Using state machines for different weapon types would be the cleanest way, I believe. This would require different animations for each weapon type. Even if you only blend the clavicles, for example, to only impact the arms and hands, you would need animations for handguns, rifles, etc. individually.
IK would help for the intersection location of the hands and guns, but I’m not sure how well it would alter the entire top half of an animation meant for a different weapon type. Though, I’m no expert.
Here’s a general tutorial:
How To Use IK To Fix Your Left Hand To A Weapon - Unreal Engine 5 Tutorial