POI change?

[ZVP]

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 Why would a very low velocity .38 load shoot more to (in my guns case) the left than a Factory load? (About 3/8").
 The revolver is a box stock Ruger New Model Vaquero .357 with a 4 5/8 bbl.
 Shot bench rested.
 ZVP

[Fox Creek Kid]

There are a myriad of factors why different ammo impacts at different points of aim. Primers, powder, rifling rate of twist, etc. There are Ph.D. treatises written on it. Suffice to say you either have to:

1.  Change your load.

2.  Start handloading to tailor a round to your sights.

3.  Adjust the sights and/or turn the barrel.

The limit is your budget & resolve.  

[rickk]

How you grip the gun can affect left/right impact.

Roughly speaking, the barrel wants to lie in the same plane as your strong arm. If for some reason it does not, it would shoot left or right.

From what you said, it is bench rested and shooting more to the left than hotter loads fired from a rest as well.

By bench rest, you mean you are holding it and resting it on something, or it is in a ransom rest?

If you are holding it but resting it, grip still plays an important part in impact. I teach NRA gun safety to about 300 people a year, so there are certain things I have learned to watch for. I'm not standing here watching you shoot, so it is harder to tell. I can throw out my most likely guess though.

If you are left handed and shooting in an isosceles position (both hands on the gun, the gun partly to mostly right in front of your chest). As the gun recoils it would pivot on your strong arm and the barrel would move to the right. Add to that a really heavy bullet (180-200 say) with even more recoil and a long barrel time and it would shoot even more to the right.  A really mild load would not pivot in your hand as much, so it would shoot more to the left than a hot load, heavy.

A weaver stance hold puts the gun more straight in line with your arm and body, so it would pivot less.

An experiment for you would be to either find a glove with a seem in the web area or make a mark in the web area with a magic marker. Pick up the gun and hold it in what is "normal" for you. Make a corresponding mark on the gun backstrap that lines up with the mark on your glove. Go shoot a bit with the light load. Change targets, hold the gun with the mark on the glove moved to the left of the mark on the gun grip. Shoot another target. Change targets. Shoot again with the glove mark moved to the right of the grip mark.

Now do it all over again with the hot load.

You will find that there is a grip position that is less sensitive windage-wise  to ammo types. That is how you want to hold the gun if you don't want to have a different left-right point of impact.

Personally, I prefer the weaver stance. It lines up the grip with the strong arm and your body better. Everyone is different though. It boils down to a compromise between what is best from a technical point of view and what is most comfortable for you. Sometimes a student in my class has some sort of physical problem that prevents the use of classic grip and stance. A missing finger or two, a recent or old injury, or sometimes they just lack the strength to hold a gun. I do what I have to do to teach them to shoot safely and still demonstrate some reasonable amount of accuracy.

There are other possibilities. That is just the first one that pops into my head. I've been teaching NRA pistol for 20 years now, and I learn something new almost every class I teach.

[Pettifogger]

3/8"  At what distance?  3/8" is barely one bullet diameter and is insignificant.

[ZVP]

 Thanks for all the great ideas, information, and help!
 The load is an ultra L/V 96 grain Lead bullet. A plinking load for my derringer. I wonder if the tiny bullets are stripping?
  I am shooting over a sandbag rest, two handed.
 The revolver naturally shoots about right on at 7 yards and to the right.
 These "Light" loads actually shoot almost centered at 7 yards!
 I am trying to avoid filing the sight notch or getting the barrel turned but I think this is going to be the answer to my problem of being off center. The revolver shoots about 3'4" right even with jacketed .357 Magnum ammo. It drives me nuts to have such a good quality revolver that shoots off center so much with all but impratical loads.
 Maybe the gun was assembeled on a Friday or a Monday as it has been back to the factory once for internal parts replacements. I asked Ruger to center the sights and they did file the front sight to be spot-on at 7 yards height-wise but the Left/right was not adjusted.
 I am going to try all the tricks I can learn and if that dosen't do it, I will have to spend some money at the gunsmiths...
 Thanks again for the help!
 ZVP

[Fox Creek Kid]

You have to regulate sights to ONE and ONE ONLY particular load. That is best done with a handload and not a factory load with fixed sights as the factories load ammo to pressure & not velocity. 

[ZVP]

  If that's so the factory machienes should be the same within each lot?
 And.
 Do you mean that if I find a certain lot of ammo that shootsw to the sights, I should buy a bunch?
 ZVP

[Fox Creek Kid]

  If that's so the factory machienes should be the same within each lot?
 And.
 Do you mean that if I find a certain lot of ammo that shootsw to the sights, I should buy a bunch?
 ZVP

Ideally, unless you're Bill Gates you'll need to learn to reload. 

[St. George]

Plus - a weapon fired from a sandbag rest 'will' have a different POI when shot one-handed.

You need to adjust for that, too.

Figure out your best load - the one that groups tightest - and then it'll be up to you, your shooting budget and range time.

Vaya,

Scouts Out!

[ZVP]

 Lotsa good suggestions there! This past weekend I tried liftimg my trigger hand's thumb from a wraparound to a sort of "Taco Hold. ' The POI did center a bit better. I may end up having a 'Smith rotate the barrel to get her centered. Any idea how they refit the ejector housing to a bbl that's been turned? Seems like somthing would end off center there...
 Maybe they cut the holddown screw hole in an oval, thus allowing it to be slightly shifted with the barrel?
 I love this Vaquero and dont want to mickey-Mouse things up on it for any reason!
 Yea I'm ready, The revolver is worth it! Good fixes cost money...
 ZVP

[cpt dan blodgett]

most right handed shooters will shoot left.  They push the gun.  Lighter slower load has more time for the gun to move in the direction being pushed by the trigger finger or by the hand clenching.