Let's go further into why this isn't really a point of discovery, research, or design. Not intending to make you feel bad for having thought it up because it's a noble concept, but to highlight why it's not a big field of study.
I will caveat this with a statement as well as a request: I was a babby machine gunner (M249) but with all but one team leader I worked for having come from a weapons squad, I was rather well trained with regards to "total utilization capacity" of my weapon system. I can still get shit wrong, and still also can forget shit, so any current or past dedicated machine gunner can pipe up to correct me, and I will thank you for it.
Having said that, in it's most common/doctrinal employment, machine guns provide a base of fire on a target to kill the enemy first, and secondly fix the enemy that are lucky enough to find cover for the duration of time that it takes for a maneuver element to come at them sideways and put lead to head. A machine gun's accuracy has a circular error probability that is known as a cone of fire. This cone of fire produces a beaten zone.
This employment includes
- direct fire, which is where you pure and simple see a point or area target and search/traverse your beaten zone across the target until your target isn't a problem anymore
- grazing fire, which is an area denial technique which provides an generally "inhospitable to meatbags" area of terrain
- plunging fire, which is fires that due to a combination of both distance between firing position and target, as well as firing position itself, allow both the possibility of firing without having line of sight on a target as well as having a beaten zone that is interrupted by a distance between the target that it's statistically safe to traverse.
You can make an analogy of putting a spraypaint can on a table, then holding a sheet of cardboard a foot in front of it. Press the spraypaint can nozzle down, it makes a circle, right? Beaten zone. Now, if you tilt the cardboard at an angle and spray, you digress from a circle to an oval to a long oval with a cutoff point (Grazing fire). However, if you extend that cardboard for say 10 feet out from the paint can, and elevate the spray can or increase the angle you're spraying at upwards from the earth, at some point the paint particles will start to impact the cardboard and will produce a larger, oval shaped pattern as well, while the cardboard can stay clean between the impact point and the spraycan. Plunging fire!
Men specifically utilize the beaten zone, direct/grazing/plunging fire often in the outdoors, sometimes also incorporating search and traverse depending on the alcohol level of what has caused the need to urinate. "That tree, and That tree, and that bush, Oo an anthill FUCK YOU lemme dig a hole in that URRGH ok done"
So we've explained in basic terms what a MG is for, and how it does what it's supposed to do as a piece of equipment. Machine gun Mungo make bullets go that way, bad guy scared, friend bayonet bad guy while bad guy scared of mungo with machine gun. Urgh, Chest thump, Weapons squad, yeerp. Right?
Well, Machine guns really aren't the most reliable pieces of kit, because for all their simplicity, they're always rolling the dice due to all the variables and things that need to happen perfectly (or in the "close enough range) for it to continue going GA-GA-GA-GA-GAOW while whomever holds the trigger down grows a deaths head grin of fuck you or a scowl of fuck this, depending if it's proactive or reactive fires.
Malfunctions fall under immediate and remedial action. Immediate, you just pullstart the machine gun like it's a huskvarna chainsaw and keep fucking shit up. Remedial, something worse has happened which can vary from quick (but not as quick as an immediate action stoppage clearing) to "it just blew up and now we need a medic for the gunner". We'll simply break it down into partial (fixable by the operator), and total malfunctions (AIRBORNE!) which render the machine gun totally inoperable.
Modern machining provides tolerances that allow for utilization of quick change barrels, which provide more functionality than an implementation of a classical fixed, cooled, barrel.
- If you have an extended string of fire, you can rotate your barrels on and off the gun in order to maintain them within a safe temperature range of operation for both safety of firing and overall metallurgical longevity with regards to wear. This in itself provides a cooling factor while feeding into the second point of:
- If you have a malfunction that causes barrel damage or blockage, you can quickly swap barrels with a minimal delay in fire duration maintained as well as minimal reduction in fire volume placed on target
Machine guns also typically function as an element, not individually. To combat individual weapon wear, heat, and ammunition expenditure, you "talk" guns, which is where one gun fires for a bit, then another one does, then another one does. This provides a steady stream of fire onto the desired area of effect while providing a gap in fire for each weapon system so they don't overheat. It also provides a system of redundancy as if one gun fails for some reason, the others remaining can simply accelerate their own rate of fire between the remainder and continue the same target effect whilst said "down gun" team unfucks its equipment.
Unless it's a totally passive system, which are typically the least effective in terms of cooling and also typically have the highest ratio of cooling effect to weight along with the capacity for long-term maintenance issues using common materials and dissimilar metal contact corrosion, you're talking a closed loop gas or fluid cooling system, possibly trying to capitalize on phase change IF there's something that will work within the temperature ranges on a machine gun.
Closed loop systems either make it a huge bitch to swap a barrel at all, or otherwise make a barrel unswappable. You also have issues with regards to weight of the system in terms of portability as well as action function, as an example the M2 MG's barrel recoils as part of the system of function of the gun. Increasing barrel weight slows action speed and can increase malfunction rates up to a point where the gun just won't work.
Without the ability to swap the barrel, a chamber jam becomes a bigger deal because you have to work it through the action in lieu of with say a M240 or M249, where you can swap barrels and the gunner can continue putting fire on target while the AG/AB busts out his leatherman and unfucks the situation. If you can't get something like a blown head casing out of the chamber, or any catastrophic barrel-related failure that doesn't outright take out other functional elements of the machine gun, a barrel-swap MG is still able to fire whereas a fixed barrel system MG is now a 30lb or more piece of dead weight.
If it's a liquid cooling system, you have to mule around the liquid FOR the cooling system, have to worry about corrosion factors on the equipment due to liquid exposure (if it's the most commonly available liquid, water). Water has the advantage that while you can pack extra water specifically to load up the MG, or constantly IN the MG, whatever you haven't run through the gun is able to be used for drinking. I wouldn't drink water that's been through a water-cooled MG as a general rule unless it's dire circumstances due to toxicity concerns from byproducts of the cooling system as well as environmental factors (heavy metals/toxic compounds) so reliably speaking, whatever you ran through the gun is now not drinkable except for emergency circumstances. You also have to factor in fluid replenishment, as otherwise you're reliant less on dissipation of heat through medium transfer to a different location and further dissipation at that point in the loop, and start towards a consumption process of water to machine gun, cooling medium phase change, and release of changed state cooling medium with introduction of fresh cooling medium. This increases the logistical load of operation of a machine gun team as now you're having to continually feed the MG cooling media as well as ammunition and associated oils to maintain proper function.
A passive fin-based system looks good on paper as it increases both overall mass for heat soak as well as surface area for subsequent radiation of heat away from the weapon system, but then you have to take into account the end user and environment of operation that these mechanical devices have to reliably function in.
It's a funny meme, but it's germane to the discussion. The keen eye will notice something that the un-indoctrinated will not... What is the ubiquitous troop carrying?
A machine gun. I myself could not, even with the most meticulous level of care and concern, for the life of me maintain a functional killflash grid device on the front of my optics. Ever. Regardless of what I did, how I did things, or even actually handling my weapon whatsoever, would have the fins of that device break and occlude my vision through my optics to where I would have to try to remove it from the front of the scope, get the notches ground down because I'm doing it mid-mission because I CANNA SEE SHIT CAP'N, and finally just dig in with my pliers and just yankfuck the shit out of it until I could see well enough to engage effectively as necessary. At one point in training, my entire weapon was replaced piece by piece with new equipment because it wouldn't perform with blanks, with the sole piece not replaced being the actual receiver. It fired fine for live ammunition, but it progressed completely through the cycle of function with regards to location/timing of malfunctions, with the final kicker being a triplefeed off of linked ammo and subsequent inability to even disassemble the weapon myself, requiring our professional armorer to totally unfuck my situation to my chagrin. My bruised ego as a SAW gunner was soothed by the fact that prior to bringing it to the armorer, my team leader and squad leader actually had worse results than I did when trying to chide me for failure to operate my weapon properly.
Plus, carried, static weight is a huge deal in the infantry, which is the largest proponent and end user of belt fed fun distribution devices.
I'm in the process of having a friend who is much smarter than I work out the actual equation and results for how much better in terms of rate of fire, a cooled machine gun would have to be, in order to actually be of benefit, given that a cooled MG *IS* going to be heavier in terms of a weapon system, than a current generation uncooled one, as well as given an equal rate of malfunction and equal rate of "catastrophic" malfunction that takes a barrel out of play. While he's still working it out and will be finishing the problem up as we discuss further over the next couple of days, it's looking pretty untenable of a position to have an actively cooled MG group beat out an equal weight.
Just so you see what I'm working with, the M1917A1 was 41 pounds with water, with a 53 pound tripod, firing at a maximum rate of 600 rounds per minute cyclic. In comparison, a M240B weighs 28 pounds rounded up, with a 20 pound tripod and associated hardware, firing at 950 rounds a minute maximum cyclic. That's 19 rounds per second, per pound, for a modern air cooled MG versus about 6 rounds per second per pound of water cooled MG. Maintaining the same weight of both weapons, but using the good idea fairy's wand to increase a M1917's rate of fire to the same speed as a modern MG and you still are at half the performance per pound (10 rounds per minute, per pound) compared to a modern MG.
Given a 3 gun team weapons squad's purely gun weight, excluding ammunition, and with rates of fire remaining similar, you're only going to field 1.5 watercooled MG's in the same "weight capacity". Even with telling weapons squad to "suck it up" in order to carry two WC-MG's, a 5 second ROF downtime for barrel changes on guns to maintain that rate of fire, and the exclusion of barrel "loss" due to catastrophic malfunctions that a modern MG with barrel change can at least work around, you're looking at over 13,000 rounds downrange for a 5 minute long cyclic burst with those three M240's, where the two water cooled MG's with no barrel changes would only be putting out 9,500 rounds.
So you'd need to be able to make up for things by being able to have an increased sustained cyclic rate of fire, in this instance, of greater than 1,300 rounds per minute, which now increases complexity and cost due to materials and design required to sustain that kind of a rate of fire for a weapons system in both the weapon and it's mounting hardware... and currently is starting to play in the arena of another weapon system which will skunk the shit out of a reciprocating bolt design machine gun...
41-some odd pounds in it's lightweight design configuration and using a 44 pound tripod, excluding the electrical infrastructure to make the spinny bits spin so it goes BRRRRRRRT, you're talking about a piece of hardware that can go up to 4000 rounds per minute depending on what you wire it up as. They even made, but never put into production, a gas operated variant which increased weapon system weight, but negated the need for heavy and cumbersome electrical equipment for operation in a dismounted role.
The minigun also has an advantage in the fact that they're "failure tolerant" in that a barrel blockage such as a stuck casing is simply bypassed, where a removable barrel air-cooled MG just goes KERPLUNK new barrel, but a non-removable cooled barrel utilizing weapon system would just be dead weight.
So let's boop the magical water-cooled gun with the GIF's wand again, and make it shoot 1400 rounds a minute so it's on paper better in a 2 gun weapons squad, than a 3 gun air cooled current weapons squad.
All of these things are neat and cool, but the end issue they all have in common, is feeding the beast. 7 pounds per hundred rounds, so a 10 man, 3 gun team, weapons squad carries in weapon and ammunition weight on the order of 44 pounds per man in the gun team (not distributed perfectly equally, but as close as possible) with a 1200 round standard load.
Even with another wand boop and fairy dust sprinkle weight reduction of 11 pounds by not using a classical M1917 as-issued tripod, instead using a modern M3 heavy machine gun tripod, you're looking at 47 pounds (rounded up) per man with all weight shifted around that you could to equalize load across all 9 men actually functioning as pack mules I mean weapons squad machine gunners.. except that it's not going to work that way, since there's a pile of individual equipment that really needs to stay on each individual, like water, medical shit, dip, skittles, charms, and that photo of a significant other (note, I didn't source who's significant other).
Final reiteration: We have ignored completely, but have mentioned malfunctions. Classically, cooled MG's had little to no means to be able to swap barrels, that I have found. A blown case head, or stuck casing, or anything that basically obstructed the barrel would flat out render the weapon inoperable, as there is no recourse to remedy that type of a situation. That takes a whole gun completely and effectively permanently out of a fight until someone can unfuck it's independent situation... where a minigun just in effect ignores that barrel, and something like a M240 just swaps barrels and then you use a lower sustained rate of fire to allow cooling time for unmounted barrels.
You can see where I'm going with this, I would hope. While there's more than likely holes in the logic used to get where we currently are in this discussion, it's a rather accepted fact that actively cooled MG's just aren't feasible for a dismounted role anymore, and really weren't generally used as such even when still in their prime of battlefield presence. That's why things like the M1919 machine gun were designed, and things progressed to where they are today with modern, air cooled, dismountable medium machine guns.
