73mm

[pmulcahy11b]

IIRC OPFOR classes in the Army:

If you try put a BMP round into an SPG-9, it won't fit.

If you put an SPG-9 round into a BMP-1, the smaller round will not develop enough pressure to launch properly, and will not even leave the chamber of the BMP-1's gun. However, the fuze will be armed, and heavy burned propellant fumes will fill the interior of the BMP-1 when you open the breech again.

Apparently, some fool has tried those things in the past!