An even simpler option:
Hard Armor has a DR 20 - this makes all but the lightest (9 points or less) hard armor safe from small arms and even heavy machine guns could only do 2 SP damage.
Mek weapons, mainly auto cannon and machine-guns, firing anti-personell ammo have AP 15 and do (max SP damage) D12 HP - this places them close to personnel weapons but the AP is slightly higher due to burst fire. They still suffer DR against hard armor.
Mek weapons firing anything but anti-personell ammo ignore DR, however they do not auto destroy the target. Instead they do normal (max SP damage) D12 HP damage. - This makes them deadly against soft targets but not overpowering. A 12.7 mm machine gun still has to shoot a few times to destroy a heavy truck, a lot of the shots go right trough doing little damage.
Personell weapons which have AP of "-" ignore DR, including that of hard armor. They do SP= HP/ 10 damage. - No change from published rules.