Never seen that before, possibly a added on counterbored muzzle, seen them before to protect the end of the rifling, pretty common on muzzle loaders to be counterbored,,for that and to help start the sabot straight. But never seen one that sounds like it was added later on anything????
Does it look like the barrel had been bobed a bit, measure if you can find length specs for that gun.
The only other thing I could think of is possibly a thread protector that blended in well after an arsinal refininsh and was tack welded on for compleyince. A real good close up pic here or inspection with an eye loop by you could definatly help in picking out a "parting line",, but if I had to guess,, it would be the latter of my guess's, a well blended in thread protector of some type. Reserch if that gun ever had a threaded barrel in it's production run and that could tell allot there. If it did I'd be willing to bet thats what it is.
You never said if the gun was original or bubba-ed or if the bore in that extension was close to being the same as the rifleing dia., could have been an attempt to change the barrel profile after removing a front sight tube, without recrowning the barrel after a cutting off, anouther possibility. Pics please, there working now.