Finally got this thing to hold gas long enough to pop off a few rounds.
I got a filler adapter from Dennis Quackenbush a while back, but the tank didn't hold worth a darn. It still has the original valve seat (horn? gutta percha?). At first I just used some 1200 wet/dry on a glass plate on the valve face to clean it up and get a nice flat sealing edge; it helped, but it still leaked quickly enough that by the time I filled the tank, got the fill adapters off (paintball tank to crosman bulk fill nipple to Quackenbush adapter), and got the tank screwed onto the pistol, it didn't have enough left to get any respectable shots off. So then I tried a flat mill file on the face of the seat, to clean it up and get a relatively clean sealing surface; that helped even more, but it still leaked enough that I could get off maybe 1 or 2 full power shots off before it pooped out. I set it aside for a while.
So today I tried wetting the face of the seal with some 30W non-detergent, wiped it off to just a film, and then waved a pencil torch over it very rapidly to heat up the seal enough to 'absorb' some oil and swell a little bit. Now when I filled the bottle and got the adapters off, I could only detect a very faint hiss if I held the bottle to my ear. Good enough to play with at least.
So I go outside and start popping off shots at a steel plate about 8' away. It's a relatively cool day here today, only 92F on my patio.
Here's the first few I recovered, next to an unfired round (.315 round ball).
The largest splat (next to the unfired ball) mic'ed out at about .670, so over twice the original size. I need to dig out Gary Barnes' old article on "Splatology" (from an early Airgun Review) and see if I can determine an approximate speed.
I got maybe 15 rounds off before it started noticeably diminishing. The last round fired was barely deformed. You can see on that last round that the ball is just engaging the rifling, so .315 seems a good equivalent for 8mm.
Here's a few from the tail end of the string (from the backside).
At some point during the middle of the string, I took a couple of shots at a flat plate about 25 yds out. The first shot missed completely, but a little Kentucky windage and I managed to get a nice solid ring with the second shot!
In the end, I managed to shoot for about 5 or 10 minutes. The tank needs to be (correctly) rebuilt, obviously, but for me it was good enough to enjoy immensely. Maybe 125 or so years after it was made, it's still shooting strong and true!