I'm assuming yours also uses a coaxial airchamber to push the piston and blade back (then venting through the piston valve piston)... mmm too many pistons

If that is the case could you not put your blowforward mechanism inside the body with just the breach sticking out ?
I was forced to rebuild mine because I actually had to use it as a nail gun, but I have used it with a 6mm barrel and its pretty impressive. (oh and it still works fine after being used at 27bar

Have you considered using 6mm fishing weights as ammo ?
Not always perfectly smooth spheres but they fly straight thanks to their weight. Pressing them back to shape isn't too much trouble, still worth it in my opinon compared to airsoft BBs.