Armor is a very tricky thing to feature in a game like Traveller… The relation among accuracy, stopping power (damage) and penetration capability is always a difficult thing to represent.
Usual ways to approach it are it either gives you DMs to hit, so making you harder to be hit, but if hit full damage is received (CT approach) or reducing damage (MgT approach), each one with its own issues.
(Disclaimer: I will not give a solution here, just pointing the problems on each one)
CT approach (DMs to hit):
While easy, it doesn’t represent the fact armor many times reduces the damage more than fully avoiding it. It also makes good penetration weapons accurate, something that is not always true.
To give an example, let’s imagine you are confronting two enemies, one in vacc suit (cloth, for armor effects) and the other in Combat Armor. You have a snub pistol with HEAP rounds. If you shoot at the one with vacc suit, you’re quite likely to hit him, and so doing full damage, while if you shoot at the CA equipped one you’re quite unlikely to hit him. Whie this may seem logical (he’s more difficult to damage), if you miss the round goes elsewhere, producing collateral damage on the bridge equipment. So, shooting at the CA equipped enemy is more likely to produce this collateral damage than if you shoot at the vacc suit equipped one, while both should be equally easy to hit. OTOH, if you, due to other DMs, hit the one in CA, the damage will be the same than if you hit the one in vacc suit…
On another approach, the possibilities to hit are the same, so being in CA does not increase the probability of equipment damage, as does if this one is used.
Another effect is that if you have enough good DMs due to skill, stats or anything, you can damage a BD equipped person by throwing stones at him…
MgT approach (armor avoids damage):
While also easy, this uses to confuse armor piercing capability with stopping power…
So, a shotgun (high stopping power) uses to be a good weapon against armored people, while the shotgun pellets are quite easy to be stopped by armor (more so if the armor is full, leaving no exposed parts, as a vacc suit, CA or BD). Of course, you can give weapons an armor multiplier (as in T2K first ed), reducing this effect…