Hornets:
are the weakest both in damage and in crit bonus, but are the quickest to fire and most accurate.
Huszars:
have the highest damage potential and the highest crit chance, but have a burst penalty and require 1 more strength (you'll have full-auto anyway).
Marauders:
are between those two; full crit bonus, no burst penalty or bonus.
Chimeras:
are craft only. They have a higher lower damage range than hornets but with the same upper damage range (aka weak). Otherwise the same as a marauder, except their crit bonus is 130% (the only one above 100%).
Hornets are for getting in more shots/bursts; the 7.62 Hornet is the only one you can burst when encumbered or dazed without a rapid reloader, a rapid 8.6 Hornet can burst as well; the only AR above 7.62 that can (also a rapid 7.62 Marauder can burst twice, or once while encumbered/dazed)
Huszars and chimeras are for Gun Rush from Commando.
The Critical Power feat makes chimeras that much better (effective 175% crit bonus), especially with an anatomical scope (262.5% crit bonus)
Personally, I think the Huszar is the better choice until you get Critical Power, and even then, I might still use it if I'm not using crit-chance boosting gear.
The marauder is the odd man out; he's balanced between them, but has no niche. The huszar competes with the chimera for the Gun Rush slot, and eventually loses (still wins if not critting; a 9mm huszar with acid bullets is your best Gun Rusher against bladelings for instance). The hornet is the main use gun, simply because of AP efficiency.
(are the shrooms immune to crits? that's two situational enemies where the huszar beats the chimera if so)
A marauder could use some combination of the following:
>more bullets per mag
>a 5% precision bonus
>a 1 or 2 tile farther range/optimal range
and to seal the deal
>lower AP cost by 1 (from 13 to 12 and from 14 to 13; with rapid reloaders that's 10 and 11)
The hornet would have higher burst precision and leave you with a few more AP after shooting, but the marauder can still get in on the juicy extra shooting. The marauder has a higher crit damage bonus.
A muzzled full-auto 8.6 marauder with a 27 round mag could burst three times with no reload. A 7.62 with 36 could burst 4 times. With ext mags, those would be 33 and 45 (the 45 is 5 clean bursts), but that means no rapid reloader.
No AR has a precision bonus, and they all have the same range; two areas where the marauder could also have an advantage.
It would then be more of a player-choice kind of thing; a bit more AP per round and burst precision, or looser reloading and/or some extra range/aimed-shooting with better crit damage.