The woodworkers' vises I am familiar with are meant to bolt under the bench with the top edge at bench level. Now that doesn't mean they can't be bolted higher but unless you much shorter or your bench much higher than average your work will be too low requiring you to bend over to do much of the work.
The ideal height for most is between elbow and nipple height for maximum comfort, i.e. not stooping or bending the neck.
Additionally, when clamped in a wood workers vise the stock is always parallel to the bench face and this is not ideal for working on a gun.
For gun work you want your bench and vise as secure and heavy as possible. You will be unbreaching barrels, pounding on things, etc. You don't want your bench or work moving.
I suggest a vise like this as it has all the attributes you should look for in a vise:
http://www.northerntool.com/shop/t..._-3485&ci_sku=3485&ci_gpa=pla&ci_kw={keyword}
The base rotates 360 degrees and the head/jaws swivel 360 degrees so you can possition your work at any angle. It is heavy. Make some removable wood jaws that rest on the shaft and it won't mar your work. You can unbreech a barrel in it no problem. As a bonus, it has a large, flat anvil surface.
Mine is on my bench mounted on MDF layered to 3" to get the height up and that is secured with tow heavy "C"-clamps....it does not move.
I have other vises that I use for lighter work if there's a gun in this vise but this is really all you need.
I am sure you will get many differing opinions. A good vise can be a major investment so choose wisely or you will end up owning several before you are done. I went through several before I saw somebody working with this type. With it set at proper height, I am fatigued less and can work longer and am not sore when I am done as I am not stooping and am not fighting a less substantial vise.
Enjoy, J.D.