Reinforcing during printing, not so much. The markforged printers can and do this by turning the printed part into a semi typical fibre composite. But no machine you're going to have on your desk for hobby use will be able to do that.

Reinforcing after printing, sure. Wrap it in fibreglass, carbon fibre, whatever you want really. You could print hollow and fill it with resin, etc. Lots of options here.

Abs is the most uv sensitive of the common 3d printing plastics, I can't think of any others that are noticeably uv reactive. That being said I doubt this is really much of an issue with airsoft items, most of the plastic parts on those are already probably abs. Nylon is hygroscopic so it will absorb moisture which affects its properties (flexibility/strength). Its relatively rubbery compared to other filaments so I personally don't find it too useful.

Finally, abs is actually one of the weakest 3d printing plastics. Pla is one of the strongest. Which one is more suitable for a given mechanical application is another question, but pla is stronger despite what so many people will tell you.

The best plastic for your applicstion (and therefore how the 3d printer part performs) really depends on what you want to print. Any examples of what you're looking to 3d print?