These guns are actually 32 gauge or .51 caliber. They were also made by several other companies. I had one made by H&R for several years. Tranquilizer darts are still available in various stengths to knock down animals from mice to elephamts! Ed.
They are very handy on working cattle and wildlife.Pnu-Dart has disposable darts to tranquilize or medicate,take blood samples ect.,even a "Bear Scare" dart that when loaded with Pyrodex explodes on impact to chase off unwelcome wild life.
Though I have retired from cattle ranching,I'v kept my H&R one shooter and just last month helped my neighbor recapture three 75-80 pound escaped Rhea birds (South American ostrich) 100 yard shots are not un heard of with the H&R,I'm not familure with the Marlin