I use 777 in my Trapdoor and it works well for me. My kids love the smoke and I like the easy clean-up of the brass and rifle.
From all I've read though, 777 and compression don't mix, so watch out for that. I once tried different rates (by feel) of compressing loads in my .45 Cherokee and the groups opened up. Have not tried this in my Trapdoor yet, and just load with the .457" ball (for plinking) or .458" bullets lightly touching the powder.
"Freedom and the Second Amendment... One cannot exist without the other." © 2000 DTH