. So, again, 25 yards from a rest I'm shooting...
124 gr. Federal HST. Also rated at 1150 FPS from their website.
Strangely, I get this nice extremely tight little group...maybe 3 inches or so....but 3 inches to the RIGHT of the bullseye. That confused me. I expected the group to tighten up with the defensive ammo. I expected the group might even be a bit high or low. I didn't expect the group to move right.
So I tried again. Same thing. So then I tried another group of the practice ammo...once again....its centered right on the bullseye.
Not quite knowing what to do at this point, I figured that because its a defensive handgun its better to zero it with the defensive ammo so I adjust the RMR to move that defensive ammo group to the center of the bullseye. I produce another nice group, now chewing the orange center out of that bullseye with the defensive ammo. Now I test the practice ammo again, and sure enough, now that group prints left of the bullseye by about 3 inches.
To throw another curveball, I also had a Sig 229 Legion RX with me. I had previously zeroed the Romeo 1 on it with that same practice ammo and it had been shooting dead center at 25 yards with that ammo just fine. I'd been through several classes with it and it has been right on the money. However, I check it off that rest with that same defensive ammo and sure enough...the group is right of the bulleye. Its not as far right as with the Glock 19 but its a good 2 inches to the right.
You'll have to forgive me as I don't reload and don't know that much about ballistics. Anyone know what might be going on? I'd normally assume its *me* causing the problem...but it seems strange that I'd produce a tight little group with the defensive ammo that is offset to the right from the practice ammo. If I were having trigger press issues or any other shooter issues I'd expect to see the problem in both types of ammunition.
Neither type of ammunition is +P or anything and both is 124 gr 1150 FPS. Its very subjective and I'm not very good at 'noticing' things like recoil impulse but I didn't notice any big difference in recoil between the two.
So my concern is that now that its zeroed with the defensive ammo and my practice ammo shoots 3 inches left...that's great on the really really off chance I ever had to use that pistol to save my life. However, I do take a lot of classes and such and I use practice ammo for those classes. So now I'm going to be shooting left for those classes and probably look like I'm using too little trigger finger.
What should I do?
Thanks!