The AG(also known as the Autogun) is designed to dish out our 6x50mm round in job lots. Gas piston-operated and belt-fed from a contained spool(using a metal belt that adds to cost but improves reliability, and drawing rounds back out of the belt and then down to the breach. The spool itself is contained within a box mounted under the gun and holds 50 rounds, although longer lengths could be used.), the autogun is designed to be truly man-portable and used almost like a rifle, retaining the layout of one(with the addition of a pistol grip). It comes with a fire-selector for the ability to put out single shots and has a full-length 559mm quick-change barrel designed to take no longer to replace than it takes to reload. An integrated bipod capable of being positioned at either the gas block or the trunnion and large diameter aperture sights(with a smaller diameter flip-up leaf for precision work) graduated out to 800m bring the feature list to a close. The firearm's rate of fire is some 500 rounds per minute.
Maximum weight is to be under 10kg and should be more along the lines of 8kg; cost should be less than 45 and more along the lines of 30.
How nerdy do you want to get?
Given how much our competition loves putting out cheap and unreliable POS, I'd include full accessories: an "oiler" oil can, cleaning rod, and most importantly one or two spare barrels specifically headspace'd for the individual gun. (Even today, most machine guns have barrels are specific to the individual gun. Adjustable headspace is a thing on the M2, but then you're making barrel changes slower as well as trusting privates in combat conditions not to screw it up.)
If you're going to do select fire, might as well do a safety setting. Also, does the action fire from a closed or open bolt? Closed is sexier, but an open bolt allows for easier cooling and prevents cookoffs. In you go open bolt, you can always put a dust cover flap if you're worried about field grime. While we're talking about action, a "last round bolt hold open" is a nice ergonomic feature to consider.
Which direction does the belt feed from, and which direction does the brass eject? Assuming a solo right handed shooter, you want it to feed from left and spit brass to the right. But if you want an assistant gunner, it might be more convenient to feed from the right and spit brass down (rather than left across the shooter's face.) (Feeding from the bottom can make obstruct prone use, and top fed can block sights and general field of view.)
Speaking of assistant gunners, the problem with making a MG with changeable barrels is that there's no way in hell a solo operator can carry an MG, hundreds of rounds and a spare barrel or two. They're called "crew-served weapons" for a reason.
Speaking of barrel changes, make sure there's an insulated (wood) grip/handle, or everybody is going to have a bad time.
The small diameter sight is kinda silly, but maybe some customer will like it. RoF and belt size looks about right. Other than being easier to carry, another nice thing about smaller belts is it keeps the operator from overheating the air-cooled barrel. (Fun fact: Modern air-cooled MG's that go through 100-200 round belts are supposed to be getting their barrels swapped every 2 to 5 minutes. This is why quick change barrels and two spare barrels are a major feature of GPMGs.)
30 credits seems pretty optimistic, but I guess we'll see, eh?