kman,
I have always believed there are no stupid questions but there are a lot of stupid answers.
While this question has been asked numerous times before, by different people, (I have appretices asking me the same question time and time again, and sometimes when I answer there questions I do learn something), so here goes.
Just about any lathe can be used for gunsmitthing and for through the headstock barrel work it comes back to how short the barrels you want to work on are.
If you are doing 20" barrels then without making some tooling a lathe with a hole thru the spindle to accomodate the barrel with clearance and short enough through the spindle so you can hold it at both ends of the spindle and still have room to thread and chamber it would be one of the requirements I would look for. If you are doing longer barrels then a longer headstocked lathe can be employed.
The lathes you are referring to would in my guess be ones Grizzly are selling, the G4003G and the G0509G, personally I like the G0509G (my own lathe is similar to this one) and I can do 20" barrels through the headstock in it (only after making some tooling to enable this to be done) my lathe (and I would assume the Grizzly G0509G) is solid and very accurate.
While some may say buy a good used lathe, most "noob's" to lathes etc don't know how to tell a good one from a bad one, I started with a bad one and spent 2 years (and lots of time and money) getting it so I could do accurate work with it........now I realise it would have been easier to just go and buy a new quality lathe and been doing what I really wanted for those 2 years instead.
If you are looking at a new lathe then ask for the names of people who have bought and used the model of lathe you are looking at, this way you can contact them and ask questions about performance, accuracy, service etc............I get phone calls every so often from people looking at buying the same machines I already own and have no problem telling them my thoughts on the machines, I even had one person come and do some work on one of my machines before he bought one.
There have been several lathes mentioned on Benchrest.com as suitable for gunsmithing work, so a search of Benchrest.com would be a good place to start looking. Some of the ones I would place on a short list to seriously look at would be ones that good gunsmiths actually use.
Remember that the cost of the machine is only part of the cost, I think I have almost as much in tooling as I do in machines, maybe more.
Hope this helps............Ian
?? joe