shadowrun-sr5 – How Do Attack Rolls Work in Shadowrun SR5?

You’re missing the two-weapon fighting combat option rules. Without those, the feat et al. won’t make very much sense.

The two-weapon fighting combat option allows you to get an extra attack when you full attack, but all of your attacks in that full attack take a penalty. The extra attack has to be with a different weapon from your regular attack (“offhand” weapon), and the penalties you take depend on whether or not that offhand weapon is light or not.

The Two-Weapon Fighting feat reduces those penalties to a point that this option is actually usable.

Another key thing to realize is that having two weapons in hand does not automatically mean you are using the two-weapon fighting option. You have to choose to use it, to get an extra attack and take an attack penalty.

As for the +6/+1 thing, that’s called an iterative attack, and that’s free just for having BAB that high. It is separate from, and can be combined with, the two-weapon fighting option. So, assuming you have the Two-Weapon Fighting feat and a light weapon in the offhand (the only case in which the two-weapon fighting option is a good idea), with +6/+1 BAB you could attack normally, for +6/+1, or you could choose to use two-weapon fighting, taking a −2 penalty. Then your attacks would be +4/+4/−1, where the second +4 attack has to be with the offhand weapon. If you take the Improved Two-Weapon Fighting feat, that becomes +4/+4/−1/−1, because Improved Two-Weapon Fighting gives you a second bonus attack with the offhand weapon, at −5 (matching the penalty on the iterative attack).

There is usually no additional penalty for a Critical Failure on attack rolls.

Per PHB p10,

Similarly, failing the check by 10 or more is a critical failure (sometimes called a fumble). This sometimes results in additional negative effects. [...] Note that not all checks have a special effect on a critical success or critical failure and such results should be treated just like an ordinary success or failure instead.

Because, as you noted, there is no Critical Failure entry for Strike Actions, there is no additional penalty without some other ability being in use.

The Fighter Feat Dueling Riposte is an example of a situation that changes this.

Trigger A creature within your reach critically fails a Strike against you.

There are also abilities baked into some Bestiary creatures (however they seem generally rare at the time of this writing). The Gimmerling (Bestiary p177) is one such example.

Trip Up [reaction] Trigger A creature critically fails a melee attack to hit the gimmerling or moves into a space within the gimmerling’s treacherous aura.