Take Michael 3 for example.  As himself with nothing in hand, you can pose his hand correctly to your hearts content. put an object in his hand and then bam!  Things get all screwed up.  For example I put Motokami_Ya's  Glock 19(gun) in M3's hand. Side to side appears to be acting correctly. But bend is now the same as twist. And Twist is still the same.  Depending on the object in hand Twist, Bend, and side to side get messed up in different ways. Its as if the hand is now using the X,Y,Z of the gun or something inbetween. Why is that. Its terribly annoying.

pjanak

Is IK chains on? Is the Glock meant to be parented?

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks

Well I would assume the Glock is menat to be held/parented. Otherwise its of little use.  some times I might use Ik. Others not.  I know you are not supposed to use both in a given animation
pjanak

Strange. I can't see why parenting the Glock (what ever a "Glock" is) to the hand would affect the rotations of the hand. Are you sure you did not do it the other way round, and parent the hand to the Glock?