Horizontal POI shift, 124gr vs. 147gr, the gun, the ammo, me or YES

[Friction]

So heres the deal, I know I'm likely doing something wrong and I'm hoping the combined details below may help someone help me diagnose and fix the problem.

The BL is that for some reason I am seeing a signifigant horizontal POI shift when I use 147gr ammo in my G34.

My G34 is currently adjusted for POA/POI with MG 124gr CMJ rounds (~130 PF, approx 1070fps). I had to push the sites fairly far to the right of the slide to get the shot placement centered on target. Obviously the quick answer is that I'm likley pushing the trigger and compensating for a bad habit with my sites. But...

When I loaded up some MG 147gr (~130pf/sub ~890fps) I saw a very clear shift to the right of the POA. If I was pushing the trigger on the 124gr I would likely be pushing the trigger on the 147gr as well since that action is prior to recoil.

If I adjust the sites back to a more traditional just slightly offset right from center position on the gun then the 147s are spot on but the 124s impact noticeably left.

The problem manifests itself on slow aimed fire or rhythm drills

Why I'm stuck:

-If the impact shift was up or down I would blame the ammo but I've never heard of a left/right shift in a pistol cartridge

-If the impact shift was consistently left and consistently compensated by the dramatic shift on the sites to the right I would blame me/my trigger pull

-If the condition only happened after using either ammo for a long time before switching I might also blame me for somehow anticipating or reacting differently to the recoil but I can litterally swap back and forth one round at a time and the condition is repeated

Does anyone have any idea how I am screwing this up or what other factors may be at play?

Gun stats:

Production legal G34, stock Glock barrel, grip tape

Vogel Competition Trigger

Dawson FO sites. 105F/115R

I'm attaching a picture of my final demo target of the day. I had to use the cardboard that came with the shoot and see targets because I used them all up during my denial phase of the afternoon trying to figure out if this was really happening. The cardboard actually shows the problem very clearly though.

15meter target/1" dots: Ammo type marked above dots. Frustration at this point in the day affected the groups but the overall shift wit the 147s is clear.

The sites on my G34 (Dawson)

[Rick88]

Your gun is telling you it likes the 124s better feed it what it wants...

[Friction]

I suspect that there is more to it than that. If I shot the 147s first I would't have moved the sights that far and the symptoms would be reversed when I shot the 124. Applying your logic to that sequence and the gun would like the 147s and the 124gr would be the problem.

[Steve RA]

Pick one and just use it.

[grapemeister]

Here's one theory. It may be total BS, but here we go.

I think if we're not careful we can learn to have a premature reaction to the way a gun recoils, makes noise, etc. For example, flinching and jerking the trigger causing the typical low left hits. Another example, when I fire a revolver, which is rare, I tend to flinch and shoot low, this especially true when shooting a revolver that's not mine.

Over time you've developed a premature reaction to the way your gun shoots 124 gr bullets. Hence, the reason you push your rear sights to one direction or the other. The way the gun recoils, or the way you perceive the gun behaving when shooting 147 gr bullets has changed. There is normally a different feel, or recoil with heavier bullets, and you picked up on that after firing a 147 gr bullet for the 1st time.

Anyhow, in my opinion it's most likely the shooter, and not the ammo. Maybe it's a gun problem, or more specifically a barrel problem, and that's certainly worth exploring, but I really have my doubts about that.

[rack&roll]

One obvious answer is to borrow a friends stock G-34 barrel, and see if it prints the same with both loads. (124-147)

[Loves2Shoot]

If you are expecting the bullets not to shift, then you are not taking into account all the factors involved. It is not just a matter of elevation, but barrel twist, bullet weight, and velocity. Try some 115's or 110's and it will illustrate it even more. Once you have that down, then you need to take into account bullet stabilization. Every bullet weight will stabilize at different velocities in different twist barrels. If the bullet goes too fast or too slow your groups will open up.

[grapemeister]

The more I think about it, the more LovetoShoot's post makes more since than mine. I don't like the idea of shooters moving their sights to compensate for bad shooting but If you've been consistently accurate or had a consistent pattern by moving your sights to one side or the other, and then all of a sudden the pattern changes with the 147 gr bullets, what Loves2Shoot said makes more since to me.

I think it would be interesting to see how the ammo does with another gun that's just like yours and/or have someone else shoot your gun and see if the results are consistent.

[JeffJ99]

This is pure spitballing and conjecture here:

Is it possible that you are inducing a slight amount of left twist when pulling the trigger and then your grip is immediately going back to neutral? If so, then it's possible that since the 147s stay in barrel just a little longer (why they usually hit higher) that you've allowed the gun to move just a little bit back to the right by the time it leaves the barrel.

That might be totally crazy, but it was something that popped into my head when reading.

[Miranda]

when I started shooting I wondered why no one said anything

about different weight bullets and speeds and those relationships

to point of aim and printing a target.

since they all seemed about the same to me, I quit wondering.

:-D one of the things I thought must happen is that different bullet weights must

impart different torque on the pistol.

Try rotating the pistol counterclockwise a little when you shoot the 147's and see what happens?

miranda

[trgt]

All other things being potentially equal, it's my understanding that because 147s are slower and in the barrel longer, both muzzle flip from recoil and shot follow through from what you do (or don't do) with your hand and trigger finger are more pronounced with 147 vs the lighter and faster bullets that are simply not in the barrel as long to be affected by either.

In my case normally 147 shoot higher at distance than 124 or 115, mostly due to barrel flip, I believe.

I saw an instructor give an example of coming off the trigger like it was a last shot, vs keeping the finger on like another shot was expected, and that had a pronounced horizontal effect on shot placement. I'd imagine that difference in follow through would be more pronounced with 147 than lighter bullets.

EDIT: I just noticed I may have taken twice as many words to say what JeffJ99 just said :-)

Edited March 18, 2014 by trgt

[Friction]

Rack/Grape: I've been looking for anyone else with a 34 barrel and haven't found one yet but I may try to have someone else shoot mine to check the pattern for them. My first assumption is that I am the problem and not the gun based on the symptoms.

Jeff/trgt,

My original though was that it might be related to the difference in the actual time in the barrel but I didn't want to lead the conversation since i would rather get opinions from others that may have seen or experienced the same thing. Based on the difference in projectiles of about 200fps the actual time difference in the barrel is around .0022 seconds.

200FPS = 2400 inches per second

a 5.31" barrel = 1/452 of 2400"s

1/452 of 1 second is .0022 seconds

So if that is the problem then the presumption is that I am pushing the gun, probably via the trigger, to the left at the break/ignition then correcting back to on target in .0022 seconds. Does that sound reasonable? If so I'm probably still doing it with the 147 but the gun has those extra milliseconds to settle back on tgt before the round makes it out of the barrel.

So from a training standpoint, if I switch to 147s I'm just getting lucky and masking the issue the same way I might be now by pushing the sites to the extreme right on the gun. In either case I need to fix my grip/trigger engagement. If I leave the sites where they are and train with 124s I should be able to detect the issue being resolved by the POI shifting to the right of the target. Does that check?

Thanks

[Loves2Shoot]

Rack/Grape: I've been looking for anyone else with a 34 barrel and haven't found one yet but I may try to have someone else shoot mine to check the pattern for them. My first assumption is that I am the problem and not the gun based on the symptoms.

Jeff/trgt,

My original though was that it might be related to the difference in the actual time in the barrel but I didn't want to lead the conversation since i would rather get opinions from others that may have seen or experienced the same thing. Based on the difference in projectiles of about 200fps the actual time difference in the barrel is around .0022 seconds.

200FPS = 2400 inches per second

a 5.31" barrel = 1/452 of 2400"s

1/452 of 1 second is .0022 seconds

So if that is the problem then the presumption is that I am pushing the gun, probably via the trigger, to the left at the break/ignition then correcting back to on target in .0022 seconds. Does that sound reasonable? If so I'm probably still doing it with the 147 but the gun has those extra milliseconds to settle back on tgt before the round makes it out of the barrel.

So from a training standpoint, if I switch to 147s I'm just getting lucky and masking the issue the same way I might be now by pushing the sites to the extreme right on the gun. In either case I need to fix my grip/trigger engagement. If I leave the sites where they are and train with 124s I should be able to detect the issue being resolved by the POI shifting to the right of the target. Does that check?

Thanks

No, your expectations of them impacting in the same spot are off. Given they bullet weight, velocity, and torque differences why would you expect them to impact in the same place? If you had a LH twist barrel would you expect the same bullet to impact the same point on the target?

[bigdave24]

Why not load the magazines with alternate 124s and 147s, and see if they truly differ? You could have two different aiming points, and evaluate the groups.

[Friction]

Why not load the magazines with alternate 124s and 147s, and see if they truly differ? You could have two different aiming points, and evaluate the groups.

I did that during the initial test and created 2 distance patterns which is what drove me to wonder about other factors.

[Miranda]

I'll repost in a minute

Edited March 18, 2014 by Miranda

[Miranda]

Hi Friction,

at some point you will have to trust yourself and what you see.

balistics of a big area of study

It is difficult to have identical projectile paths for a 124 versus a 147.

they can be close and when they are it takes more energy to get the 147 on the same path.

so the pistol will recoil more and torque more.

make sure the pistol frame and slide have no cracks.

once done, try as tight a grip as you can manage and see if the two groups move together.

about all I can think up...

edit to fix spelling....

Edited March 18, 2014 by Miranda

[grapemeister]

Whoops, wrong topic.

Edited March 19, 2014 by grapemeister

[Jshuberg]

Here is another piece of wild conjecture. It may be a combination of trigger control, timing and grip. As you press the trigger, you are overcoming the weight of the trigger spring. When the shot breaks, the resistance of the trigger drops, and when this happens there is a tendency for the trigger to snap rearward through the overtravel until the trigger reaches it's rearward limit and stops.

If you were putting a lateral force on the trigger, but masking it with the right amount of grip strength, what may happen is that your sights remain correctly aligned until the shot breaks. Then the trigger resistance drops, and any lateral movement would be introduced into the frame when the trigger hits its rearward stop. The grip strength that was able to overcome the gun moving laterally before the shot breaks may not be able to overcome the gun moving when the trigger snaps rearward unrestricted and then stops abruptly. It may be a perfect alignment of timing, grip strength, and trigger press mechanics.

Again, this is just a very wild guess, but play around with your trigger press and see if any changes in it's mechanics effect your groupings. Another test would be to load a few 115g bullets light to achieve the same muzzle velocity as the 147s and see if a similar grouping occurs. If so, you are likely inducing a lateral motion on your trigger that only evidences itself under very certain circumstances. The solution would be to work to correct your trigger press so that it is perfectly rearward.

Edited March 19, 2014 by Jshuberg

[Religious Shooter]

I experienced the same thing with 115's and 147's. I was shooting minor 115's for three gun and wanted some major rounds for MGM spinners. I tried 147 major and found a similar shift at a relatively close range. Discovered the difference and tried 124's for major round. The 115's and the 124's had pretty much the same POI at that close range. So I ditched the 147's.

Gun was a 6" SVI.

[JAFO]

I was taking a class with a double-action only pistol and having a hard time. The instructor let me borrow his DA/SA gun and try it with my ammo. The gun consistently printed groups to the left of POA. I thought it was me, but when I mentioned it, he tried and it did the exact same thing for him. Then he remembered that the gun had been zeroed for his 125gr carry load and I was using a 115gr factory load. So yes, I have seen lateral shifting resulting from bullet weight/load changes.

[BeerBaron]

given you can shoot a group I would say it's not you. it's the different projectiles. as love2shoot mentioned there is a point at which different projies stabilise. some people get caught up loading for power factor, and end up with bullets that have some fliers or wide groups. another little bit of powder and suddenly the groups tighten up.

I would experiment a little with different loads. if the 147gn is just loaded to power factor it will be going very slow. maybe too slow.

but if the group is not large, and they are hitting to the left consistently then there's no real problem. you've just found the POI for that load. adjust the sights accordingly and enjoy.

[chowellacsbd]

Just relaying info from Taran Butler when I had him install TTI sights on both, my g34 and g17...so please don't kill me on this

He said that he installs his rear sights slightly to the right. I forgot exactly why, but he said this was discovered to help during filming of a past season of Top Shot.

My TTI rear sights are both slightly to the right and they shoot spot on with 115gr MGs.

Sent from my iPad using Tapatalk

[bountyhunter]

This is pure spitballing and conjecture here:

Is it possible that you are inducing a slight amount of left twist when pulling the trigger and then your grip is immediately going back to neutral?

That was my thought. The heaveir bullet accelerates more slowly so it gets out of the muzzle slightly later. If the shooter is moving the gun slightly with the trigger pull, the muzzle will have moved farther off line when the bullet leaves the barrel.

The heavy bullet probably also has more initial recoil so it would aggravate a problem if the gun is not locked on line.

Edited April 3, 2014 by bountyhunter

[lawboy]

Your gun is telling you it likes the 124s better feed it what it wants...

Bingo. What you are experiencing is not uncommon. Guns frequently shoot different ammo to different places both horizontally and vertically.