So..long story short, I've been reloading for 10+ yrs and had a major wakeup call this evening. I'm only posting my findings to help others from making the same mistake I did. This is not intended to bash any manufacture etc.
I bought and reloaded a box of 140 amax in my 260. Everything went well. Ran out of amax so I switched to 140 smk for the time until I found more amax. Obtained and Reloaded some of the new amax and went to range tonight. These rounds were loaded to the same specs as the previous load with amax.
Was shooting 1087 yards. Had both loads with (140 amax and 140 smk) 140 smk was the standard expected accuracy. The amax were shooting everywhere, several feet off in every direction etc.
Went to reloading room and gave the rifle a thorough cleaning. I then decided to remeasure the overall length to the lands with the new amax to compare them to the previous loads that were .020 off the lands, measured with Hornady oal tool to ogive. My findings were rather disturbing.
The previous lot of amax measured 3.311 to ogive when contacting lands with 25 cal insert in oal tool. Rounds were loaded at 3.291. The new amax however had an oal of 3.275-3.291 when touching the lands. (About 20 bullets measured). Meaning I was unknowingly jamming my rounds up to .016 into the lands when I thought I was jumping the same .020 as before (the seating die was adjusted so the oal of the rounds from new lot was 3.291).
I confirmed the smk measurements as well. Both previous and current measurements were 3.350. This was for 5 bullets all exactly 3.350.
Has anyone seen this big of a difference in bullet lot numbers? FYI each amax was measured several times to confirm each measurement was correct.
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