Things are shaping up real well with respect to weapon balance thus far, we need one last pass to inspect ammunition however, if I may say so. Particularly:
Armour Piercing (AP) VS HEAP VS APDS
First off, APDS should be without a doubt available to rifles. A few Rifles today use APDS ammunition, and Gauss rifles make perfect sense to use that Ammo too.
Now, Value comparison:
Armour Piercing, TL6, adds 1 "level" of AP to the weapon. Level being as much AP as the dice of the gun firing.
HEAP, TL7, adds 1 die of damage AND 2 levels of AP to the weapon. 1 die of damage is functionally 3.5 AP on average. The assumption here is the AP value you get is based on dice BEFORE the +1d to damage. Otherwise, APDS is simply inferior 100% of the time.
APDS, TL7, adds 3 levels of AP to the weapon.
Example A - 2D Weapon:
3D AP4 (HEAP)
2D AP6 (APDS)
Example A - 3D Weapon:
4D AP6 (HEAP)
3D AP9 (APDS)
Example B - 4D Weapon:
5D AP8 (HEAP)
4D AP12 (APDS)
Example C - 5D Weapon:
6D AP10 (HEAP)
5D AP15 (APDS)
Example D - 10D/1DD Weapon:
2DD AP20? (HEAP)
1DD AP30 (APDS)
Example E - 20D/2DD Weapon:
3DD AP40? (HEAP)
2DD AP60 (APDS)
We have two issues to address then:
a) HEAP, is clearly the better choice in almost every circumstance. HEAP has better max damage, better average damage on small arms, and barely noticeable disadvantage on 5D weapons and above. Then you hit DD weapons and HEAP just flies away with it
b) +1D on DD weapons, and AP with DD weapons.
I don't mind if +1D works with DD in the same way. Logical and simple. Same with DD and AP. 1DD = 10 AP.
Proposed Solution:
i) Clarify HEAP AP bonus is based on the BASE dice, before the bonus +1D.
ii) Change HEAP to SET (not add) the AP value of the weapon equal to it's base dice. I've been playing around with many models - I've tried using just (i), but HEAP is still king. I've tried even using (i) and saying HEAP adds just 1xAP- but with inherent AP (Gauss and so on), it's still better than APDS.
There really is no way around it, unfortunately as HEAP is now, it makes APDS obsolete.
With the (i) and (ii) in place, we have the following:
Example A - 2D Weapon:
3D AP2 (HEAP)
2D AP6 (APDS)
Example A - 3D Weapon:
4D AP3 (HEAP)
3D AP9 (APDS)
Example B - 4D Weapon:
5D AP4 (HEAP)
4D AP12 (APDS)
Example C - 5D Weapon:
6D AP5 (HEAP)
5D AP15 (APDS)
Example D - 10D/1DD Weapon:
2DD AP10? (HEAP)
1DD AP30? (APDS)
Example E - 20D/2DD Weapon:
3DD AP20? (HEAP)
2DD AP60? (APDS)
As you can see - HEAP and APDS are now both viable, and meaningful choices with strengths and weaknesses, rather than just "Pick HEAP, all day every day!"
Armour Piercing (AP) VS HEAP VS APDS
First off, APDS should be without a doubt available to rifles. A few Rifles today use APDS ammunition, and Gauss rifles make perfect sense to use that Ammo too.
Now, Value comparison:
Armour Piercing, TL6, adds 1 "level" of AP to the weapon. Level being as much AP as the dice of the gun firing.
HEAP, TL7, adds 1 die of damage AND 2 levels of AP to the weapon. 1 die of damage is functionally 3.5 AP on average. The assumption here is the AP value you get is based on dice BEFORE the +1d to damage. Otherwise, APDS is simply inferior 100% of the time.
APDS, TL7, adds 3 levels of AP to the weapon.
Example A - 2D Weapon:
3D AP4 (HEAP)
2D AP6 (APDS)
Example A - 3D Weapon:
4D AP6 (HEAP)
3D AP9 (APDS)
Example B - 4D Weapon:
5D AP8 (HEAP)
4D AP12 (APDS)
Example C - 5D Weapon:
6D AP10 (HEAP)
5D AP15 (APDS)
Example D - 10D/1DD Weapon:
2DD AP20? (HEAP)
1DD AP30 (APDS)
Example E - 20D/2DD Weapon:
3DD AP40? (HEAP)
2DD AP60 (APDS)
We have two issues to address then:
a) HEAP, is clearly the better choice in almost every circumstance. HEAP has better max damage, better average damage on small arms, and barely noticeable disadvantage on 5D weapons and above. Then you hit DD weapons and HEAP just flies away with it

b) +1D on DD weapons, and AP with DD weapons.
I don't mind if +1D works with DD in the same way. Logical and simple. Same with DD and AP. 1DD = 10 AP.
Proposed Solution:
i) Clarify HEAP AP bonus is based on the BASE dice, before the bonus +1D.
ii) Change HEAP to SET (not add) the AP value of the weapon equal to it's base dice. I've been playing around with many models - I've tried using just (i), but HEAP is still king. I've tried even using (i) and saying HEAP adds just 1xAP- but with inherent AP (Gauss and so on), it's still better than APDS.
There really is no way around it, unfortunately as HEAP is now, it makes APDS obsolete.
With the (i) and (ii) in place, we have the following:
Example A - 2D Weapon:
3D AP2 (HEAP)
2D AP6 (APDS)
Example A - 3D Weapon:
4D AP3 (HEAP)
3D AP9 (APDS)
Example B - 4D Weapon:
5D AP4 (HEAP)
4D AP12 (APDS)
Example C - 5D Weapon:
6D AP5 (HEAP)
5D AP15 (APDS)
Example D - 10D/1DD Weapon:
2DD AP10? (HEAP)
1DD AP30? (APDS)
Example E - 20D/2DD Weapon:
3DD AP20? (HEAP)
2DD AP60? (APDS)
As you can see - HEAP and APDS are now both viable, and meaningful choices with strengths and weaknesses, rather than just "Pick HEAP, all day every day!"