It is always best practice to stay within the limits given by loading manuals. They do have pressure equipment and there has been a trend in today's manuals to recommend powder charges that are not highly sensitive to changes in primers and cases, and recommend charges that don't have wide pressure excursions.
As for your loads, I tested 69 Sierra matchkings with 23.0 grains at the low end and ended up at 24.5 at the high end. All of the rounds functioned.
I really like IMR 4895, it is a medium burning powder, and it shoots well in the 308 at lesser charges than what are listed in reloading manuals. For whatever reason IMR 4895 does very well even in reduced charges, but I am not taking about cast bullet reduced charge levels, but reduced from book loads.
In very cold weather you might have a cycling issue in the AR15, but probably not.
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If I'm not shooting, I'm reloading.
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