I have only participated in the forum rarely, but the .577 has turned out to be my favorite rifle and I had a couple of comments and questions.
I have a Bland .577 hammer gun manufactured 1905 and the proofs are for a 650 grain bullet with 75 grains cordite. In the gun case was a note from the previous owner that stated the best load was 75 grains IMR 4198. I loaded working up to and beyond that level and the gun regulates quite well with 77 grains 4198. My velocities are around 1850 which seems to be ok as far as I can tell reading Graeme Wrights's book. I use 650 grain Woodleigh sp's (black powder), federal 215 or other magnum large rifle primers- I cannot discern much difference- and I use backer rod to fill the air space with just a touch of compression.
I have used dacron as well, but it takes so much that it looks like the cottonwood trees are shedding. I have never had a misfire or hangfire with this loading.
I recently purchased some Varget as Graeme's book suggests that you might get good velocity with lower pressures, but I haven't yet tried it. I also read somewhere that I could also use 4831, but I'm a little leary of that. Since 4198 performs well, perhaps I should stick with that.
My question is: am I pushing the loads too much? I don't want to damage the rifle and I note that others use lighter loads than I have been using. Even though the rifle says the load is cordite, I assume that it is designed only for nitro for black loadings.
Incidentally, the rifle weighs just over 12 pounds.
You've got to love that 577.
Ron