using @ReinMcDeer [weapons balancing database V2] to convert guns caliber to damage, then converting damage to hp.

WEAPON

Weapon damage & impactForce =
(x)mm x 2 = (result) damage &impactForce

• 30mm in real life
  30mm x 2 = 60 damage & impactForce

• 20mm in real life
  20mm x 2 = 40 damage & impactForce

• 12,7mm (0.5 in) in real life
  12,7mm x 2 = 25.4 damage & impactForce

• 7,7mm (.303 in) in real life
  7,7mm x 2 = 15.4 damage & impactForce

ARMOR

1cm = 10mm
8,8cm = 88mm
2cm = 20mm
3cm = 30mm
0.5 in = 12,7mm

Armor thickness in (x)mm = (x) hp

Metric:
Normal = 100 hp
1,25mm = 2,5 hp
2,5mm = 5 hp
5mm = 10 hp
10mm = 20 hp
15mm = 30 hp
20mm = 40 hp
25mm = 50 hp
30mm = 60 hp
35mm = 70 hp
40mm = 80 hp
45mm = 90 hp
50mm = 100 hp
100mm = 200 hp