I recently got a thousand rounds of Brown Bear 115 gr. 9mm. I ran a box of fifty today through my P30LS, and came back with curious results; while the groups are what I expected myself to be shooting, they are consistently centered about 1 inch above my intended POI. When I shot the Winchester 124 gr. NATO 9mm, the POI was what I expected it to be. All shooting was done at 10 yards. I'm curious if this is normal, and if so, what creates this difference in POI. Thanks.
Weight and speed.
Think for yourself. Question authority.
That was my first guess, but I wondered if such a noticeable difference would appear at just 10 yards. Also, I've wondered if the underpoweredness of the Brown Bear would have negated the fact that it was a lighter round, coupled with the fact that the the Winchester is basically a +P round. Still, prior to this, I've only shot 230 gr. .45 ACP, so I really don't know shit about this stuff.Originally Posted by orionz06
Just this Sunday I had a box of Remington 115gr shooting about 3" high at 7-10 yards and ~6-8" high at 25 yards out of a gun that shoots everything else just fine.
Think for yourself. Question authority.
Cool, sounds like it's nothing to be worried about then. Thanks for the info.
Sometimes you'll get a right or left shift in POI too.
Can someone please explain this to me? I've heard this before, but can't understand why/how it can happen.
Thanks.
Joe Mamma
If I had to guess, I'd say it's because when you change the length of time the round is in the barrel from the time of firing or change the force imparted by changing the bullet weight, the way the barrel, slide and all the other parts lock up becomes different. All those parts are all moving during the firing process even if it's infinitesimally. The barrel has play within the frame in all directions, not just up and down. Since all the tolerances of the parts aren't exact and there's flex happening in the frame and other parts this would account for the difference.
As for the vertical dispersion between different bullet weights it's a result of the recoil force and time in barrel. As soon as the round fires it imparts recoil which in handguns starts upward movement because the bore axis is located above the hand. The less time spent in the barrel, the less the barrel will get to rise before the bullet leaves the barrel. Slower loads hit higher because the barrel has a chance to tilt upwards more before the bullet leaves.
This is all at close handgun distances though, when rounds are subject to gravity longer as is the case with rifles at farther distances this principle generally reverses dependent on the velocities and BC.
I have no explanation why the POI shifts right/left, some other factors I "think" might come into play are the twist rate of the rifling, the length of the bullet's bearing surface, bullet jacket alloy, bullet diameter, and velocity. Also let's not eliminate powder burning rates and the amount of gas generated by different powders. I just checked some of my load testing data and found that changing powder would change left/right zero with same bullet. I've also documentd more right left changes with factory ammo.
I don't know about pistols, but in rifles, as you work up a load - only changing the charge - the POI can make figure 8 pattern. This is partially caused by the whipping motion of the barrel during the firing process.
You try to find a load at the top or the bottom node in the figure 8 - you do this so temperature changes (which have the same impact as changing the charge) have the least vertical impact on your POI.
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