Just a thought, but we could just basically copy/paste the basic rank structure of the US Marine Corp.http://en.wikipedia.org/wiki/United_States_Marine_Corps_rank_insignia
I would assume most player characters are probably enlisted, given that those are the ones who are generally sent into the field to actually shoot stuff. Obviously, the higher you go in the chain of the command, the more your duties involve sitting at a desk doing paperwork.
Junior enlisted are basically the grunts, NCOs are generally squad leaders and such, and COs are the ones who plan the strategies and give the big orders.
I dunno if you'd want the fancy pants commanders and people giving the orders to be GM controlled NPCs or what, but I'm thinking my character would, if we use Marine ranks, probably be either a Corporal or a Sergeant.
Also, technically, she'd be more correctly a "Designated Marksman" than a "Sniper", the key difference being that DMs act as part of squads and do general infantry stuff, whereas snipers usually work as part of 2-man teams and are more involved with recon work. Snipers are also specialized in the use of sniper rifles and fight from a long distance. DMs, on the other hand, usually use modified battle rifles or assault rifles, and are, in general, more versatile than snipers.
So, she'd likely be equipped with a modified infantry rifle, with enhanced accuracy and long range capability, semi-automatic. Less powerful than a fully specialized sniper rifle, but with a significantly faster rate of fire and ammo capacity.
That's sort of the kind of gun she'd probably have... though more science-fictiony looking I'd imagine xD
(With a scope, of course. xP)
Granted, full disclosure, I'm not military and never have been, I've just done a lot of military roleplay over the years xD
Some important bits from that wiki page for rp purposes:
Marines address all enlisted personnel by rank, and all Commissioned officers with "sir" or "ma'am". Warrant Officers, regardless of rank, are addressed just as commissioned officers, but may also be addressed as "Warrant Officer", or "Gunner", although the latter is sometimes considered improper unless the officer is an Infantry Weapons Officer (MOS 0306). During recruit training, recruits are indoctrinated to address all superiors as "sir" or "ma'am". Addressing a commissioned officer (or any rank) as "Mister" has long been considered a grievous insult towards the individual. The most junior ranks between pay grades E-1 and E-3 (Privates, Privates First Class, and Lance Corporals) are commonly referred to by their last name only, using their rank only in a formal situation. "Marine" is also a common form of address for junior Marines.
Informally, some enlisted ranks have commonly used nicknames, though they are not official and may be improper for use in formal situations. The acceptability of nickname use by juniors is at the discretion of the individual rank holder. A Gunnery Sergeant is typically called "Gunny" and occasionally "Guns", a Master Sergeant is commonly called "Top", and a Master Gunnery Sergeant is "Master Gunny" or "Master Guns". Differing from the Army and Air Force, all ranks containing "Sergeant" are always addressed by their full rank and never shortened to simply "Sergeant" or "Sarge". A Private First Class is usually referred to as a PFC, instead of simply "private"; similarly, Lance Corporal is not shortened to "Corporal". Senior Officers may informally address junior officers by first name. Marines of the same rank may also address each other by first name when among peers only and never in the presence of junior or senior Marines.
Finally, Marines consider it a grievous insult to be called a "soldier" or to be referred to as "troops" as soldiers and troops are army terms; the proper term is always Marine. When writing journalism or scholarly references to the Marine Corps, its elements, and/or individual Marines, the correct attribution is Marine(s).