I agree with ATP's list, although I sometimes waffle about separating Shotgun into its own skill.
My distinctions when a TL-10 soldier is handed a TL-3 weapon are on the roleplay level; unless it is loaded and ready to fire, he/she has to take time just to figure out how the thing works, and even then will have an unfamiliarity penalty until enough practice or combat has gotten him/her accustomed to this new/old weapon.
That is the other means I add a touch of realism to this simplified skill list, the unfamiliarity penalty; picking up a weapon you've never seen before, even if similar enough to your own weapons that it doesn't take much time to figure out, will impose a penalty of -1 or -2 (-3 in really extreme cases) until you've gotten used to it.
As long as I can dream
...
I like to view a skill as 2 years of work experience ... about 4000 hours of accumulated skill and knowledge ... so I paint skills with a broad brush. Where I like to throw detail is into expertise ... the higher the skill, the narrower the field of expertise. I like to use nested cascades to define that expertise.
HANDGUN-1
So using the weapons skills, I would expect someone with Weapon Skill-1 to choose one of the four broad classes of weapons [Handgun, Longarm, Support or Gunnery] ... So for the sake of argument, let's say he picks Handgun-1. IMO the character has about 4000 hours of experience handling handguns - some of it at the range, some in training seminars, some possibly in combat situations. He knows enough to pick up any handgun, point it at a target and get it to fire.
HANDGUN, PROJECTILE-2
For skill-2, the character will now be more familiar with handguns and has begun to specialize. Either Energy handguns (practicing line-of-sight shooting and energy management) or slug thrower Handguns (dealing with recoil and parabolic trajectories) ... and in either case familiarity with the basic weapon system internal components (part identification - strip and reassemble). So the character selects Handgun, Projectile-2. This gives him skill-2 with any projectile handgun and skill-1 with any other handgun.
HANDGUN, PROJECTILE, GAUSS-3
For skill-3, the character will need to specialize further. At this skill level, the difference between a traditional bullet, a self propelled gyroc round, and a gauss round has a significant impact on his performance and the character can probably perform simple repairs on handguns and may begin to request special modifications based on his 12,000+ hours of experience shooting handguns. So the character selects Handgun, Projectile, Gauss-3, which gives him skill-3 with any gauss handgun, skill-2 with any projectile handgun and skill-1 with any other handgun.
HANDGUN, PROJECTILE, GAUSS, TARGET-4
At Skill-4, the character will start to break the 2D6 curve, so at this level of skill mastery the specific weapon matters. 'Gauss' is no longer good enough, so the character needs to decide wheter he has further specialized with a compact "pocket gauss pistol", or a standard "service gauss pistol", or a competition "target gauss pistol". So the character selects Handgun, Projectile, Gauss, Target-4, which gives him skill-4 with a target gauss handgun, skill-3 with any gauss handgun, skill-2 with any projectile handgun and skill-1 with any other handgun.
HANDGUN, PROJECTILE, GAUSS, TARGET, LING STANDARD-5
At Skill-5, the character is a world-class marksman and using his gun or another gun makes a difference. At this point, he has a favorite make of Gauss Target Pistol, that gives him a slight edge over other brands. So the character selects Handgun, Projectile, Gauss, Target, Ling Standard-5, which gives him skill-5 with a Ling Standard target gauss handgun, skill-4 with any other target gauss handgun, skill-3 with any gauss handgun, skill-2 with any projectile handgun and skill-1 with any other handgun. The brand stuff is just chrome ... recognition of the character's skill achievement.
HANDGUN, PROJECTILE, GAUSS, TARGET, LING STANDARD, "BLACK MAMBA"-6
At Skill-6+, the curve is broken, you can do the impossible ... as long as you have your specific, custom made weapon ... which you named "Black Mamba".
So that's how I would do it, if I were king of the universe.