Things get weird, however, when you start putting buck and ball shotshells in guns that get more than one per round.
With buckshot, shotguns are treated as recoilless. Every further degree of success gets you another hit. With buck and ball, the first hit is always the slug, and any further successes are buckshot. No information is given for multiple shots.
My leaning is this; in a repeating gun, you get another slug on target every time you reach the slug recoil. First success gets a slug, then you get buckshot with all further ones until you get to the required 5 (even if this has buckshot from the second shell using successes, as would be the case in a slug+3 buckshot shell).
However, if you go for a double barrel (or bolt two shotguns together), it's straight up recoilless. Fire both barrels at once, first two successes are slugs, and beyond that is buckshot.
It gets even weirder when you start bolting guns together into twin-linked configurations. GURPS
This seems like something of a can of worms, and I may well just declare "screw it" and call it close enough at basic unchanged recoil.