Hotter rounds shoot at 10 o'clock from center?

[myt30]

Hey folks, I had my Savage MKII FV SR out for some load testing last week and had some odd(in my mind anyway) results. I went through every type of ammo I could find looking for the tightest groups.
Odd thing was that while with standard velocity ammo i was zeroed dead on at 50yds, the higher velocity i went, THe rounds crept up in the 10 o'clock position. Winchester wildcats were about an inch from center, powerpoints were 1.5" from center, and cci stingers were 3.5" from center at 50yds.
I had expected vertical drop and rise from the velocity changes, but why does it go off to the side as well?

 

[Jgoulin]

Just a stab but with increased velocity you get increased spin which may result in some lateral movement in the rotation direction.

 

[grauhanen]

Gravity is not the only physical force at work when shooting. In a simple and straightforward world, a slower round would simply hit the target with more drop than a faster round. Unfortunately rimfire ammo is not always simple and straightforward.

Barrel harmonics differ with different ammo as all barrels vibrate and will do so differently with different ammo. As the barrel vibrates according to the nature of the round passing through it, the muzzle changes position ever so slightly as the bullet exits. Different ammo brands that are the same in velocity -- and you don't really know what the muzzle velocity is unless a batch is chronographed -- can behave or respond differently. Different ammo that is around the same velocity, such as various match ammo, may well have different POI. Spin drift may also be a factor, where a combination of spin and gravity cause the bullet to yaw slightly and drift sideways.

The bottom line is that it is not surprising that you have to resight your rifle when changing ammo.

 

[buji88]

Another factor with your rifle is the factory stock. The Savage synthetic stock is pretty bad for causing POI shifts with different ammo. I would say re-torque your action screws before worrying about it too much and play with the settings as well. If you have never touched them, this could explain why it's pulling to the left with the increase in velocity. The high velocity could be causing the action to rock or the barrel to deflect off the stock.

 

[myt30]

I really appreciate all the detailed replies. I never thought of harmonics having to do anything with it.
I'll check the torque on the action screws, and see if it makes any difference. I still love the rifle, all the groups are still tight(or as tight as my poor shooting will allow), I just have to pick an ammo and sight in for that specific one, which is not a problem.
Thanks all for your help.