Thanks for the feedback. I do not yet have a mould (first I need a barrel around which to build a mould, one I'll probably hone for my needs as I need a good fit). I have a lot of ideas running around in my head, things with which I would love to tinker, but I first need a platform. I've always been a big bore fan, and even though the 577 NE is a better gun, I fell victim to the 600 NE just like pretty much everyone else (just hold one of those 900-grain slugs in your hand and you know you've got something!). By the way, thanks for the link to nitroexpress.com. I just went on and registered. I am an old-school gun nut and used to have pretty much every book, magazine etc. published on the subject of the old Nitro Express calibers, their rifles, etc. But, some time ago I left the field and passed on most of what I had (guns and all). But now I am back, and my how the world has changed!!! What I used to pay dear $$$ to read about and learn is now often freely shared online, or so it would seem. Of course, discernment is always a much-needed needed skill. In other words, some share true gems, while others just share bull, but it is all good fun and hopefullly we all learn something in the process, for the better of the shooting community.
However, as I think about it further, with regard to the 10-bore I'll probably just work with the barrel I have. I'll either lop off the end, or hone out the choke (and the whole barrel to some extent, giving the inside from end to end a VERY, VERY slight taper toward the muzzle). I also might do this to save money because I'd like to get a Tracker II. I have some ideas I want to work on with a rifled bore, and the Tracker II would be a great platform. The Ultra would be an even better platform (accuracy, recoil, etc.), but I want to hunt with the gun as well and the Tracker II is much more my style when it comes to something I am going to have to carry around the woods.