First consider most 28 loads for that gun would have been 5/8 ounce not our 3/4 ounce load so keep pressures down. It could be reproofed for nitro and 3/4 ounce. You must examine what it is proofed for. It is reasonable to say the 28 was proofed to the same level pressure wise as a 20 if nitro proofed. But to be sure you must look at what proof marks are on the gun.
Payload is not as critical for barrel safety as for keeping stress down on old stocks. Pressure would stress the barrels. In 12 we go from 3/4 ounce up to 1 1/2 ounce with little issues as long as we keep the pressures in line. So I doubt a shift from 5/8 to 11/16 or 3/4 ounce would be an issue as long as velocity and pressures are watched.
You can come up with a safe load and have them tested. If you keep your pressures down to the 7,000-8,000 psi and keep the payload at 5/8, 11/16 or 3/4 ounce you should be fine. Testing loads at $25.00 per string is very much the thing to do. In a 28 2 1/2” Damascus I would be looking for 7,000 psi with any payload from 5/8 to 3/4.
