I know that the 590a1 has a metal trigger guard assembly compared to the plastic trigger assembly of the 500. Are there any other parts in the 500 trigger assembly that are plastic? Is the trigger plastic too? I'm contemplating another purchase but of a 500 or 590 this time to avoid the weight of the heavy barrel on the a1. A bit concerned about durability of the parts in the trigger assembly of the 500/590. Thanks