if not, if you can find another magazine cap its a pretty easy thing to chuck it in the lathe and bore it out to accept a factory remington tube, which simply uses a modified magazine cap to hold it in place.
No that won't work because the magazine cap on a JC Higgins Model 20 isn't threaded, it's a simple cap that's screwed on from the side.
I know of no way to extend the magazine tube on a JC Higgins Model 20. Too bad, because there are few pump shotguns that are as smooth and reliable as a JC Higgins Model 20; they're smoother and more reliable than a Winchester Model 12 (and this coming from a model 12-a-holic). Your only shot would be to pick up another model 20 magazine tube and have someone tig weld it to the original one. Then add in an extended spring. But this would be a tough job and a costly one.
I'd say the best alternative is a side saddle and stock pouch for extra shells. Keep 4 in the tube, hammer down on an empty chamber and you're good to go. Don't be tempted to swap it off for a Remington 870, because you won't end up with as good of a shotgun, seriously.
I have two of them and would not part with either. The one that i use for HD stock will have to be replaced soon it has a crack in it. I was hopeing there would be a mag tube that could be made to work instead of haveing to go to my gunsmith. He does great work I am just straped for cash at the moment is all.
Hey I've been working on a JC Higgins model 20. It was my grandpas and when I recieved it, it was in pretty rough shape but now that it's almost done I would put against any mossberg. Anyway I have read that the threading will accept a mossberg magazine tube. I'm going to test it out with an 8 round tube. If it works I'll probably install the S & J hardware bayonet lug for fun!