Not all mods are gun specific. Some general mods work for most guns, and help a little; usually not alot.
If your gun needs to be cocked, you can use the rubber band method. The way cocking guns work is by the plunger (part you "cock") pushing air out behind the dart. This is all nerf does to make guns? Yes. Add those little springs, and you have a gun. Back to modifing... put one end of the rubber band around the front of the gun (or other sturdy place) and the middle through the "Cocker"loop your fingers in. the other end goes aorund the front of the gun again. Easy, even Stephen Hawkins could do it NO hands! This is suppoused to help the cocker compress faster, but I dont think this works, and people that say it improves their distance by 30 feet are probably overexagerating. The most extra distance you will get is 5-10 feet tops. Ok, 15 if you're lucky.
This is s t r e t c h i n g the main spring bigger to cause it to be tenser when compressed. Also, stretching a spring will reduce it's lifespan. This is actually a scientific principal; Isn't Nerfing educational? Be carefu l taking those guns apart, as clumsy schmucks can break them easily. This is a very effective mod and will increase distance by at least 10-15 feet on most guns, but probably not in the ones that suck in the first place.
Silly Nerf, you keep putting holes in your barrels! No problem, we can fix that by applying duct tape over the holes they leave to make the gun shoot, um, less far.
On air pump guns, you may notice holes in the pumper (I like to use as scientific sounding words as possible). Don't worry, you can cover that in nice hot glue. Man, they just keep making holes for us to fill with glue and duct tape.
Unnnnnn....tor want sandwhich.