Assuming we're ignoring select fire attributes...
Military classes to what it would most likely be in civilian categories:
Assualt Rifle -- magazine fed, semi-automatic small to medium caliber rifles.
Battle Rifle -- bolt action or semi-automatic mag fed medium bore iron sighted rifles.
Sniper Rifle -- medium bore, scoped, bolt action rifles.
Machine Gun -- nothing relates to this in civilian terms.
Submachine Gun -- pistol cartridge, magazine fed, semi-automatic carbines.
Civilian Categories as what they'd be in military categories:
Scout rifle -- nothing relates to this in military terms.
Varmint rifle -- Perhaps a sniper rifle.
Hunting rifle -- assuming the hunting rifle is scoped, then a sniper rifle. If not scoped, then perhaps a "battle rifle"
Express rifle -- nothing relates to this in military terms unless you go back to the 1800's and the .45/70.
Of course, there's problems with all of this. For one, many of these terms are not real. *** is a "battle rifle"...that is falls more along the lines of the internet jargon of armchair generals who hate the 5.56 or 7.62x39mm. Why? Because the characteristics of all rifles commonly called "battle rifles" are wildly different. So much so, that the only common feature is that they shoot a caliber .30 or similar, but not "big bore"..
Sniper rifle is another one...sniping is action, not a feature. So any rifle used by a sniper is by default - the sniper rifle or the sniper's rifle. Most people associate a sniper rifle with the following characteristics: 1] Bolt action, .30 cal or at least 7mm to 8mm, accurate, magnified optics from 4x to 10x. Of course, there's the Dragunov, which is semi-auto...but that is probably more of a designated marksman rifle that was ahead of its time.
The main differences between a varmint rifle and a hunting rifle, assuming the hunting rifle is a bolt-action scoped rifle, is that varmint rifles are often 1] heavy barreled (whereas a common deer gun does not have to be) 2] usually .20cal to .25cal 3] have stocks suited for bench or rest shooting 4] optics are usually 12x or greater. Beyond that, they are similar. Hunting rifle, which could commonly mean "deer rifle" is typically anything from .243 to any .30cal, typically 3x to 9x optics (or sometimes irons), and bolt action. Like sniper rifle, hunting is an action...not a feature. So anything you hunt with - is a hunting rifle. Hard to define it.
The main characteristic of machinegun is belt-fed and fully automatic. Can't get around that. What is belt-fed in the civilian world assuming you can try and ignore the automatic fire? Nothing really.
Submachinegun, if we can ignore the full-auto fire which is a huge part of what a submachinegun is and does...could be related to high-capacity, magazine fed, pistol caliber carbines. Like the Beretta, Kel-Tec, etc...