Trap loads shoot the cleanest, field loads are dirtier, heavier field loads are even dirtier, and magnums are the dirtiest in any particular gauge. I don't care who makes the rounds, that's just how it is. 410's shoot even diriter than the rest.
Trap loads use lighter charges of fast burning powder, field and heavy field use heavier charges even slower powders, and magnums use heavy charges the slowest powders*. To use one brand of powders as an example Alliant / Hercules Trap= Red or Green Dot, Field / Heavy field= Unique or Herco, Magnum=Blue Dot.
*410's use the slowest powders of all in this example 2400.
As has been hashed over numerous times on the forums, the slower powders don't "clean up" residule wise unless you are running them in their preferred pressure curve. Shotguns by their very nature are LOW pressure systems with the 410 being the high pressure king at 13K psi.
Now the question with the reduced recoil loading of the buck and slugs starts with: what powder did they use in the first place. Did they use a trap style powder? From your question it seems not. Did they reduce the charge of the powder usually used in their buck and slug loads? Possibly since standard buck loads are in the same class as Heavy Field; that would explain real bad fouling as a light charge of that powder would burn like dirt. OR they got some lousy powder in that particular batch.
