This is the beauty of spitballing ideas on a forum with knowledgeable members who can point out things you may not have considered. I had not considered whether the motor can handle continuous duty by running 8 hours at a time. I would think the reduction gearset might be the weak link there.
One problem I did pickup on with the motor I was looking at is it's speed. I was thinking that since it's listed as 435 rpm no load speed it would be perfect hooked up direct to 12 volts DC since I calculated my present driveshaft speed at 405 rpm and by all accounts that's slower than most guys are running, so a slight increase might even be a good thing. But then I saw that the loaded speed was listed as 389 rpm +/- 10%, so it's probably too slow. There's a similar motor listed with a no load speed of 612 rpm with a loaded speed of 540 rpm +/- 10% that might be a better choice, but if I go that way I think I'd want a speed control in case it spins faster than I want. Another problem with the 612 rpm motor is that it used brass and nylon gears and carries a warning that stalling the motor may result in gear damage, while the 435 rpm motor uses steel gears and doesn't have that warning so I assume it's more durable.
I'm thinking that if I go with the 435 rpm motor I won't need a speed controller as it won't be too fast and if it's too slow I can use the trick Doublebuck mentioned of slipping tubing over the driveshaft to increase drum speed.
I'm not real worried about a spark hazard. As you said most studies have shown it's really hard to ignite black powder from an electric spark and also with the powder being inside that rubber drum any spark near the closed end couldn't reach the powder in any way I can think of.
I'm inclined to give this a shot as I think it will be a fun project even if it doesn't work out the first time. I could probably figure out what needed changed and tweak it till it does work. In the end it might not be real cost effective but I like to tinker with things like this. I previously built an annealing machine to prep centerfire rifle cases for reloading and that was a fun project. It used two smaller DC motors with speed controllers to regulate the rotational speed of it's two drums and it works great. Here's a short video of it's operation:
https://youtu.be/4_cmGs1TUVw