I couldn't think of a better forum than here, so I hope it isn't inappropriate to post here. I have been putting the finishing touches on a Jap Arisaka in order to make it a hunting rifle. I mounted a one piece Weaver base on it, installed medium rings, and slid an old 4x Weaver into place. The rifle shoots well enough at 100 yds, so I sighted it in for 3" high and then tried 250 yards. The puzzle is- my groups were 6-8" low at 250 yards. In my experience, the load I am shooting should land them close(r) to zero at that range. The line of sight above the bore is about 1.6". The load is:
6.5x57 Mauser
R-P Case (resized 7x57)
Rem 9 1/2M primer
48.0 H4350
129 gr. Hornady Spire Point
2953 fps- 5000 feet above sea level

Any ideas?

did you try an in between range such as 175 or 200 yards? Also, did you just go up 12 clicks or whatever? It may not be moving them up 3" if you just click it, if it moved the group 1.5" that might explain some of it. If it groups well just low I think you could be happy with it.

Mark,

Thanks for your reply. It did occur to me that the scope was made with 1/2 minute clicks. In the interest of brevity, I didn't mention that I verified the sight- in of 3" high at 100 yds before shooting the 250 yd group. I can be happy with a lesser trajectory, but there is a rule of thumb (Wayne Van Zwoll, I think, is the author) that any cartridge flinging spitzer bullets downrange at @ 3000fps MV will behave very close to my desired result. I have seen it with .270's/ .280's/ and 30-06's enough to know its true. This 6.5 is shooting nearly as fast as a .270 and faster than my .280 Rem. Hence my puzzlement...

Is that 2953 mv chrono or a load manual guess?????
Your actual mv might be much lower than you think.
Lyle

As a general rule, rifle trajectories APPEAR FLATTER when the scope height is increased. Your results are just the opposite of what one would reasonably expect! It is possible that the MV of your load is less than you think, but since every rifle is different, one cannot rely on trajectory tables to tell you what each one will actually do at extended ranges. You just have to shoot them to really know for sure. This is why sighting in at, say 25 yards, and then thinking that the bullet will be somewhere specific at 200 yards or more without testing it to see is really a big joke!

Based on your load, your scope height and the Sierra-3 ballistics program, you should have had to zero at 2.6" high at 100 yards to be dead on at about 250, as you thought.

A parameter is likely incorrect and/or the velocity is suspect. Maybe time for a chronograph session.

To zero at approximately +3" at 100 yards, and then be 6" low at 250, your velocity would have to be less than 2,400 FPS, a pretty substantial drop from what you expected.

Are you sure your scope-over-bore height is correctly measured?

Thanks for everyone's responses. Yes, it was chronographed, that's why I threw in the 5000 foot elevation number. PACT Model 1 with 24 inch skyscreen spacing and 10 feet from the muzzle. It COULD be a chrono error, but doubtful, since I see the "correct" MV when shooting other rifles. The 1.6" figure from bore C/L to Line of Sight is a ruler measurement, so its rough. Being a disciple of Wayne van Zwoll is what prompts me to do such testing of loads at extended yardages. I guess more testing is in order...this rifle has caused me mucho headaches!

Thankd for everyone's responses. Yes, it was chronographed, thats why I threw in the 5000 foot elevation number. PACT Pro 1 with 24 inch skyscreen spacing and 10 feet from the muzzle. It COULD be a chrono error, but doubtful, since I see the "correct" MV when shooting other rifles. The 1.6" figure from bore C/L to Line of Sight is a ruler measurement, so its rough. Being a disciple of Wayne van Zwoll is what prompts me to do such testing of loads at extended yardages.

You have verified scope/bore axis are parallel? Just a thought.

8mm

Good thought! How would I do that? I know that sounds stupid, but I bought the correct base called out in the Weaver charts. There are no shims under the bases, and the scope laid nicely in the ring saddles while mounting it. Any other ideas on how to verify this?

Bore sight compared to scope sight over varied ranges, using LED type light sources, two, spaced in relation to each other at the muzzle, maybe. I honestly never had to chase this one down, maybe a good gunsmith.

Good luck!

Paralax error in the scope could account for this. At 250 yards move your eye around behind the scope and watch the crosshair move all over the target. It is only a 4 power and bullet placement at 250 can be a little tricky to nail down with so little magnification.

Fairly small chance but the receiver threads could be cut canted in the front bridge making the barrel point down relative to the axis of the action bore?