I had no problems with my 336 30/30, removed all the spacers. IIRC it goes behind the band on mine. Moves it closer to the muzle but it worked. The mounting rig is kinda mickymouse though.
Just use only the rubber cap for the spacer stack and none of the spacers, or maybe the thin spacer and rubber cap, and use the square rod per instructions to position the bayonet under the muzzle like normal (bayonet on the mag tube, strap around tube and barrel). You can't mount it sideways on the barrel only unless there's room to get the strap fished between the mag tube and barrel...some leverguns have enough room and some do not.
Or just get a LabRadar and chronograph any firearm and get results of velocity and accuracy at the same time since the LabRadar doesn’t affect barrel harmonics.
MS blade sitting on my desk now. Wondering if I permanently fix it to a muzzle device that screws on. Other choice is reinforce the bottom with plexi and put a metal band around it with tension device. Hmm. Only use it on threaded barrel rifles anymore anyway.