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!

3" off center at 25 yards? Not a huge deal. There can be such differences between loads, but...left side groups are typical of a Glock shot by a right handed shooter. Most likely it's your trigger finger not the ammo. Put more finger over the trigger, not just the tip (contrary to typical rifle shooting for accuracy).

Set up a target at 15 yards. maybe a blank piece of notebook paper. Draw from a holster or from low-ready if no holster, drawing not allowed at your range. And put 6 shots on to the target in under 6 seconds. That will give you your typical "combat group".

I know nothing about zeroing an RMR on a pistol, sorry.

Sucks, but I doubt it's an uncommon issue.

That is really weird.  I was shooting my M&P Shield after voting on Tuesday with the same ammo.  It has upgraded night sights not an optic.  I started shooting with the magazine of carry ammo (124gr HST) at 10 yards with typical spread in the X ring of the target.  I then switched to the Federal 124gr FMJ (ballistics same as HST) for the rest of the session and I was shooting the same typical spread with no windage or vertical offset from the HST.  This is how it always goes for me, satisfactory range session.  I can’t imagine why you are getting such a huge windage offset.  My stash of HST is about 16 months old, but the FMJ was bought from Target Sports USA this past summer.  It’s got to be ammo related, no?  Have you tried without the optic (can’t be the RMR, can it)?

Simply a case of barrel harmonics, slide speed, and powder burn rate.

Although not desirable, some guns shoot wildly different with different ammunition.

I have a .270 rifle that will shoot 90, 110, 130, and 150 grain bullets to almost the exact same POA at 100 yards.

My brother's 30-06 exact same model bought at the same time won't even shoot 150 grain flat based and boat tailed bullets to the same place.

Just depends on each particular firearm.

It's not the trigger finger. He shot it through a rest. If it were his trigger finger, the same results would happen with both brands of ammo.

His POI shift should only be elevation and not windage when using 2 different types of ammo if there is supposed to be any POI shift at all.

Even if the bullet and velocity were to remain nearly the same - A simple change in brand / type of powder alone can easily cause a change in the POI. Not at all uncommon.

In the same respect you could be using the same exact bullets, powder brand, cases, etc. and it would not at all be uncommon to see groups shift POI or increase / decrease in group size as one works up a load for his particular firearm from min to near max powder charges.

If you were a reloader and 'if' you chose a SD factory round that was commercially available in component form you could possibly do enough load development to be able to get pretty close to having a home rolled round that was hitting at the same POI if that was your goal.

Or you could buy your factory SD ammo in bulk so you could 'practice with what you carry'???