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Old October 11, 2012, 10:48 PM   #13
JohnKSa
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Join Date: February 12, 2001
Location: DFW Area
Posts: 18,565
Quote:
I would add that there is enough difference in velocity to change your trajectory slightly. my 9mm (115gr @ 1150fps) that hit roughly 2" above the sights at 25 yards will hit just under the top of the sight blade at 7 yards while my 45 (230gr @ 800fps) with the same 25 yard zero will hit right above the top of the front sight blade. It's a very small difference but it's there.
Right, one could reduce the slope of the line and the bullet would stay below the top of the front sight for a longer distance.

However, even in that case, the point of impact won't be very much below the front sight because it's still traveling on an upward path over the distance out to 25 yards and it only started maybe a half inch below the sights to begin with.
Quote:
I saw john's chart, and assumed satellites everywhere are in danger.
Yeah, the scale is amplified to make the graph clearer--but it does give the impression of an inevitable upward trend, doesn't it? Of course, if I were a good enough shooter to make decent groups out past 25 yards, and took the time to collect data at the longer distances we would eventually see gravity take over and the trajectory would bend downward.
Quote:
POI is a factor of grip, recoil, and bullet velocity. A mechanical alignment of bore and sights is not necessarily a useable sight setting.
That is true, however it's usually not a major factor in locked breech autopistols because the recoil of the pistol while the bullet is still in the barrel consists primarily of slide/barrel movement and not much muzzle rise. There isn't much muzzle rise until the barrel impacts the frame and its momentum is transferred to the frame which causes muzzle rise. But before the barrel unlocks and hits the frame, the bullet is long gone. So it isn't affected much by recoil-induced muzzle rise.

(Of course, there's more momentum transferred--and more muzzle rise induced--when the slide is stopped at the end of its travel but that's much later in the cycle and not particularly relevant to this issue).

The graph shows that there isn't much muzzle rise in this particular pistol while the bullet is still in the bore--at least not the way I was holding it. Remember that the 0 yard figure is the actual distance between the top of the front sight and the centerline of the bore. Had muzzle rise been a big factor in the point of impact for this pistol, the trajectory numbers wouldn't have lined up well with the 0 yard figure. The 0 yard figure would appear to be low because the other numbers would have been bumped upward by recoil-induced muzzle rise.

Perhaps at very long ranges, the small effect would be magnified enough to show up.

I'll repeat this someday with a revolver so we can see the difference.
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