Very sorry, you missed the original proposal that had nine different specialities - this is about as far away from that as one can get.
I like it because it kills the skill bloat and puts a de-emphasis on combat skills in the game.
However, it is not the only way to achieve this. Presuming no one has issues with the Heavy Weapon split (man portable, artillery, vehicle), then another way to handle Gun Combat, as someone has already suggested, is make a shortarm/longarm split. Or, as you suggested later, slug/energy - in terms of game design, now we have pared things down to two main specialities, I don't actually mind which way we jump;
1. Civilian/Military (splitting between easy to use and stuff that needs proper training - I think this will be on the bottom rung for most people though).
2. Shortarm/Longarm (as advocated by Mongoose's Official Weapons Person - ex-sergeant French Army)
3. Slug/Energy (strong arguments for this one too)
Shortarm/longarm are key - but so is energy/slug/indirect.
Anything less than 4 ( Slug Pistol, Slug longarm, Energy Pistol, Energy longarm) is too few for anything approaching realism - but, thanks to gun combat being a cascade, the "bloat" is minimal even if you use 9.
Slug should probably also have SMG, since the firing stances are different for them. Energy weapons don't have a compelling reason for an SMG skill equivalent. And the early SMG's, like the Thompson .45ACP or the Sten, really fit well with Rifles, but modern ones like the Ingram MAC-10 and MAC-11, and the Skorpion vz. 61 lack the grip points to use it like a longarm, but are too heavy and recoil intensive to use like a pistol.
The arguments for shotguns are, like SMG's, about the difference in posture, aiming techniques, and recoil compensation techniques - but if you're going to consolidate, it's the one that is least special.
The Grenade Launcher should fall under Heavy Weapons, but if it falls under Gun Combat, then it should be a separate specialty. Again, it's a matter of stance, aim, and recoil.
So, I see 5 as key: Slug Pistol, Slug Rifle, SMG, Laser Pistol, Laser Rifle.
I see two others as "should be" - High Energy Weapons, Shotguns.
HEW Because firing a bolt of plasma isn't the same as a slug of lead/copper/steel, nor is it the same as putting a laser on target. You have to lead differently.
And yeah, that's 7 specialties. Most people do one or two - most competitive shooters compete in one.
I see what you are doing, and it has elegance, but it is also too complicated for what we are looking for and would send the page count rocketing in Traveller books. It would also mean a lot of book consulting during play, whereas there is next to none at the moment.
Don't think it will fly
for the clueless, Art was suggesting the system from 1st ed Paranoia.