I've been doing a bit of math. Math is fun.
A cubic meter is a million cubic centimeters. A metric ton is 1000 kg or 1 million grams. Basic stuff, but it leads to this: if a ton of liquid hydrogen occupies 13.5 cubic meters (13.5 million cubic centimeters), then a gram of liquid hydrogen occupies 1 millionth of that, 13.5 cubic centimeters. That's in it's ever-so cold liquid state. Our guesses for the PGMP hydrogen charge have ranged from 2 to "a few" grams. That means a starting fuel charge occupying a minimum 27 cc. Now, that's approaching the size of a shell casing from the venerable Browning .50, and there's 40 such charges, 1080 cc. That 3 kg battery pack likely include a hydrogen reservoir. For later purposes, we will apply the 20mm diameter mentioned in book 4 and assume optimistically the fuel charge is a cylinder 2 cm in diameter by about 9 cm length (Pi R^2 H).
Next:
About the biggest kicking rifle out there is, or was, a Holland and Holland .600 Nitro Express "elephant gun". The Nitro Express fired a 58 gram round at a muzzle velocity of 620 m/s, kinetic energy in excess of 11000 joules when it left the barrel. Basically, it made the TL8 Light Assault Gun, with its 30 gram 500 m/s round, look like a wimp. There was later a .700 Nitro made special for some rich boy: it put out a 65 gram round at 610 m/s for 12000 joules and could best be described as a bruiser - literally.
Now, let's say 12,000 joules is the maximum KE you can spit out without doing yourself a hurt. Let's say, for the sake of argument, that your PGMP-12 is firing off 2 grams of hydrogen plasma. To have the same kinetic energy - and therefore the same kick - it's top velocity has to be no more than ~5.66 times that of the .700, or 3450 m/s. At 3450 m/s, your bolt goes from muzzle to its 750m max range in less than a quarter of a second. (To get speed up to 8000 m/s without losing our shoulder, we'd need to cut mass to 0.375g.)
(Incidentally, that 3450m/s speed is about the exhaust velocity of a good rocket. That becomes important later.)
Let's look at it in Newtonian physics. What are we looking at? 2 grams of propellant leaving a 20mm aperture in about a quarter second at a temperature of several thousand degrees and a speed of 3450m/s. In other words, a very tightly focused, very very brief, rocket. A plasma torch? By comparison, the Space Shuttle Main engines punch out about 1300 kg of fuel per second at 3560m/s through a 2.4m diameter bell at temperatures around 3300 degrees C. That's 162,500 times as much propellant through 14,400 times as large an area. In other words, the PGMP's a little rocket with about 1/10 the mass output per square centimeter of a Space-Shuttle main engine.
What will it do to the target? I don't honestly know. That's still 12,000 joules - elephant killing power - but it's behind 2 grams of a sun-hot, rather thin gas with a slightly larger cross-sectional area, travelling at mach 10. Striker says it punches through 6 cm of steel at 250m. I don't thing my poor body would live through something like that, but I can't see it punching through 6 cm of steel to kill me on the other side.
So, let's try a different tack. It's a game, after all. Let's look at it in Traveller physics, ignoring the elements of canon description that seem to be leading us to disaster. At the core, we're dealing with a plug of liquid hydrogen, undetermined quantity, superheated and blasted out of a gun with our human limit of 12,000 joules of kinetic "punch": at 250 meters, it can penetrate 6 cm of steel (Striker); at 450 meters, it's 2 cm; by 750 m, (about a sixth of a second later), it's down to 0.25 cm - it's about spent. So, it's reduced to 1/3 of its "punch" in a bit under double the range and 1/24 of its power at triple the range. Not much spread.
In fact, if we try to apply just enough of Newton for the thing to work, let's ditch that 20mm bit - it's killing us - call it a typo and let's say it's a 2mm pulse; that gives us the kinetic power of an elephant gun behind a jet of plasma with 1/80th the cross-sectional area. Now it's the rapier to the elephant gun's mace. Now what we appear to have is an unusually long range, very tightly focused, very brief and potent plasma torch, an intensely tiny rocket blast that by virtue of concentrating massive kinetic "punch" into a very tiny area, punches through steel like a nailgun through wood.
