OK, done another little test, based on Mac's RL example above.
A 7.62mm LMG, which for the purposes of this exercise I will regard as a belt fed ACR. 3D, +6 AUTO
A .50 cal MG, which here we will stat as the
Mercenary LMG. 4D, +6 AUTO.
Will treat both as having Range (Rifle), and at 500m (I will do this twice, once at Distant, -4, and once at Very Long, -2).
Skill 1, no other factors. There is no size mod in MGT, and special actions like aiming to kill balance with movement and dodging.
Target, TL6 APC, armour = 3 x TL, therefore AR18.
Firstly, at Distant (500+m).
30 attacks 7.62mm: 2 hits, none penetrate for 0 Vehicle Hits.
30 attacks .50 cal: 2 hits, 2 penetrate for 4 Vehicle Hits.
Hmmm, reasonably similar to Mac's account...
Now, at Very Long (250-500m).
30 attacks 7.62mm: 6 hits, 3 penetrate for 3 Vehicle Hits.
30 attacks .50 cal: 6 hits, 3 penetrate for 3 Vehicle Hits.
So the 7.62 does penetrate here, but this also includes much closer distances than the RL example.
In pure damage terms, just recording actual hits (as effect can only be determined by making attack rolls, and I don't have time for all that!

):
Out of 20 hits, 7.62mm, 5 penetrate for 5 Vehicle Hits.
Out of 20 hits, .50 cal, 12 penetrate for 18 Vehicle Hits.
So, can MGT small arms damage the stated armour values more than they can in RL?
Yes, arguably, but you'd have to fight a whole war to see small arms destroy a TL6 APC.
Is this worth pulling down and rebuilding the MGT damage/armour system?
Nope, especially as there are much simpler ways of fixing it, if you feel the need.
1. Bursts: the above examples get a whopping +6 damage from the auto value. If you convert that to an improved chance to hit (halve the auto value and use it as a bonus), and not as a damage bonus, the perceived problem almost totally goes away. There is a vanishingly small chance of a 3D weapon doing damage to AR18 with single shot or full auto, and hardly much better for 4D weapons.
2. Vehicle Armour cannot be reduced by projectiles from weapons using the Gun Combat skill. (this slot can be used to hit exposed gunners instead, or just bounce off armour with a satisfying ricochet noise).
3. Tauten up and hone the weapon descriptions to reflect the game mechanics as stated. (Though maybe mod some of the ranges, ie: Machine Gun, range (rifle).)
Much easier than rebuilding the whole combat system, or introducing unnecessary clunkiness with different scales (and games that use this method often suffer from this very same issue).
Simpler to introduce a couple of house rules rather than having to almost rewrite the game!