07-12-2018, 01:59 PM
grayfox Wrote:I do a couple of things when looking for good loads, MV with SD<10 and ES in the teens, and a linear portion of load grs vs MV. Ideally powder, for typical ranges in a typical case, should behave pretty much in a linear fashion unless you get into real low or really compressed loads (but I avoid those types of loads), plus the SD/ES requirement usually shows you are in an OBT node, meaning the barrel muzzle tip is "far away" from the longitudinal pressure wave in the barrel steel... all of which are good things.
I'd estimate your 22" barrel has a node in the range 2480-2520 ("node #7" in the xls below). There are definitely other nodes as well but this one works for the 2 combos you show. Note the 2477 for CFE and 2509 for ARC (I'd also say you probably would find low SD/ES at 27.4 of AR comp, around 2490 ft/s, I interpolated that value on the graph). So for either or both of those loads play with coal in 0.010's of difference to see which coal will group the best. 2.250, 2.260, 2.270... or maybe 2.255, 2.265, 2.275 etc, something like that. My bet is that one of those will group tightly. What does the Coal variation do? It doesn't appreciably change the MV but it does change the timing of the bullet's arrival at the muzzle tip... and since these scenarios deal in microseconds of time, a 0.010 delta will allow for slight bullet tip length deltas but show timing changes by tighter or looser groupings.
[ATTACH=CONFIG]12222[/ATTACH]
[ATTACH=CONFIG]12225[/ATTACH]
Thanks for this post sir.
Being new to loading in general this puts the effects of changing seating depths in a format my poor old Cajun brain can understand for effect on groups.