Does the gun.dts even have collision?
I don't remember if only bricks have the "setColliding()" method or not, but you could try this:
%static = new TSStatic()
{
shapeName = gunItem.shapefile;
}
%static.setTransform(%x SPC %y SPC %z);
%static.setColliding(true);
MissionCleanup.add(%static);