Does POI Correlate with MV?

[grumpy one]

Not having a chronograph, I tend to notice things like the way my point of impact on the target rises as I increase the powder charge without changing anything else. In fact, the amount the POI rises per grain of powder added seems to increase pretty rapidly as I get near maximum load. According to the literature the pressure is doing much the same. Furthermore all other things being equal, at the low to moderate velocities we are talking about with cast bullets I would expect the vertical position of the POI to correlate pretty well with muzzle velocity, just on the basis of the shape (i.e. how flat) the trajectory is.

My question is, provided we are talking about the same gun, same bullet, same fit, same cartridge etc, can I regard vertical POI as a useful indicator of MV? If so, this would be pretty useful for comparing different types of powder.

Geoff

[ANeat]

If you had a point of reference for yourself you probably could translate some point of impact change to a change in velocity. I dont think you could come up with an across the board rule for everyone as everyone reacts to recoil in a different way. One person can have a gun sighted in dead on and another person can shoot the same gun/bullet and the point of impact may be way off.

For yourself if you had a known load/velocity and a ballistic chart you may be able to get a ballpark estimate of velocity from a change in POI.

Adam

[LET-CA]

This may be an over-simplification, but think of the bullet falling to the ground from your hand. As you add forward speed to the bullet, it still falls at the same rate, it just travels further forward before it hits the ground. So, the faster the bullet is moving, the less it falls for any given distance. Hence, when you are shooting at a 100 yard target the Point of Impact moves up as your velocity increases. The bullet just hasn't had as much time to fall before it reached the target. All of this ignores any aerodynamic effect of the bullet flying because of it's speed.

All the best

[44man]

A lot depends on the gun you are shooting and how the recoil effects barrel rise. A revolver usually shoots high with light loads because the boolit has more barrel time and lets the barrel rise more before it exits. But then, boolit weight can alter this also so that a heavy boolit, heavy recoiling magnum load can also hit higher. Handguns are totally recoil dependent as to where they hit.
With a rifle, there are many other things that can effect POI. Barrel vibration, recoil, how it is held, how it is rested, weight of bullet, velocity, powder speed, amount and weight of powder charge, barrel bedding, barrel heat, mirage from a hot barrel, bullet RPM's, BC of the bullet, shape of the bullet and I am sure there are more.
Best to stick with the chronograph. Even better to go for accuracy first and forget about velocity as long as loads are safe. It is easy to adjust scopes and sights.

[RangerBob]

Yes . . . But . . . .

One powder type, with one bullet type/weight, in one rifle . . .

In this case I believe that you can judge velocity (relative, not absolute) by POI.

If you change to a different powder type or bullet weight, then all bets are off. The pressure curve changes and barrel time may be quite different for a given velocity.

I have a .458 that hits noticable lower with increasing velocity.

[44man]

As you change the powder amount, you are still changing the muzzle whip, RPM's and recoil plus adding to the total weight being shot.
It is normal for faster loads with a given bullet to hit lower. But every gun is different and even the rifle's weight can change things. You should be able to tell if velocity goes up but never how much. You might be able to plot what your rifle does as you increase powder but still need the chronograph, so why bother? Your chart will only be good for the one powder and bullet out of your rifle, won't work for other guns, with a powder, bullet or primer change. You are looking for an easy way but are actually making it harder.