The Clark recoil/elevation unit comes with just one breech size, so far as I know. So, it's the job of the modeller to modify the breech assembly; to make the unit accommodate the barrel and not vice versa. At least, that was
my experience with the Clark unit. I guess some would say it's much simpler to use the IR/Recoil/Flash type barrel than the airsoft one, you have...
But if you're saving the pennies....
Anyway, with a barrel whose diameter is greater than that of the unit's breech, it's simply easier to enlarge the breech, than slim the barrel down.
This can be achieved, by reaming the unit out with abrasive paper (wrapped around a rod of some sort), or files that work well on plastics. Or..well.. whatever you find that works.
In any event, the Clark unit needs quite a bit of sanding to get it to work smoothly.
A small diameter barrel would require a sleeve in the breech to reduce the bore, and in all cases, the barrel will have to be bonded, or pinned to the breech assembly somehow.
Here's a Clark unit in my Cromwell turret:
Hope this gives you food for thought..
Note: I had to
make the trunnions supporting the gun, for this model. Mounting the gun+recoil can be a real head-scratcher, unless there's a readily available adaptor on the market (even the Tank's original one, but modified to suit)
