What is the strongest back up gun?

Answer 1

Generally, you should never need a backup gun in game. However if your primary breaks, its best to have either a quick replacement, usually a rental type like a Tippmann 98 (or your first gun) for the next game.

If you're referring to sidearms, the best pistols are the Tippmann TPX (my preference and the T8. Its really a decision between having co2 in the gun separate from the magazine (tpx) and having the co2 in the magazine (T9/8). This means that you get full use of the co2 in the TPX, but you also may have to load two separate things. Also the TPX magazines are cheaper due to them having no valves or pipes.