Once you get a good square start on the lathe with a taper tap then it is no big problem to get the 2nd and plug in straight. The trouble with singlepointing is that one slip into that blind hole and the whole job is screwed. Plus this way the thread tapers out and does not need the tool clearence groove of the SR way. IMHO this is one less stress point.

My sizes were diferent to yours, so lets say for 12G. Take 1 1/2" long brass rod, turn it to bore size. Drill it for 1" and tap a 1/2" thread, drill the remaiming 1/2", 1/4" dia, slit this up both sides till you meet the thread. Take a 1/2" bolt and turn a taper 3/16 to 5/16 on the bottom 1/2".
Put bolt into brass mandral, nip it up and the tapered bolt end expands the last 1/2" and it locks tight. The more force you have to turn with to put the barrels in the tighter it gets. When you are finished and it is all col it should come out easy. If not just tap it out from the breech end with a cleaning rod.

I confess not to using a T wrench but whatever ring spanner I had to hand

I dident peen the joint in because it was for myself as a personal Africa gun and not a client, it was a very tight joint anyway, just a hairline. I have a little 30-30 jones underlever fungun DR on the bench however on which I have done so. If it is for a client and an expensive job I would build shoe lumps or chopper lumps everytime, that is what I am doing on a 470 at the moment.

Yes I glassed the head of the stock, it was just not up to the job.

BTW are they rifled 12G barrels of tubes?

Regards