Taipan veteran 22 JSB 5.53 mm

No doubt about it, I shoot JSB 18's and Crosman domes almost exclusively in my Veteran, does best with the JSB's but Crosman's are less expensive. Note five shot single group at 25yards /18's @ 710fps, one turn of HSA get it to 900 and just as accurate, group above the dime is "10" shots with the 18's, superb trigger and 30X scope, one hole groups are easy with the 18's. The group below the dime was with a Streamline with the 18's. Didn't know the 18's came in different sizes...
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Thought I better clarify, they both shot any true JSB pellet with a hollow skirt very well.

Neither liked the JSB 25.4gr Monster RD.

But, 18.1, 13.4, 14.3, 15.89 and 15.89 Hades were all really accurate. 

Guys are shooting slugs through Veterans with factory barrels. I'd assume that most slugs are tighter to chamber and tighter at the choke than the 5.53mm JSB's you're talking about. If anything you might seem more leading but I doubt any harm from using the bigger head size pellets.