Larry,
Is the 303 barrel .313 diameter at the lands or grooves? Muzzleloaders are measured at the bore diameter (land to land), while I believe center fires are measured at the groove to groove diameter. Please correct me if I am wrong?
If you are looking for a volunteer to shoot this rifle for you, I guees you could twist my arm.
I would love to be able to buy a smaller caliber barrel to fit the Huntsman/Pardner/Topper/Handi/Ultra frame. I just wish H&R/NEF would have made some. I would also like to be able to purchase a .45 caliber barrel in a faster twist 1:18 to 1:20, NEF listed one in the Manual back in 2003, but it never came to fruition.
If you want it to shoot conicals, DO NOT put a false muzzle (counter bore) in the end of the barrel, they can ruin accuracy with full bore bullets. The short piece of barrel to pre-rifle would be a much better option IMO.
If you ever decide to sell this barrel, or just want someone to shoot it, please give me a shout, I'm interested.