I've played with spreaders in a couple of 16 ga M-12s. One runs .021, the other .012 (likely not too far from your IM and SK2). Loads made by placing Polywad discs on top of the shot column gave me unuseably open patterns from both guns. I read Mike Campbell's technique (he will not credit himself - if you do a quick search on 16ga.com you will find it well described) of placing the disc on top of 1/8 oz less than a full payload, followed by the remaining 1/8 oz shot placed above the disc. I like a 7/8 oz drop into a SP16 wad followed by the disc, followed by 1/8 oz. I use a 9mm empty ground down to hold 1/8 oz. RGL hulls. Crimps end up just right. This technique results in patterns that, for me, run around 45% with both guns. .012 ought to be pretty close to SK2.
I've tried the X-type cardboard inserts and gotten unuseably open (ie., voids) patterns at just 21 yds.
Sam
