Glock Grouping Problem

[tiriaq]

A friend is experiencing a peculiar problem with his 9mm MOS. He has other Glocks but this is his preferred competition pistol, and the only one which has the issue described. He is a very experienced, outstanding shot. The pistol has a Leupold DeltaPoint Pro installed.
The pistol consistently fires the first shot loaded from the magazine low and right from the group formed by the balance of rounds in the magazine.
This was first observed at 35m, but is also noted at 15m. At 15m, the first round group forms about 2" from the group of autoloaded rounds. More pronounced at 35m, of course. First shot is a flier.
Careful testing has confirmed the pattern; this is not imaginary, pulled shots, etc. The first round manually fed from the magazine has a different point of impact than rounds fed when the pistol cycles.
Usually the first round is fed by inserting the loaded magazine, then racking the slide. It makes no difference if the slide is locked back and then released.
Last night we experimented with easing the slide closed on the first shot. Rounds loaded this way also grouped to the right.
Manually fed rounds have a different point of impact than ones fed when the pistol is fired.
As I mentioned, the shooter has other Glocks, but this is the only one which demonstrates the pattern.
The pattern is observed with different makes of ammunition.

Any suggestions?

 

[Still alive]

Sounds like you’ve tried everything.
Switch out the barrel? Or get an exorcist?

 

[Kratos]

Have you tried inserting a full mag, chambering a round, then manually cycling out a live round, then firing? It’s possible the spring pressure with the 9 rounds in the mag is pressing up on the slide and its ever so slightly binding something, after the first shot that pressure is relieved. Very unlikely but the only thing I could think of.

 

[jparent]

I have heard of this phenomenon, (similar to the rifle that all ways shoots the first shot out of a cold barrel to a different POI than the subsequent shots) with semiautos, but usually rifles, never pistols.

Something is causing the force of the slide being racked and chambered to index the barrel differently than when the force of recoil does it.

That said, that is a huge difference, i would look at the barrel, but something that pronounced should be noticeable, there is no pivoting link like on a 1911.

Very strange.

 

[Ganderite]

Does it do it if the mag is loaded with 5 rounds, not 10?


if so, try clipping the mag spring 1 coil on one mag and see if that cures the problem.

 

[Still alive]

Does it do it if the mag is loaded with 5 rounds, not 10?


if so, try clipping the mag spring 1 coil on one mag and see if that cures the problem.

Won’t downloading to 8-9 rounds accomplish the same effect, as Kratos pointed out?
Clipping the mag spring may increase the mag cap to 11 if it’s a pinned 10/17 mag and, oh! the horror! Hide the children!

 

[kivas]

I don't see it stated but I presume you've tried other magazines and same outcome?. Like others have suggested don't fully load the magazine - or even rack one then drop the magazine out I would think that would remove any possible pressure completely.

 

[Still alive]

Regardless of any mag pressure issue, the gun shouldn’t behave like that. There must be something inherently wrong with it. What? No clue!
One should not have to fuss with a Glock to have it work properly.

 

[teltec01]

Have you tried removing the red dot and shooting the pistol with the iron sights to see if you get the same results? Maybe it has something to do with the DeltaPoint Pro....

 

[tiriaq]

Sounds like you’ve tried everything.
Switch out the barrel? Or get an exorcist?

Have you tried inserting a full mag, chambering a round, then manually cycling out a live round, then firing? It’s possible the spring pressure with the 9 rounds in the mag is pressing up on the slide and its ever so slightly binding something, after the first shot that pressure is relieved. Very unlikely but the only thing I could think of.

I have heard of this phenomenon, (similar to the rifle that all ways shoots the first shot out of a cold barrel to a different POI than the subsequent shots) with semiautos, but usually rifles, never pistols.

Something is causing the force of the slide being racked and chambered to index the barrel differently than when the force of recoil does it.

That said, that is a huge difference, i would look at the barrel, but something that pronounced should be noticeable, there is no pivoting link like on a 1911.

Very strange.

Does it do it if the mag is loaded with 5 rounds, not 10?


if so, try clipping the mag spring 1 coil on one mag and see if that cures the problem.

Different barrels, different magazines, single loading, full mags, less than full magazines have all been tried. First noticed starting with 5 rds in the magazines, but it does the same thing with 10.
Consistent performance - the first shot goes right and down. First shot groups are respectable - just away from all subsequent shots.

 

[Still alive]

Tiriaq,
You are 100% sure it’s not the shooter, right? Is your friend re-adjusting his grip after the first shot?
Low right is classic for a left handed flinch.
Nevermind! Reread your OP!

 

[Suther]

This is a very odd issue.

Have you tried firing a round (so the next one is chambered by the gun) then swapping in a different mag?

 

[tiriaq]

About every possible loading sequence has been tried. Manual loading produces a different poi than auto loading, whether snapped shut or eased closed. Nice tight autoloading group, separate manual loading group. Three different loads have been tried - factory ball, commercial reloads, handloads.
We're stumped.
After working with the MOS, a P320 was shot; nice tight centered group, no fliers. It isn't the shooter.
Maybe its time to try a different optic. Perhaps the different vibration pattern is affecting the DeltaPoint Pro. I have a NIB Pro. It might be worth swapping it onto the MOS.
I'd initially thought that the cause was how the pistol went into battery, manual vs auto.

 

[Winchester-1897]

marked for interest

 

[Rocket an]

Replace red dot to rule it out. Conversely you can hang a rail laser(if you have access to one), center before first shot to optic, and see if there's any difference going to subsequent shots. Strange problem.

Best

 

[tiriaq]

Owner has a light/laser. Would be worth trying.

 

[B..]

Try a ransom rest or pistol vise to rule out human error....then warranty.....

 

[tiriaq]

The owner is going to try with the laser, bypassing the optic.
We don't have access to a Ransom Rest or pistol vise. However, neither of these duplicate a hand held situation, which could be part of the equation. I'll post the results of laser in lieu of optic.
Appreciate the suggestions that people have made.

 

[Scott Bear]

Won’t downloading to 8-9 rounds accomplish the same effect, as Kratos pointed out?
Clipping the mag spring may increase the mag cap to 11 if it’s a pinned 10/17 mag and, oh! the horror! Hide the children!

The follower would be stopped by the pin or whatever block is used regardless of the spring length. Unless they block it in some strange way.

 

[tiriaq]

Well, the pistol was tested with a different barrel, and also using a laser instead of the sight.
Same pattern; first shot out of the group, low to the right.