I keep one on my .45 and .50 inlines. The one on my .50 sticks past the end of the barrel just a hair but nothing major. The one on my .45 looks like it was made for it and sits under the barrel nicely.
I don't know if you are using a .50 or .45 BP gun, but when I bought mine it came in a pack of two. To make it fit my .45 barrel I just took a little sand paper to the outside of it. Didn't take much to make it fit. Would like to have one for my .50 sidelock also, however this would require me to buy another ram rod to use for hunting with it. I am just lazy and haven't got around to it yet.