You need to slug your barrel, my Marlin in 38-55 came out .375, exactly as my Model 375 is, makes me wonder if Marlin is using the same barrel for both at one time. My neighbor has build a 375 using a 38-55 barrel from Marlin and gained 4 more inches of barrel length. The brass for 38-55 is not as strong as is 375 brass but have used both in either gun with no problems. Last yr MidwayUSA had a lot of 375 brass and I purchased 500 of both and now keep them seperate. I shoot only paper patched bullets in the 38-55 and works great. Get the book "paper patch bullets" author Paul ?, more info than you can ever use and soon you will understand how deadly these bullets are on game. If I remember right Paco did a lot of testing with the 375 a while back and a search on internet will locate his work. Also Ken Waters did a lot of work on the 375, and althought did not use AA1680, I have used his loads with H335 with great success