Never tried the WLBXR in my 12ga. 870 SM (23" barrel). But the Kicks GT .670 paired with this gun produced very good (pre Longbeard) patterns with the 3.5", 2oz., Winchester HV shells (black hull shells). It would average low 90's in the 10" at 40 yards. I killed a few birds with this setup, but preferred the higher pellet counts in #5's and #6's. Out of this same 870 SM, the Kicks GT .665 did very well with both HV #5 and #6 loads. I suspect you will get an easy 40 yard killer by shooting the GT .670 in your SM with any Longbeard shell - obviously verify this before you hunt.