In a Marlin you can go with the 18.5" Guide gun... I have .45/70's, .45/90's and .45/120's in 18", 20", 24", 26" and 32"... I personally favor the shorter lengths. The .45/70 does well in short barrels as do most straight wall cartridges... They give up less velocity per inch than do bottleneck cartridges IME.