Your gun smith is right....the LR ammo was really made for the XLR Marlins...I tried some of them in my 45-70 and they jam also.I called marlin and they said the carrier in the XLR guns are made to handle the Hornandy Ammo.
Some older guns they feed ok, some they don't
You can change the carrier to make them work.
I didn't
I wasn't really impressed with the LR ammo anyway.