Hello, I popped down there (SMC) a few weeks ago as it happens.
Regarding pellet weight it seems with these 1377s the muzzle energy can be higher with a lightweight pellet. The gun I've just tuned is a bit naughty with H&N lead free pellets (5.5gn). 5.7 ftlbs might be a little high on reflection - I'm in the process of ordering some brass transfer ports from GMAC, these have small holes in but because they're brass they're easy to drill out to tune the power.
The alternative would be for me to pull my finger out and turn some up, but I use the lathe as a workbench and there's a TS185 engine in the middle of a rebuild.
PM me an we can meet up to show off our toys.