Pattern shift with velocity of ammo

I have been refining my shooting for a while now.  Things are coming together well.  I have observed a pattern shift that is associated with ammo velocity.  I am shooting an IZH35M, the gun can make one hole at 10 yards if I shoot it off bags.  As the ammo I use decreases in velocity (from 1125fps to 1007fps) the pattern I shoot shifts to the right.  from the extremes of velocity change the shift is about 1.5 inches at 25 yards.  Pattern is still nice and round but simply shifted right the slower the bullets go.  I am wondering if this is a grip problem as the rounds are in the barrel longer at the slower velocities allowing more deflection (the IZH has a 6 inch barrel with a distance between sights of about 8 3/4 inches).  I can understand a vertical shift but the horizontal one has me wondering if I have a grip problem.  Larry

If the groups are tight, but just shifted, it is the ammo. Don't obsess on why.

trotterlg wrote:I have been refining my shooting for a while now.  Things are coming together well.  I have observed a pattern shift that is associated with ammo velocity.  I am shooting an IZH35M, the gun can make one hole at 10 yards if I shoot it off bags.  As the ammo I use decreases in velocity (from 1125fps to 1007fps) the pattern I shoot shifts to the right.  from the extremes of velocity change the shift is about 1.5 inches at 25 yards.  Pattern is still nice and round but simply shifted right the slower the bullets go.  I am wondering if this is a grip problem as the rounds are in the barrel longer at the slower velocities allowing more deflection (the IZH has a 6 inch barrel with a distance between sights of about 8 3/4 inches).  I can understand a vertical shift but the horizontal one has me wondering if I have a grip problem.  Larry

You sound like a rifle guy.  

If Precision Pistol is the game you want to play.   Worry about what your ammo does at the 50 yard line, and the 25 yard line.   Testing at ten yards on sandbags is just a waste of time and money, and it isnt teaching you the three most important factors in Bullseye.  Trigger, trigger, and more trigger.  Set your sights for the 50 yard line with a reliable ammo, and then work on your shooting.  The ten ring is almost four inches across.  Poor triggering will cost you more points by a factor of hundred than different velocities in your ammo will.

different brands or even different lots of the same can shoot to different poi. find what you like best and adjust dot or sights to hit center and then practice doing it.

Heck you can shoot a group, put the pistol down, pick it up, regrip and shoot a same ammo group into a different spot.

I shot it off bags to make sure I was not having a hardware problem with the gun.  Clearly it has the ability mechanically, so I can rule that out.  The group is nice and round, today I just clicked it left a little bit and things are good with this ammo.  Can't notice any difference between S&B subsonic and SK pistol match, I do see a POI shift if I get up around 1,100fps+ with the ammo.  It is just curious to me to get a horizontal shift with velocity.  I have an old person match to shoot in Arizona next month so we will see how it does for real.  Larry

Larry, Something that can happen is that your pistol can be slightly canted. A canted pistol will have horizontal shift with changes of velocity. Much more noticeable at longer distances.

Len

Would something like a nick in the crown have this effect?

From a diagnostic point of view, it might be helpful to list all the different types of ammo that you tried, and where each one shot.  

Trying what Jack H suggested seems like a good test to see if it's really your grip.

If I'm doing the math correctly, 1.5" at 25 yards is about 6 MOA, or, conveniently, 0.1 degrees.  At these velocities, the bullet is moving in a 6" barrel for only about 1 millisecond, and the difference between how long it's in the barrel for a 1000fps bullet versus an 1200fps bullet is going to be less than 0.2 milliseconds.  For the point of aim to move 6 MOA, rotating around the grip (assumed to be 7" from muzzle), the muzzle is moving about 0.0122" (0.3mm).  To move that far in 0.2 ms, the muzzle would have to be moving 5 feet per second, which is pretty quick.  I'm having a hard time envisioning how that could be a grip issue.

Yes muzzle damage can effect accuracy, but you need a better testing procedure than a sandbag.  You need a HEG or ransom to truely test.  But not many of us have access to that stuff.

LenV wrote:Larry, Something that can happen is that your pistol can be slightly canted. A canted pistol will have horizontal shift with changes of velocity. Much more noticeable at longer distances.

Len

Much better decanted.

james r chapman wrote:

LenV wrote:Larry, Something that can happen is that your pistol can be slightly canted. A canted pistol will have horizontal shift with changes of velocity. Much more noticeable at longer distances.

Len

Much better decanted.

Lol

Would you like to recant that answer?

Well, I decanted the gun over night and went to the range today.  The S&B subsonic and the SK Pistol match both shot to the same POA.  Only shift I get is when I shoot Aguila Standard plus which is 1,125fps.  So I guess I will just not shoot that.  And guess I will continue to decant the gun overnight before any match.  Larry

It's probably you or ammo and not the gun. If there was a crown issue it would not hold under .5 an inch at 10 yards.

Zero could change with 22LR even with the same brand but a different case lot which is why 22lr rifle guys and Free Pistol guys will buy cases from the same production run.

You are probably right about the ammo, it is just a curious thing to me.  Larry