quote:

Originally posted by Loswsmith:
I have a .22 Ruger American that I'm trying to sight in. My easy peasy mostly anytime range is indoor and only to 25 yards. Is there a easy drop compensation to calculate what POA/POI I can use at 25 yards to get a 50 yard zero?

quote:

Originally posted by Loswsmith:
I'll bust out the measuring tape for the sight/bore measurement tonight.

quote:

Originally posted by fritz:

quote:

Originally posted by Loswsmith:
I'll bust out the measuring tape for the sight/bore measurement tonight.

Did you make the measurements? With the height-over-bore and some ammo information, I can get you through JBM ballistics data.

quote:

Originally posted by fritz:
OK, that's enough info to get me going. Even if the webz set you down an incorrect path. The true measurement should be center of bore to center of scope.

Since you gave me the scope model, I went to Nikon's site and found that the eye piece is 44 mm diameter, or 1.73". Divide this by 2, and subtract from 2.5" total. Now I come up with a eyesight height over bore of 1.6". My assumptions in JBM ballistics are:
- height over bore of 1.6"
- air density altitude of 3,000'. Guessing that's a reasonable air density for your shooting, although with such short distances it won't matter all that much.
- muzzle velocity of 1075 fps. Assuming you're bumping up close to Mach 1, but staying a little under.
- CCI standard bullet, per JBM's table of bullet ballistics.

JBM predicts that for a zero of 50 yards, your impact should be .2" above point of aim at 25 yards. Just so you know, the combo of you gun and ammo may not be accurate enough to truly judge an impact of .2" high at 25 yards. Or, it may be. Anyway, just think of being about 1 bullet diameter high at 25 yards, and you're good to go at 50 yards.

FWIW, the above assumptions predict impact at 75 yards being 2.5" below point of aim, and 7.5" below point of at 100 yards.

quote:

Originally posted by sigcrazy7:
I do believe the 2-7 Prostaff is a centerfire scope, so note that it will be parallax adjusted to 100yds instead of the more common 50 of a rimfire scope. This should give it something around .3 parallax at 50yds. Not a big deal at low power, but could get a little furry at say 15yds if you crank it up to 7 and try to shoot at paper.

At my age, everything is getting blurry all the time anyway.

quote:

Originally posted by Loswsmith:

quote:

Originally posted by sigcrazy7:
I do believe the 2-7 Prostaff is a centerfire scope, so note that it will be parallax adjusted to 100yds instead of the more common 50 of a rimfire scope.

Mine is advertised as a rimfire scope

quote:

Originally posted by fritz:

quote:

Originally posted by Loswsmith:

quote:

Originally posted by sigcrazy7:
I do believe the 2-7 Prostaff is a centerfire scope, so note that it will be parallax adjusted to 100yds instead of the more common 50 of a rimfire scope.

Mine is advertised as a rimfire scope

It doesn't matter if a scope is designed/designated as rimfire or centerfire -- parallax is parallax. With parallax fixed at 100 yards you must be careful on eyebox position for target distances different than 100 yards. This means that it will be more challenging to produce exacting aim and precision impacts at close distances -- to a certain extent at 50 yards, but even more so at 25 yards. A consistent head position on your stock is really important.

Understand that parallax challenges increase as magnification increases. As one approaches 10x magnification, fixed parallax becomes a major pain for precise shooting. So....if your target groups aren't going very well at shorter distances, consider reducing magnification levels.

quote:

Originally posted by Loswsmith:
Thank you, this I did notice when getting the group for 25 yards but was able to get the sight dialed in for that distance without too much difficulty. Only after I did that did I clue in to the fact that at 50 yards that zero was going to off pretty significantly.