but being a break action you can't set the barrel back a thread to shorten the headspace.
I'm not sure i understand what your saying?
On factory loads in the imp chamber, they will be very low pressure because of the bigger chamber/smaller case, and there won't be any problems with "first time" fireings. (read new ammo)
As for reloading, you just reload them as you SHOULD, and set the head space on the "new" shoulder, so still no problem!
I've done this many times in break open actions and it works perfectly...
Drilling Man