In the CoCW, under MGs of the old USSR, there is a 14.5mm HMG weighs around 108 pounds. (listed at 49.1 kg)
For ammo weight, my rough guess is that a 15mm round, with round, ram-jet part and propellant, might weigh a nice 80-100 grams, or up to a 1/4 of a pound. That means 400 rounds are going to come in at close to 100 pounds.
That means realistically (IMHO), the weapon should weigh around 100 pounds, and a 400 round clip should also weigh around 100 pounds.
I say that because light weight materials and HMGs don't mix well. Little weight and a lot of recoil doesn't seem very fun.
On the negative side, the Mercs book says ram-jet rounds can only do 5D6 MD for a 40 round burst from a 14.5mm weapon, so the 1D4x10 MD from 20 rounds seems a little high.
Since there was mention of pre-rifts stuff, here is an old pre-rifts idea I had a while back, complete with made up history.
The stuff is kind of rough around the edges, and was made with the idea that they were made when true MDC armors were still a bit rare or overly expensive.
ML-1F
Man Portable Laser weapon.
Main Purpose: Troop support
Secondary Purpose: Anti Armor support
Crew required: min of 2, 1 gunner and 1 power source carrier, 1 cooling system operator optional.
SDC: 3d6x10 SDC
MDC: 2d4 vs. MDC targets.
Range: 5000'
ROF: 1 per melee, 3 if connected to a cooling system (requires another person)
Payload: Power pack holds enough power for 30 shots.
Size: Laser w/ mount is 4ft long and 10in wide/tall and weighs 89lbs, power pack is 3x2x2 ft and is 117 lbs, optional cooling system is 3x4x2 ft and is 132lbs and adds a 1ft-4in-3in 12lb module to the bottom of the laser weapon.
It takes at least 15 seconds to set up the laser, 30 seconds in rough conditions plus an additional 10 seconds to set up the cooling system.
With the starting use of energy based weapons being tested for standard armored vehicles in early to mid 2030s, it was decided by the US and Japan military to develop a man portable versions of one of the small model laser weapons to give troops a new type of support weapon.
In the ealry 2040s this weapon was fully built and ready for field testing. While it is effective, it lacks a certain punch when it comes to damaging targets with heavy armor. It was the correction of minor mistakes made in this weapons designing that lead to the development of effective handheld energy weapons in the next two decades.
2038 AD: Military contracts several companies to develop an infantry support laser. The weapon must be small enough to be carried by no more than 3 soldiers and it must have enough power to damage armored vehicles
2039 AD: The first prototype design is made, it is calssified as the ML-1X, or the Man-portalble Laser Experimental. (Maybe Wilks was the one awarded the contract?)
2040 AD: The ML-1F is built and ready for full field testing and unlike the previous versions barely meet the steep requirments the US Military.
2041 AD: After a few minor errors are found the ML-1F is deployed by the US military. It only needs two soldiers to carry and operate, but to improve performance a third soldier was added to carry a bulky coolant system.
Break throughs in a Laser technology bottleneck casused by attempts to create the ML-1G upgrade allowed for test to be made to make an actuall single man laser system.
2043 AD: The true first man portable laser is made. Designated the ML-2A, it's only flaw is limited power and overheat problems.
2047 AD: The ML-2R is made. It has better rate of fire, a moderate power supply and packs enough power to damage even heavily armored vehicles.
2054 AD: The MLR project is started, it stands for Military Laser Rifle. It is an attempt to make a Laser Rifle without the need for a large power pack of heat problems. The prototype can only fire one shot per large energy clip.
2057 AD: The MLR-5D is tested and found to work better then expected. It is effective against troops, but to their surprise, the weapon actually can cut into heavy tank armor giving a platoon or soldiers with this weapon the ability to stop even a Main Battle Tank or Powered Armor. It is soon modified to have to firing modes. The main mode had little effect of heavy armor, but was still good for taking out enemy soldiers. The secondary mode uses much more power and can only be fired once every 10 seconds, but can cleanly punch body armor or the armor of a light armored vehicle and even cause signifigant damage to heavy armor. It is durable enough to be mounted with an under barrel grenade launcher on a mount that hangs 30mm below the main weapon casing.
Militaries around the world leap at the chance to buy a small but potent energy clip design being sold by Japan. It is suspected that several world powers have already got their eyes set past Lasers and were already testing rail gun adaptations to accelerate Ions or even super heated plasma.
