Weapons skill does not affect critical chance or damage in any way. Having a 95 in a weapons skill equates to having a 95% chance of hitting a target under perfect conditions. Conditions in Fallout however are not always perfect. There are combat penalties that lower your effective weapons skill. For example a Combat Shotgun has a minimum strength requirement of 5, if you have a character with a strength of 4 they would incur a -20% penalty for using the combat shotgun, that Small Guns skill of 100% would really be only 80%. For a strength of 3 the effective skill would have a -40% penalty. There is also a penalty when shooting in the dark, an example of this can be found by targeting an enemy in a cave, then dropping a lit flare and targeting again. You will notice there is a difference. The distance to a target also affects your chance to hit, try making a long range shot with a short range weapon. All weapons have a maximum range but it is up to your perception stat as to whether or not you can hit something at that range. Low perception adds more penalties. You also have to overcome your opponents armor class. Armor class is the evade/dodge chance of the character, this number is in direct opposition to your weapons skill. There is also cover to consider, like when a target is standing behind a rock. And then there are the aimed shots. Having a Small Guns skill of 100 is fine if you are shooting at unarmored targets 3 hexes away in clear daylight, but if you are in a cave and your target is wearing combat armor behind some rock formations about 22 hexes away and you are aiming for the eyes then you will find that 100 is not enough. Having a combat skill around 120%-130% is definitely not an overkill.