IS ( i s ) i, standing for infantry, (not JS) are 323 Bullet size Barrel 7.89mm lands and 8.20 grove diameter.
I or ( i ) are 318 bullets barrel 7.80mm lands and 8.07mm grove

The added "R" on both version is for Rimmed ( Rand ) Cases.

Bullet diameter is on all "S" loadings 8.22mm for non "S" diameter is 8.09mm

Standard Pressure for "IS" is 3400 bar
" for "IRS" is 2900 bar (Rimmed)
" for "I" is 3300 bar
" for "IR" is 2800 bar (Rimmed)

to complicate the issue you also get a 8x57R_360 that uses only a bullet diameter of 8.09mm and is loaded to a maximum of 2200 bar

1 bar = 14.50PSI

1 mm = 0,03937008 inches

Best slug the barrel if there is not a "S" with the caliber marking on the barrel!

and you cant use load data for rimless cases for rimmed cases pressure would be to high!

Hope this make sense to you