I think Mick is pretty much right; there was a thread about this some time ago -
Assuming the Technics.ini parameters are the same as ships.ini
The first two values are for projectiles, ie bullets and shells
MinShotCaliber 0.00762 - the minimum size calibre that can cause any damage
NumShots 80 - the number of (the above) hits that can cause sinking damage
The other four values are for bombs, torpedoes and rockets
MinHitExplTNT 4 - minimum amount of TNT in kg that will cause visible damage with a direct hit
NumHitExpl 4 - the number of (the above) direct hits that can cause sinking damage
MinNearExplTNT 8 - minimum amount of TNT in kg that will cause visible damage with a near miss
NumNearExpl 14 - the number of (the above) near misses that can cause sinking damage
So, for example, the above craft would sink with one direct hit from a bomb containing 16kg equivalent of TNT (4x4)
But, it would take a bomb containing 112kg equivalent of TNT (8x14) to sink her with one near miss
Stock Hull Durability values sometimes seem to be quite unrealistic
For example the IJN battleship Musashi can be sunk with 2 - 3 torpedoes at stock values in the game, whereas it took something like; 14 torpedoes and 15 x 1000lb bombs to sink her at the Battle of the Sibuyan Sea
https://en.wikipedia.org/wiki/Japanese_battleship_MusashiJust to illustrate this - here are my altered Hull Durability values for the Musashi (Yamato Class) which gives the realistic number of bomb and torpedo hits before sinking.
[strength_HullEnormYamato]
MinShotCaliber 0.203
NumShots 148
MinHitExplTNT 132
NumHitExpl 25
MinNearExplTNT 152
NumNearExpl 25
Compare with the stock "HullEnorm" values, which are dramatically smaller
[strength_HullEnorm]
MinShotCaliber 0.203
NumShots 148
MinHitExplTNT 40
NumHitExpl 25
MinNearExplTNT 80
NumNearExpl 25
I think for the landing craft the min calibre should be at least 50 cal. (0.0127) or even 20mm (0.02)
I don't know the thickness of plating on these IJN landing craft but surely it would take an awful lot of 50 cal. bullets to sink one, whereas it is feasible that reasonable number 20mm AP cannon hits on the water-line could possibly cause the boat to sink fairly quickly.
Hope this helps