Anyone here ever threaded a Handi-rifle? I've been wanting one in .357 for shooting suppressed .38's. But I'm not sure if the barrel will fit in my lathe with lug on it. I would want to cut it to 16" so it wouldn't fit through the back of the spindle. I'm not sure how to do it while kepping the threads concentric.

I'm an H&R collector, and you know what I have a single shot H&R in a 357 and it is an amazing hunting gun. Also it is pretty quiet the way it is. But I had never thought of putting anything on it to quiet it down as it is very quiet already. But now that your talking about it maybe a flash hider would be cool on it...

Put it between centers to thread the muzzle.

they make a great host, but you would need a pretty large lathe to be able to fit that lug through the spindle. I typically thread any barrel between centers anyway though. its faster, easier and less likely to damage the finish IMO. if you need to cut the barrel off you could set it up on a steady rest.  I left mine full length, keeps it a little quieter and I can still get good velocity with hot 357 max loads. (heres a couple pics my rifle shortly after i threaded it (i have touched up the blueing since then)

So basically all the barrels I've threaded by chucking them in the four jaw and indicating off the bore could have been done much easier by turning them between centers? I've always been worried about not having concentric threads by threading between centers.

as long as your tailstock is aligned with your spindle then yes, it is MUCH easier and generally more accurate to do between centers.  to check alignment, chuck a piece of stock and cut it to a point (i usually do a 60 degree)  and then put your live center in the tailstock and check the alignment of the two. The old machinist trick is to use a business card or piece of brass shim stock for the gauge, if the two points hold the card perfectly straight then is perfectly aligned, if its holding the card canted or if it wont hold it at all you need to adjust the tailstock. I like to check alignment at both extremes of the adjustment in the tailstock just to make sure.  hope that helps.

It pays to know your machine.  I've threaded multiple barrels by chucking the action in a three jaw and turning threads out at the tailstock.  No baffle strikes yet.  I wouldn't attempt it on a lathe I hadn't checked out.

bore concentricity has nothing to do with knowing your machine, while modern bores are usually fairly concentric with the OD its almost never perfect and in some cases is off by a large margin. the bore is almost never perfectly centered  in relation to the OD and its also not usually aligned perfectly over the length. thats why the standard practice is to thread between centers (takes out both variables in one step)  or to indicate it in with a 4 jaw chuck and a spider on the back of the headstock.  this is time consuming and requires the use of a range rod and bushings to get measurements 6'-8" down the barrel (this is required to get the entire length of the bore running true)

Knowing your machine and whether or not your tailstock is in line with your spindle, or what areas of your bed it is in/out of alignment given it's particular twist if you haven't anchored it and straightened it out.  Even then you're only dictating taper and not runout.

Everything you said is true.  I don't do it because I've never found it to matter.  I even did the trig to show why, and also why turning between centers doesn't guarantee anything if the bore isn't a straight line between the two planes you're constraining with your centers, but I don't care to argue about it, since how you do it will never impact me and vice versa.

you are correct, I dont care to argue about it either, I only care because other aspiring machinists/gunsmiths (like the OP) may read this and its important to me that misinformation isnt spread on the subject. you may continue to have good luck with the process you have chosen but i didnt spend years learning and thousands of dollars on machines to count on luck, and I hope others learning will take the time to to a job to their best ability and come out with the best result.