Mooner,
Some drillings can be lengthened, and some can't. Many can't due to the way drillings are made. The barrels are struck down as far as practicable, usually, to save weight. Most other things about the drillings are the same. Accordingly drillings are not as strong a gun as a simple SXS shotgun or double rifle. They were made for the typical German hunt...shoot one or two rifle rounds and maybe the shotgun once or twice. The shotgun loads were milder, also.
That being said, you can buy 2 1/2" shells for the older guns from Gamebore. I don't recommend that you shoot 2 3/4" mild loads in the 65mm chambers, but I do all the time and have for 51 years. I also have a stash of old paper hull shells with felt wads that are easier on the very tight chokes these guns have.
The best solution is to get an early post-war gun like JBs sweet little Sauer 16/16/6.5X57R. They have all the pre-war craftsmanship but with the more modern steels and chamberings. Many of the later pre-war guns had 2 3/4" chambers, but most were still 65mm. The 12/12/9,3X74R Sauer I am getting might be either. These guns will usually re-chamber OK to 2 3/4".
I have a second solution that works well. I have a long forcing cone reamer for 16ga. It doesn't give a 2 3/4" (70mm) chamber, but it sure relieves the chamber pressure, lessens recoil, and actually improves patterns most of the time.
That 7X72R would be no problem to load for, and it is about on par with the old 303 Savage. You simply size 9,3X72R brass down and load 139-140gr bullets with 28 or 29 gr of IMR 4198. I owned a plane-Jane snap action drilling in the round years ago and took an old dry mulie doe with it.
