Benchmark Powder

25g Benchmark with 55g bullet is a pretty standard and stable load for many rifles. Your small change in OAL should not have made that kind of difference.

Something more significant between the two loads? How many firings on first trial brass? Did you full length resize both sets with the same setting on your die?
 
Have you checked your case (trim to) length? They may have "grown" and be choking down on the bullets. This will raise pressures substantially on the cases that are over max length, but not on those not quite as long, possibly causing a vertical spread. If so - you're getting onto dangerous turf! A trimming and chamfering may be all that's needed.
Mark
 
How about temperature differences? When I first sighted these bullets in, it was during our Arkansas summer. Temps were 90 or above. The day I was at the range and noticed the huge difference in shot grouping the temp was, as I recall, 52. Could that 40+ degree change make that big of a difference?
 
Not in my opinion, but you know abouts opinions. Benchmark has been very stable for me and I don't see much change on the chrono or groups, hot or cold.
 
How about temperature differences? When I first sighted these bullets in, it was during our Arkansas summer. Temps were 90 or above. The day I was at the range and noticed the huge difference in shot grouping the temp was, as I recall, 52. Could that 40+ degree change make that big of a difference?

that temp difference could make a difference even though BM is less temp sensitive than other powders

you mentioned vertical stringing.... sometimes this is solved by increasing the powder charge slightly.... but be careful and go in small increments if you do.... like .1 or .2 increments
 
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