Getting 80 or 82 grains of 1.5 or 2f Swiss in a .45-70 case is not a problem and it shoots well when using a PP bullet seated shallow on a .06 wad.
But when using a GG bullet say like a postell to chamber the round it needs a lot of compression and this might cause a problem bulging the case wall.
The pictures below settled my curiosity what different levels of compression does to the powder. Starting on the left top to bottom I started with .050" of compression with the top and from there on I went with .100".
.050", .100, .200, .300, .400 and .500"
.500" in a .45-70 case will make a solid mass of powder top to bottom.
Using the Goex powder compression was needed to get a clean burn and I think the heavy compression that solid plug of unburned powder scraped the bore. Shooting over snow you will find unburned powder and also there is a point shooting over the chronograph you will see the velocity decrease when the compression increases but I have never seen a point where the velocity leveled off, it just slowed after a point of compression.
Loading any caliber black powder cartridge GG or PP accuracy comes usually with a minimum compression with a good grade of powder. 1150 FPS will reach 1000 yards in fine shape using a .45-70.
If you feel you need to get 80 grains in a .45-70 case to reach 1000 yards get a .45-90 :D
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