Re: Building a Rifle
Lathe size is composed of two numbers.
First is swing. this is the max diameter the lathe can turn. I think for gun work you may get buy with a 12" swing but 13 is better and 14 about perfect. You will never need to turn anything this large but the lathe will be able to use a larger chuck to hold the barrel. The larger the swing the larger the internal bearings and the "better" the lathe will cut especially during threading operations.
Next is the max distance between centers. This is the lenght you can turn. You will need at least 36 inches.
I think the 14 X 40 lathes are the best for barrel work.
A very important thing to consider is how you will hold the barrel during chambering. I much prefer to hold the barrel in the headstock and therefor require a lathe with a short enough head stock to hold both ends of the shortest barrel I will chamber. I use a six jaw set tru chuck and build a spider to hold the muzzle at the back of the headstock. If you want to chamber in the steady rest you won't need to worry about headstock length.
Also make sure the through hole in the headstock is big enough for any barrel diameter you will ever want to use. Most 14X40 lathes are fine but check before you buy.
Another aspect is weight and a rule of thumb is the heavier your lather is the better it will cut.
I would not pay any extra for a gap bed and would avoid it if possible. No need for gun work and just one more place to have a problem.