I've been trying to figure out/solve a problem I'm having where a particular type of bullet is changing impact horizontally from 200yds to 300yds by a significant amount. I'm shooting an A-Bolt II 6.5CR with a 22" barrel, 1-8" twist. I'm reloading 143gr ELD-X's and have tried a few powders (IMR4350, RL26, RL17, Superformance) but get the same basic results. My point of impact moves 6-8" horizontally when I move between 200yds & 300yds. Groups remain generally tight. Different powders give different results obviously, but I've been able to find a node that exhibited sub 1" at 200yds and about 1" at 300yds. I haven't tried altering seating depth yet but I'm very far off the lands as the magazine length limit won't allow me to get anywhere close ( I am at the limit there, so seating them deeper would be the only option). I've worked through ladder tests for each of the powders and checked the 200 & 300 ranges at the same time so there is no particular node that doesn't exhibit this tendency (i.e. I've shot 3 shot groups at 300yds for the ladder test then fire a 4th round at the 200 range - all of the 200yd shots move to the left the 6-8"). I've chrono'd all but the RL26 (I need a new chrono :-( ) and was achieving between 2700-2800fps for the other powders. I have a load worked up for 123gr SST's that shoots clover leafs at 200yds and ~0.5" groups at 300yds (when I do my part) without this phenomenon so I know the gun is capable. I've worked on some loads for the 130gr AB and ABLR without showing this phenomenon although I wasn't achieving groups as good as the SST's. While I'm probably ready to just give up on the ELD-X's for this gun, I still want to figure out the underlying physics of problem.
My initial thoughts on the problem are:
1). While 1-8" twist rate should be better for heavier bullets but combined with a drop in velocity (heaviest bullet I've tested) is this causing the bullet to not keep it's spin rate up causing it to "roll" to the right? That was one of the reasons I tried the RL26 as lots of people seem to be getting up into the 2950-3000fps range (with longer barrels). This speed would have been closer to the other loads that don't show this issue. Without the chrono I don't know how fast these loads were traveling however I went from 46.0gr - 49.0gr and there wasn't a significant difference in the horizontal stringing. If speed were the case, I would think it might get better/worse over a range and I don't think ~2" difference is enough. I've seen the same variation on other guns/ladder tests.
2). Seating depth - since I'm pretty far off the lands and the bullet may not be entering perfectly aligned I could understand a lot of variation. Could the particular curve of the ELD-X contribute to that? However, since I am getting good groups out of them at both the 200 & 300yds this seems unlikely. I would expect the groups to open up if there was inconsistency in how the bullet aligned itself.
3). maybe it's just the barrel and the heavier bullet. It is a sporter contour (not sure of the specific contour) but thin and light. The whole setup with scope (Leupold 4.5x14 VX3 CDS) is about 6.5lbs. I've read (probably on this site) that part of the movement of the barrel is not only vibration but an unwinding and stretching as well. I would assume that this unwinding/stretching would be effected much more in a light/thin barrel with a heavier bullet. Since I've varied the powder and done this over a ladder test, I would think I would see greater variation in the effect. I'll also try some other 140ish gr bullets going forward to see if they exhibit the same effect.
OK, anyone else have a thought on what is happening here?
My initial thoughts on the problem are:
1). While 1-8" twist rate should be better for heavier bullets but combined with a drop in velocity (heaviest bullet I've tested) is this causing the bullet to not keep it's spin rate up causing it to "roll" to the right? That was one of the reasons I tried the RL26 as lots of people seem to be getting up into the 2950-3000fps range (with longer barrels). This speed would have been closer to the other loads that don't show this issue. Without the chrono I don't know how fast these loads were traveling however I went from 46.0gr - 49.0gr and there wasn't a significant difference in the horizontal stringing. If speed were the case, I would think it might get better/worse over a range and I don't think ~2" difference is enough. I've seen the same variation on other guns/ladder tests.
2). Seating depth - since I'm pretty far off the lands and the bullet may not be entering perfectly aligned I could understand a lot of variation. Could the particular curve of the ELD-X contribute to that? However, since I am getting good groups out of them at both the 200 & 300yds this seems unlikely. I would expect the groups to open up if there was inconsistency in how the bullet aligned itself.
3). maybe it's just the barrel and the heavier bullet. It is a sporter contour (not sure of the specific contour) but thin and light. The whole setup with scope (Leupold 4.5x14 VX3 CDS) is about 6.5lbs. I've read (probably on this site) that part of the movement of the barrel is not only vibration but an unwinding and stretching as well. I would assume that this unwinding/stretching would be effected much more in a light/thin barrel with a heavier bullet. Since I've varied the powder and done this over a ladder test, I would think I would see greater variation in the effect. I'll also try some other 140ish gr bullets going forward to see if they exhibit the same effect.
OK, anyone else have a thought on what is happening here?
