The base recoil is determined by (30-stability)/10. The allowed stability values are (12.5 and 17.5 shown as 12 and 17 in the inventory):
0, 3, 6, 8, 12.5, 15, 17.5, 19, 20, 20, 21, 22, 23, 24, 25
Therefore a weapon with the maximum stability has a base recoil of 0.5, whereas a weapon with 0 stability has a base recoil of 3.
The plateau at 20 stability means that you depending on your current index, using a weapon mod adding or subtracting 1 to the index may have no impact whatsoever. This plateau makes it rather easy to illustrate the mismatch between what mods claim to do and their actual effects. The Commando 553 without mods has a stability of 20 (the lower one). Mods adding +1 to the stability index will still result in the same stability value, so the inventory does not state that such mods affects stability. Mods increasing stability by more than +1 similarly show 1 less. If you have not attached any other mods to the gun, this is indeed what happens. However, the claimed effects of weapon mods do not consider already applied mods. If you have increased the stability index by +1 with other mods already, the stability will be increased by 1 for every further index shift of +1, though the individual mod keeps insisting that it increases stability by 1 less. Thus adding the Tactical Stock, which seems to do nothing but reduce concealment, in fact adds +1 to the stability index. Unmodded weapons with 20 stability are the only case where such latent effects exist, all due to this plateau. Damage, accuracy, etc. may not always give the right numbers, but they do always give something to quantify the actual effects.
Source: The Long Guide