Hi Bill, the length of the extension you added to the outter barrel is the amount you want to add to the new inner barrel. You can do an internet search for people selling their original 6.08mm airsoft barrels or buy the tightbore 6.03 and 6.04mm barrels. The 6.08mm will be more tolerant to dirt and mis-shapen bbs then the others at the cost of a few fps in velocity.

If you buy a long enough barrel, you can cut it down into two, maybe 3 barrels for the tanks. The part that's cut, smooth and chamfer the bore. It's easily secured inside the gun's breech up against the bushing with the black CA glue. It has a little give to it. The smaller inner diameter of the new barrel provides more surface area to keep the rubber bushing in place. I have yet to lose one or have it come out of place when using an airsoft barrel.

The stock Pershing inner airsoft barrel is 174mm. Corrected length should be 207mm.

The stock Tiger inner airsoft barrel is 212mm. Corrected length should be 250mm. (These are my measurements using brass to lengthen the outter barrel, the prefabricated extensions may be slightly different)

The HL Panther inner airsoft barrel is 175mm. Corrected length should be 238mm.

I have not completed conversions for the Walker Bulldog (Correct length for outter barrel with muzzle brake is 203mm) and various models of the PZIII yet.