Help with sighting in

M

mickrazz

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Hi All, I'm new to the forum but not new to shooting and have run into a situation that I can't figure out.

I recently purchased a browning buckmark .22 and opted to go with a tru glo open red dot sight. My eyes are old and I can't get a good sight picture anymore.

First let me say that I shot the gun with the iron sights and even with a blurry view of the sights the gun grouped very well. No issues with the gun at all.

I mounted the red dot and went to the range to sight it in. Starting at 7yds the gun shot 4" low and 2" left. After adjusting the sight to hit dead on at 7 yds I move the target back to ten yds and shot a 3 shot group. This group was 1" higher. I then moved the target to 15yds. This group was 1" higher than the last. I then moved the target to 25ys and the group again moved up, this time about 3".

After scratching my head I tried sighting in at 25yds and then worked back to 7 yds. When I was on at 25yds I was again 3-4" low at 7yds.

I repeated this procedure two more times with the same results and then called it quits. This is my first handgun with a red dot sight. Am I missing something?

Thanks for any help or suggestions anyone might have.

Mick
 
It's perfectly normal for a gun with optical sights to shoot noticeably higher as the target distance is increased.

Because the sight is higher than the axis of the bore, the bullet has to "rise" (relative to the line of sight, that is - all bullets start to drop relative to a line parallel to the ground as soon as they leave the barrel) to hit where the sights are looking. If you then move the target further away without adjusting the sights, and therefore without moving the gun, which is still aiming at the target center, the bullet is still rising relative to the line of the sight, and now crossing above it because of the added distance to the target, so will strike the target above the point of aim.

At typical handgun distances, the drop of the bullet due to gravity is minimal so doesn't really enter (much) into the calculation.

All this is true for iron sights as well, just not so pronounced because they're closer to the bore axis.
 
At typical handgun distances, the drop of the bullet due to gravity is minimal so doesn't really enter (much) into the calculation.
Click to expand...

Just as a point of reference, a free falling body which starts off with no vertical velocity component (flying parallel to the earth's surface) will fall about 1.08 inches in the time it takes a bullet to go 25 yards at 1000 ft/sec. The vertical distance increases as the time squared, so at 50 yards the distance fallen is about 4 inches (it's actually a bit more than that since the bullet slows a bit).

In mathematical terms,

distance fallen (in feet) = 0.5 * 32 (ft/sec^2) * (time to fall in sec)^2

Multiply the result by 12 to convert to inches

time to fall is approx. 0.075 sec (75 ft / 1000 ft/sec), neglecting velocity loss due to air friction.
 
Good clarification, and I probably shouldn’t have written off the acceleration of gravity so quickly, even at handgun distances.

Looking at the whole system more quantitatively then:

If the line of sight is 1.5” above the bore (don't know if that's correct for the OP's setup, but it's commonly used for calculations involving scoped rifles), and using the nominal 1000 ft/s muzzle velocity and the gun sighted for 7 yd (21 ft), the bullet must “rise” 1.5" in .021 s for an initial vertical velocity component of

(1.5 in / 12 in/ft) = 0.125 ft/.021 s = ~ 6 ft/s.

At 25 yards, that means the bullet will “rise” 6 ft/s * .075 s = .45 ft, or about 5.4 inches relative to the LOS. So, subtracting from that the drop due to gravity (1.08") (and similarly ignoring air resistance, etc., etc.) makes the point of impact about 4+ inches high at 25 yards, which is pretty close to what the OP was seeing.

ETA: Correction: The 4+" high impact is actually above the elevation of the muzzle, which is already (per the assumptions) 1.5" below the line of sight, so the impact at 25 yards is actually closer to 3" - still reasonably close to mickrazz's experience.
 
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Thanks guys, that all makes sense. So now what do I do? It looks like I won't be able to be "on" at two different short range distances. Maybe pick a happy medium, say 10yds, and see where I hit at other distances and be happy with that.
 
About the only thing to do if you expect to do a lot of shooting at different distances is to pick one (maybe the one you expect to be at the most) and then compensate for other distances based on experience.

Alternatively, if you don't change all that often (say, one place you shoot has the targets at 7 yards and the other at 25, or whatever) you could change the elevation adjustment on the sight according to the distance. That's what I do with my Bullseye gun - most of my competitions are at 50' and from time to time I have to shoot at 25 yards. I know from testing that to compensate for the additional distance I need to move my elevation 4 clicks down, which only takes a moment to do.
 
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