kgb41,
Haven't used 4166 in the .300 Savage but have worked with it extensively in eight other cartridges from the .204 Ruger to the .45-70. Like many newer powders, it has several small advantages over older powders. Most handloaders (especially deer hunters) don't see these advantages because their experiences, and needs, are pretty limited.
Aside from being clean-burning, it's very temperature resistant and contains a decoppering agent. The temp-resistance is helpful for some hunters and not for others, but at typical .300 Savage ranges usually isn't a big deal--though contrary to what many hunters believe, the big disadvantage of more temp-sensitive powders isn't lost velocity in cold weather, but potential changes in point of impact at 100 yards. I've seen as much as three inches change from 70 to zero degrees Fahrenheit, which doesn't matter in deer hunting if you only shoot at relatively close ranges but could beyond 100 yards.
It's a "short-cut" extruded powder, resulting in more accurate metering through a mechanical measure, but the granules aren't noticeably different in size from IMR4895. However, 4166 is double-based, resulting in more energy than single-based powders like IMR4895 and Varget, so somewhat more velocity can be attained, though as you noted in the .300 that's pretty meaningless. So is the decoppering agent, since .300's usually aren't used for extended shooting sessions.
I also found 4166 very accurate in all the cartridges I tried it in, and in fact turned out to be the most accurate powder I've ever tried for certain bullet weights in a couple of cartridges. But again, how much does a little more accuracy matter in a modest-velocity big game round typically used at "normal" hunting ranges?
Such incremental advances have been occurring ever since the development of practical smokeless rifle powders in the 1880's. The earliest smokeless powders were so temperature-sensitive they often locked up actions in warm weather, and wouldn't always go bang in cold, and fouled barrels in a number of interesting ways. Early British Cordite was so sensitive ammo companies often offered two loads, one for cool weather and one for the tropics.
After that powders improved so rapidly that many shooters decided they were were perfect by the 1930's, when Dupont introduced most of their IMR powders--though many had faults, including large granules, and were still somewhat temperature sensitive, though many hunters don't shoot in conditions where it's obvious.
Eventually we'll probably end up with powders that provide exactly the same velocities at -20 and 100 degrees (so far none I've tested have), and do not leave any copper or powder residue in the bore, making barrel cleaning obsolete.
To most deer hunters the latest advances don't mean much, but they're there for shooters who can use them.
