Seems like we're all posting at once, so this duplicates some of the above.
For a 50 grain bullet in Nosler brass with a Remington 7 1/2 primer, Nosler #6 shows IMR 4198 at 20 to 22 grains. They don't give the COL they used. Varget is really a bit slow for this bullet weight, IMHO. Nonetheless, the manual has it at 24 to 26 grains, with 26 grains being 2% compressed in their brass. The peak loads for both powders are given as the most accurate in a 24" tube. If shooting in an AR type action, keep in mind that Stoner designed that action to use 4198 originally, and you will get design gas port pressures with that powder.
I've always found IMR-4198 and H4198 to be the best accuracy producers with 50 grain bullets in both the .222 and .223. It also costs about 15% less to reach the same velocities with it than it does with Varget. Reloader 10X is also capable of good groups, but I haven't finished wringing it out to say how it compares on peak accuracy loads? Varget works well with the 69 and 77 grain Sierra MatchKings, but your gun needs a fast enough twist to use those.
The only caution that goes with it is the possibility of creating so-called "secondary pressure spikes" with slow powders and light bullets. I've concluded these are not true pressure spikes, but happen when a mass of accelerating gas slams into the base of a bullet that has slowed after initial acceleration. That happens because a slow burning powder hasn't been able to make gas fast enough to keep up with the expansion of volume behind the fast moving light bullet. Thus, pressure drops and the bullet is then slowed by friction. That collision, when the gas catches up, upsets the bullet diameter. The radial impact hammering effect swells the steel. That swell spreads out the length of the steel, and back near the breech it produces strain same as would be produced if actual pressure were stretching it radially.
The risk isn't in pressure rise, therefore, but occasionally that the bullet upset can actually bulge or even crack the barrel at the bullet location. Texas gunsmith Charlie Sisk has been able to break the muzzles off .338 barrels this way on demand. It doesn't seem to happen in barrels shorter than 18" or 20", as the bullets don't usually have time to slow down enough and/or the gas to catch up enough in those and shorter tubes. But you can see strain gauge pressure curves showing the apparent pressure spike by scrolling down to the last trace and second from last trace, here, even though its really just a reflection of that momentary stretch up nearer the muzzle, and overlayed on the actual reading of pressure-induced strain as it travels rearward in th barrel.
The fired cases from these readings don't show pressure signs, which is one way you know it isn't caused by true pressure back at the chamber. The stress on the barrel at the actual event, though, is great enough to discourage me from running slow powder with light bullets.
FWIW, Nick, I have recorded secondary pressure spikes that go off scale (above 105Kpsi) yet velocity is average and no effects seen on the brass. If such were real, the barrel would be spaghetti-like shrapnel. Nada.
Your explanation is as good as any, because on most shots where I've seen such spikes, the bullet also disintegrates. It is dust and jacket shards even before it makes it to my first chrono screen, judging from the lead and copper specks/dents I see on the plastic parts. No velocity reading on those, obviously. The ones that make it to a range of 12' before coming apart do produce a reading, and they aren't far above normal.
Very odd things happen in some loadings, for sure.
I've heard about this stuff, but thought it was relatively uncommon?
Anyway, I was able to try out the Varget loads today. The loads ranging from 24-25 grains all grouped under 3/4". As I got up higher, they started to spread. By the time I got to 25.8 grains, I had a 3" 3 shot group. I didn't get to try 26 grains, but at 25.8 grains there were no pressure problems yet.
If I have time tomorrow I'm going to try a 26 grain load, and then go into the 4198. I could always go with 25 grains of Varget, but of course it would be nice to get a bit more velocity as well, and that Varget was insanely expensive. If the 4198 turns out better, I'll be ditching the Varget I think.
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