Skeletal mesh flicking as long as there is a phisical asset

Hi I’m recently working on a turret defend prototype and I’m using the battleships main turret as the stationary weapon.
In 3DSMAX I make the turret rotating base, turret itself and three gun barrels separate object.
I didn’t use bones, instead I use the old “link tool” and when I import as skeletal there is still hierarchy in UE4(I’m using the unreal tournament editor actually, but I think they are same)

This is the schematic view of my stationary gun

However, the UE4 online manual says that traditional collision models start with UCX are not applicable to skeletal mesh. So I have to create physics asset. However, as long as I create one, the skeletal mesh start to flickering. It is flashing like this

This problem is really horrible and I don’t know how to deal with it.
I tried to re-import like this ( but the problem is still there.

Could you help me?