Some Suggested Changes

I'd thought I'd throw out a few idea on how the skills can be "fixed", as opposed to just suggesting they be changed in general.

Max Ammo Capacity

The foremost task here is adjusting the percentages to a point where the ammo gains are useful across all the weapon types. If it needs a boost to 8% per level, or such, that shouldn't be hard be hard to implement on an individual basis. Remember, the skill is at a disadvantage in utility when its effects are outdone by militia nanohives (because it currently gives about 1 clip at best).

Further give the benefit on rounding on the last rank. No one wants whether or not they're going to get an extra bit of ammo on the last (and most expensive rank) to be a question mark. If you can't just do +1 ammo per level on the low ammo gear, at least make sure the rounding benefits them on rank 5.

Finally, and I think most appealingly, add a percentage gain to max clip size (e.g. +5% max ammo, +2% max clip per level, etc.). This is harder to balance than simply more ammo, but I think at a low percentage it would make a useful and desirable addition to every weapon. The caveat is this: round down until rank 5, if they haven't gotten an extra round in their clip by 5, round up at that point. So, a 2% gain in clip size on a swarm launcher per level wouldn't result in an extra round till rank 5 (1 million SP or so).

On other weapons, this could similarly be both conservative but useful. If a new TAR has a 12-14 round clip (like another game's DMR), then they'd gain 1 or 2 rounds at most for rank 5. The standard AR would 6 more rounds in a clip at rank 5, etc.

I think it would give the skill desirability and usefulness beyond the more questionable value of a clip's worth of ammo to carry with you.

Fitting Optimization

If you can't adjust the skill for every weapon, consider making it a flat +1 to PG while that weapon is equipped. The skill retains its exorbitant cost, but gains a direct and appreciable benefit across the board. This makes the use of lower-tiered weapons in the class much more efficient as well.

Now, this would appear to have a side effect of actually "adding" PG to your suit on the lower tier (e.g. if you hit rank 5 with +5 PG, and equip a pistol using 2 PG). However, I think this is actually something that fits with the 6x cost you've given the skill. The person that get to use this skill at all has already hit Proficiency 4 - an extremely high bar in weapon specialization. It would cost them 2 million SP to actually reap that benefit, and it would ONLY come when they: A) Are using that weapon type. B) Are using low-tiered versions of that type. I would however limit the bonus to being in effect a single time per weapon type (e.g., no double bonus when dual wielding).

As in-universe justification, at the weapon mastery of capping a 2x Op, 5x Prof, and 6x Opt skills... you can feed the excess power from the fuel cells in your weapon back into your suit.