I had a similar "incident"[:@] It is a 3mm shaft. I used a guide rail from the inside of a cd rom drive( Most that I've seen are 3mm, and there hardened ) and just cut it to length. Mine is a rimfire 28-30-1250 and has a collar on the back with a set screw and has the two screws on the front so I didn't need the c clip. Works great and it's a cheap fix.
Hope it works for you.
I you still insist on trashing the thing, you could allways send it to me.
Don