Yeah, as it mentions on Jane's, the 73mm round for the 2A28 gun has an added "stub" that helps propel it. That won't work for the SPG-9. And vice versa, without that added propellant stub, that round from an SPG-9 won't make it out of the barrel. And I wouldn't want to be the one that has to get that live round out of the barrel so you can fire another shot at the thing you originally tried to shoot at (which now probably knows where you're at and doesn't want you to try to shoot it again).