What defines this? I have a hunch its joint strength or joint stiffness under the weapon, but I just want to clarify. Also, what's the difference between the two?

If I recall correctly, a higher Joint Strength will result in a stronger bond between hand and gun.

JointStiffness is the factor by which force is multiplied when it is transferred from the gun to the actor's arm.

Its been a while since I modded but If memory serves, there's 2 variables. One is, as 411570n3 said, joint-stiffness which is, the best I can describe is 'how much wiggle-room' does it have, the other is how much force it takes to detach the object. Although, that might just be for all attachables. I tried making a bunker module out of MOs that where pinned together, and I remember that if you hit it hard enough the pieces would be moved, but not detach. and after fiddling I got them to detach before they moved.

I don't remember if they both apply to guns, but I know they applied to at least random attachables, I just thought it would be interesting to know.

Keep in mind, though, that if the joint strength of the hand's grip on the gun is higher than the joint strength of the shoulder socket, your arm will get ripped off before you let go of the gun, when the gun gets hit by craft debris, coalition revolver cannons, other actors, etc. So for the health of your operators, you should probably limit the grip's joint strength to around 150.

Also, -1 joint strength is always fun for confusing enemies.