I'm not quite sure what you were intending with the tinker arm, but I just went and grabbed a rifle and tried to figure out how I'd use it if I only had one arm. The answer is that I wouldn't, because it felt like a horrible horrible idea, but here's some advice anyway.
Sitting down and leaning back slightly, while shouldering the rifle and resting the front on top of a bent knee, worked kinda well. I was using a scoped SVD, which is both too long/heavy to feel steady like this, and would've definitely given me a black eye if I actually fired it, but a lighter rifle without a scope (or just a reasonably long stock...) might work okay. It was at least possible to line up the irons, though it still wasn't steady. If you're gonna tinker up a way to make this better, I'd recommend a sort of knee-guard which extends high above the knee, and has a u shaped notch in the top to keep the gun secure. It might actually work okay that way, though you still wouldn't be able to shoot upwards.
Secondly, get a loose sling, if the rifle doesn't come with one. You need one anyway to carry the weapon, but it would also allow you to comfortably hipfire it. Hang the sling from your good shoulder, and fire/manipulate the rifle with the arm. I've never tried this exactly, but holding the SVD like this felt pretty similar to just hipfiring the rifle normally, which isn't at all accurate but could still hit a person at longer ranges than a melee weapon, at least with semiauto fire.
...Finally, if the rifle's very light, just shouldering it and firing it normally, the front unsupported, might work. The only firearms that've been light enough for me to comfortably do that with were submachine guns / carbines, but I'm fairly small, and the ethral rifle is made for str:7 moths, while Kara has str:9, so it might work. I must stress that this is a bad idea, but it's a game, so it's probably fine.
All this probably pales in comparison to the best advice, though: trade the rifle for someone else's revolver.