Buckler:
: This small metal shield is worn strapped to your forearm. You can use a bow or crossbow without penalty while carrying it. You can also use your shield arm to wield a weapon (whether you are using an off-hand weapon or using your off hand to help wield a two-handed weapon), but you take a –1 penalty on attack rolls while doing so. This penalty stacks with those that may apply for fighting with your off hand and for fighting with two weapons. In any case, if you use a weapon in your off hand, you lose the buckler's AC bonus until your next turn. You can cast a spell with somatic components using your shield arm, but you lose the buckler's AC bonus until your next turn. You can't make a shield bash with a buckler.
"...You can cast a spell with somatic components using your shield arm, but you lose the buckler's AC bonus until your next turn."
Interesting corner case - casting and fighting are standard actions which would remove the AC bonus granted by the Buckler. I would say that the Move Action would also deny you the use of the Buckler, but that a Swift or Free action would not.
Instead of using the hand with the buckler to reload the one-handed firearm, just switch the firearm to your buckler hand (free action), reload with your non-buckler hand, and switch the firearm back to the non-buckler hand (free action).
Also, don't forget that using alchemical cartridges (including paper cartridges), reduce the reload times even further (full round to standard, standard to move, move to free; see Ultimate Combat pg. 140).