The juice probably isn't worth the squeeze. I'm really not sure how you'd make that hot air balloon idea work for one thing. And honestly, getting ahold of guns in post-Soviet countries isn't exactly hard. There's also the problem that a lot of people would just sell guns and ammo for money to buy essentials like food. You're basically just taking a whole bunch of guns and putting them on the black market.

This idea has been tried previously, with concepts like the Liberator in WW2 and the general conclusion was that its just not worth the effort.

You'd be better off funnelling these weapons directly to resistance groups, but if you say you want to do that it kind of tips your hand.