I have played around with this before and had guns jumping out of the rack, I managed to get 2 to stay but the butt of one ended up outside the rack and on the floor - the other moved a little but stayed in place.
It seems very tempramental, the weapons sometimes fall through some tables or items to the floor so they have their own properties, I am unsure if it is when they come into contact with other weapons properties (too close in proximity) if that pushes them apart more or if the weapons just don't work very well with specific items / objects.
Have you tried to rest the weapon against a wall to see if that works better ?
It is nothing you are doing wrong - I played around for a while but found no pattern to have the guns looking neat on the rack.