Each weapon is designed to be use in one or more specific regions.  Each weapon is labeled “R1,” “R2,” “R1-3,” etc.  “R1” means that the weapon can only be used in the first region.  “R2” means it can only be used in the second.  “R1-3” means it can actually be used in Regions 1, 2, and 3.

If you have the correct region gun, you must also satisfy the power requirement.  The story campaign missions have strict requirements.  If you don’t meet all the power requirements, you cannot enter.

The individual weapon missions have looser requirements.  If it’s only a little bit more powerful than you, you can accept the challenge.  Be warned, the difficulty is much higher if you do not meet the requirements!