Be very careful lengthening chambers. Older drillings have barrels that were struck about as thin as safe to keep weight down.
I have used 2 3/4" ammo in my 65mm chambered 16 and 12 gauge drillings all my life with no problems.
REMEMBER...DON'T SHOOT MAGNUM LOADS IN YOUR OLDER DRILLINGS!! They probably won't blow up, but they'll sure as heck get as loose as a sailor with a three day pass!
I have a 16ga long forcing cone reamer I use on some guns with plenty of meat to allow for this. MANY don't.
Use a good quality field load like Remington Sure-Shot ammo, and you won't have any problems. Don't force heavy shot loads through the tight choked barrels. You'll get "ringing" an inch or two back from the muzzle, and this could vastly change the rifle barrel regulation.
