Originally Posted by Dave Cameron
You can use nail polish to create one or more "bumps" in the barrel right where the ball should sit when the marker is cocked. Or just cut one or more thin strips of electrical tape about 1" long and fold them in half over the breech-side lip of the barrel so that when you screw the barrel in the tape is in the aforementioned spot where the ball rests and the "tails" of the tape on the outside of the barrel hold it in place. Tape detents are very temporary, nail polish less so. Phantom detent rings probably will not work (assuming that they fit properly in the breech) - the phantom only loads the ball partially into the barrel, which is why the rings work in that particular gun, and I'm pretty sure the sterling bolt will push the ball past them.
I just took some quick measurements and it does not appear that the bolt pushes the ball into the breech of the barrel. It appears to be sitting about 3/8" behind that point. So it seems that the Phantom detent rings may actually work. Placing nail polish into the STP body may actually be the best solution.