Supermaxx 750 Mod!

The SM750 in all its purple glory

Here you can see the universal coupler and dart stop (unnecessary)

This is the Supermaxx 750--one of the most powerful pistols ever to be produced by Nerf. The internals are dead simple, and the modding turned out to be very easy for this one. In fact, the hardest part of this mod was getting the blaster apart! You have to remove the orange muzzle on the front and the orange ring on the back by the pump handle. To do this, I dipped the muzzle in boiling hot water for a few seconds then used a mallet and slotted screwdriver to "chisel" the orange muzzle off. For the orange ring on the back, I first used an xacto knife to cut the dividing line between the purple and orange. Then I used the mallet and screwdriver technique to finish her off. Once the hard part was done, it was super easy to do the mods. Here is what I did:

1. Epoxied a universal coupler to the air outlet of the air tank. Turns out the air outlet has the perfect diameter to fit right into a PVC coupler! If you cut the CPVC coupler down to the perfect length, you'll leave a little room in the PVC coupler for slipping over the air tank's outlet. Glue it up and MAKE SURE IT'S STRAIGHT! I can't tell you how many people have ruined their mods with a crooked coupler or barrel (myself included).
2. Use a dremel to widen the hole in the orange muzzle and the two halves of the blaster. If you shave a few millimeters off the air tank outlet, you won't need to do this, since the universal coupler will be far back enough in the shell. I glued mine straight on so I had to dremel the shell. On the plus side, if you dremel a perfect hole, then the internals sit perfectly in the shell with no movement. I should mention that the internals are all one piece in this blaster. 
3. Hot glue the spring on the end of the firing rod to the rod itself. Whether this actually does anything has been debated on the forums, but I think it helps the air release faster even though you have to jerk the trigger a little.
4. Plug the pump. The end of the pump has a small hole in it. The purpose of this hole is to relieve excess air pressure so you don't blow up your air tank. By plugging this, you are now able to pump as many times as you want. Beware--I have already ruined one SM750 with excess pumping!

Congratulations, you now have a fearsome sidearm! Limit your pumps to 4 or 5 if you plugged the pump. As far as barrel length, I have had success with anything from 6-12'' depending on how many pumps you use. I ended up damaging my air tank and now it doesn't hold air as well. I can still pump it up and quickly fire for a laser blast. I've clocked it on the Chrony at 220 feet per second with a 12'' barrel and 8 pumps. Hot damn! Do not pump yours up 8 times--remember mine is already damaged so no harm, no foul. And finally, remember that a dart stop is not necessary on air blasters, since there is no plunger or plunger tube!