I bought a sxs at the Southern and it was labeled as IM and IM. I measure the constriction as .011 and .013 until the very last fraction of an inch where it tightens to .020 . A friend researched the gun for me and it was proofed for steel. Could this account for the odd shape of the chokes?

Assuming 12 gauge .020 is modified and I have always heard that is an appropriate choke for steel shot. About what "fraction" of an inch is the.020? I'm just curious being that the gun is marked IM .025. The shot clumping together and forming, in effect a solid hammer that bulges barrels is the issue, as I'm sure you know. I don't see any reason for steel shot to do better through a modified choke marked improve modified than a modified choke marked modified. Is everything factory marked and machined?