I shan't address everything here, but I have a few small tidbits:

Ball bearings in a paintball gun will go nowhere. Paintballs are very light compared to any metal projectile. If you had a lighter hard projectile like a hopper full of plastic marbles or maybe some frozen (stock) paintballs you could use the thing as a weapon, but at that rate any jack-booted thug with a real gun would convert you to a wall splat in very short order once you pissed him off with it.

Even then, if they outlaw guns they'd probably outlaw paintball guns and airguns first. Keeps the kiddies safe and all, don't you know.

For ranged combat I think bows and arrows would be the most feasible. With modern tools and materials any fool can make an accurate and very deadly bow or crossbow, and if faced with a reasonable number of gunners could stand a chance of emerging victorious. You'd have to play your cards differently with a bow, but there's no shells to improvise, propellants to find, or compressors to lug around.

As I said before, any buffoon can whip up any manner of nasty hand-to-hand weaponry. As long as you know how to do it you could take someone out ith a folding chair. if you were so inclined, anyway...

~Zero

I would assume the most important thing would be proficiency with the weapon/body. It may not matter what your are using gun/bow/explosive if you use up all your ammo or blow yourself up in the process. Your body/mind is probably the most deadly thing you have, but when its 1 Vs 20 your not going to have much of a chance.
Your best bet would be some solid training and maybe some durable pipe bomb/grenade like weapon you can carry to even the odds. Although I wouldn't know how to go about making such things, I'm sure someone around here does.

when guns become illegal then only criminals will have guns... or so the saying goes..

if you want to get a gun when they're illegal go see a criminal... they'll have tons of them...

in Great Britan handguns are illegal... but criminals still have hand guns... smuggled in of course... just like drugs...

You'll never have to improvise a weapon unless you're really pressed for time... in which case sucks for you, you should have thought ahead.. not to say that creative improvisation is useless..