It's not uncommon for the lengths to vary about .015", but the ogive lengths run very consistent. I've never had any stabilization problems with a 1-8 twist in any temperature or altitude. I've run them anywhere between 3000 and 3600+ fps.
The batch I have is probably 10-12 years old, and the lengths run 1.070-1.080". I had the same trouble with 80 gr A-Max's, unexplained flyers, but the 75 A-Max seemed to be alright. I've also shot 80 Bergers, which shot just as good as the SMK, but since it was a wash, the SMK's were cheaper.
80 gr JLK's had a small edge on the other 2, but it took about .3 grs more powder to get the same speed. I attributed that to the difference in bearing surface.
I wouldn't put too much faith in those twist calculators, as every barrel is different and there are too many other factors to figure into a real world scenario. The best way is to shoot them and find out first hand.
I remember when Sierra came out with the 90 gr SMK's, they recommended a 1-6.5 twist. I got about 35 bullets from Carrol Pilant, plant manager, from the initial pilot run. Of coarse I shot them from a 8" twist just to see if they would stabilize. They stabilized from my starting load @ 3000 fps, and shot good all the way up to 3550 fps. Once I figured out the powder that gave the best velocity, the first group on paper was less than 1" @ 300 yds. I was impressed, but also out of bullets at this point. So I got a 500 ct box and started over. That's when I noticed that something had changed. The accuracy was not that good, and in hot weather, some bullets never made the target.
Upon more investigation, I found out that the pilot run was made from different jackets than the first production run. Some were complaining about losing bullets @ 2550 fps with a 1-6.5 twist. After I spoke with Rich Macholts, we determined one of the common denominators was a 3 groove barrel coupled with that 6.5 twist. He said they never actually tried that combo.
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