Credit to:
Altimit01- Eschaton 0.7 Bitmap Demo.
Philly- His duplicating tutorial.
1. You must Duplicate the Plasma Pistol using THIS Method.
For Mac users you would use HHK to extract the weapon (PMI).
For the EOF trick you'll have to use Alt's Eschaton 0.7: Bitmap Demo.
When you inject the Skin you click the "Inject as new bitmap data" box and that should work.
2. Open up your map with your duplicated Plasma Pistol in Eschaton,HMT should work to but for this tutorial I will be using Eschaton.
3. Go to the dependancy swapper and choose your Duplicated Plasma Pistol.

4. Change all the Models, effects, animations, and projectiles to the ones you want.
5. Go back to your main Eschaton / HMT window and choose Weap > Duplicated plasma pistol > main > 32-bit bitmasks > Flags > and check Secondary fire overrides grenades.

6. Also check off Prevents Grenade throwing.
7. Go back to Weap > Duplicated plasma pistol.
8. Choose Projectile > projectile 1 > Floating point numbers.
9. Change the charging / charged times to 0 so you do not charge the weapon.
10. Change the rest of your options to whatever you choose. These include Rate of fire, Spread, acceleration / deceleration times etc.
11. Repeat steps 7 through 10 for projectile 2.
I'm pretty sure that's all I'll double check over it later, PM or ask here if you need help.