I was out shooting my 300 Winchester Magnum again this morning. I was shooting the 180 Swift Scirocco again with 75 grains of RL22. First I tried the 3.38" seating depth again. I have fired 3 groups with this exact load now. The first group was at 100 yards and I got 0.43". There was about 0.2" of vertical in the group and 0.4" of horizontal. The second group was at 250 yards and measured 5.5" with 4.25" of horizontal and 3" of vertical. The third group was at 120 yards and was 1.3" all of which was horizontal spread. In looking at all the groups I am firing I am mostly seeing a lot more horizontal dispersion than vertical. That is not always the case but it does seem to be a trend. So I am wondering what the cause of that could be.
Another example of that from this mornings shooting (all done at 120 yards) was when I tried the Scirocco with 75 grains of RL22 at a seating depth of 3.475" and shot a 0.82" group at 120 yards. The group had 0.7" of horizontal spread and 0.48" of vertical spread. Overall a pretty good group (0.68 MOA). Its group center was 1" high and 1" left of point of aim. Then I tried 75.5 grains of RL22 and 3.475" seating depth. The first two shots were 2" high and 1" left and were 0.46" apart center to center. No great surprise that the first two shots were higher than with the 75.0 grain charge, since velocity was higher. The third shot of that group though was 1.5" high and 0.35" right of point of aim. Then I tried the 75 grain load again at 3.475" which had given me 0.82" group centered 1" high and 1" left. This time I got a group of 0.35" (again at 120 yards). It had equal vertical and horizontal (a beautiful little group). The most interesting thing was that its group center was 0.5" high and 0.25" right. So its group center was basically 1" below the third shot of the 75.5 grain loads group. It seems as if internally the scope moved about 1.25" to the right after the 2 shots fired with 75.5 grains. I made no adjustments to the scope between firing those two groups. There was no wind this morning.
I have two guesses as to the problem of the horizontal. First guess is something screwy with the scope. It is a Swarovski AV 3-9. It is supposed to be parallax free at 110 yards. However, I would think that if the scope is faulty (other than parallax), I would be very hard pressed to get any good groups. If the problem is parallax I am not sure how to deal with that. I am trying to place my eye (and cheek) in the same place every time but maybe I am goofing sometimes. My other guess is that my bench is moving side to side as I shoot. It is an Armor Portable Shooting Bench. It is wood with 3 metal legs. When I lean into and away from it the reticle clearly moves a lot. Naturally I am stable as the gun fires (reticle in centered on aim point as trigger breaks), but perhaps with recoil (there is not much recoil as it has a muzzle brake) I am moving some. I really do not know what is going on and it is very frustrating. What was really weird though is the group center changing so much. That is not something I have noticed before and that occurrence makes me think something is wrong internally with the scope. There was not any wind this morning. Any ideas? Do you know of a different portable shooting bench that would be more stable than what I am using? I do plan to mount my Leupold 8.5-25 on the rifle and that way I can adjust the parallax. Rufous.
Well, I took off the scope and discovered that the front ring (bottom half) was very easy to turn in the dovetail base. Time for a new set of rings and bases. I think I will go with the new Talley aluminum one piece ring/base setup. Hopefully this solves the problem. Does it seem likely that it would cause horizontal dispersion in my groups? The rear ring was tight. The bases were tight and the top halves were tight. Rufous.
Some thing closer to a Weaver, Kelbly, Picatinny rail design will be a lot more reliable than what you have now. I take those spin lock jobs off all the time and replace them with stuff that works. Even when you locktite them they work themselves loose. Go to a better design, end of problem. [img]images/icons/smile.gif[/img]
Probably not your problem but in response to dealing with parallax on a non adjustable scope. The only way that I have found to be consistent is to pull back until the outer ring of the sight picture starts to blacken, and maintain an even darkness around the outside of the sight picture for each shot. If your eye is off center you will see it. Could be your cheek weld is consistent but you lean left/right?
It works with a Tasco, should probably work real well with a Swarovski. If that didn't help I'd bed it, recoil lug bedded tight laterally,if it isn't already. If the problem still existed I'd get another scope.
Hoyteman brings up a good point, this is what I do to be repeatable without an adjustable parallax scope. With the rifle not moving, if there's not any movement in the hairs when moving your eye side to side you don't have a problem. Whatever range it begins to move is where you have to start watching it more closely and it'll just get worse the farther out you go.
I think your new base and rings will solve your problem here.