Woodchopper, don't know how many of them are on NEFs, but the smokeless forum at modernmuzzleloader.com has a bunch of guys using SMI barrels from Ron Name. Ron has an excellent reputation in both his word and work, and I've yet to read a report of someone not happy with their barrel.
I talked to Ron awhile back about getting a smokeless ML barrel for an H&R. Ron told me that his barrel and sealed breach take all the stress, so he can even put these on NEF/H&R shotgun frames.
I'm still considering it. Strongly