Pointing at an object and holding 'A' universally means "grab" in the current full motion controls. When you have a weapon equipped to a hand and you aren't pointing at something that can be grabbed, holding the 'A' button "arms" the equipped weapon. You keep holding down 'A' as long as you want the weapon "armed".
Here is how you would use a pump-action shotgun in full motion:
Realize that a pump-action shotgun requires two hands to use. We are using the Wii MotionPlus in this example. First you'd equip the shotgun to your dominant hand and hold 'A' to arm it. While the shotgun is armed, the pointer for the dominant hand becomes the gun's targeting reticle.
Now that the shotgun is armed, you point at the shotgun's pump with your off-hand. The pump on the shotgun counts as a "grabbable object" so you hold 'A' with your off-hand to grab the pump. Now you can actually fire the gun. Additionally, since your dominant hand is already holding onto the shotgun, the off-hand's pointer disappears since it serves no purpose at the moment. Likewise, it doesn't matter where the off-hand wiimote is physically as the game only cares about sliding the shotgun pump.
To load the shotgun, you have to pump the gun by physically pulling backwards and forwards (in that order) once with the off-hand wiimote. Then you fire the gun with your dominant hand by (while holding 'A') tapping 'B'. (which is conveniently shaped like a trigger) You pump the gun again to reload.