I am having my M16 sent off to get some word done while I wait, I was going to add it to my firearm specific insurance next year but I'm thinking it may be worth adding it now

Do I physically own my serial number after I buy or after my stamp clears?

Possibly a stupid question, I am prepared to be told so

I would add it to insurance as soon as you pay for it.....  the SOT(s) who hold it do not own it, they simply posses it until your paperwork allows you to take custody.  

You paid for it, you own it... you just can't possess it until Uncle says OK.

Yeah if you paid for it, then you own it. Just can't take it home until the paperwork clears. 

To paraphrase the Patek Phillipe slogan, "You never really own a machine gun.  You just take care of it for the next generation."

But any way, sort like when you're buying a house and there is a 30-90 day period during which the prior owners are clearing out.  Usually a good idea for you to buy insurance in case the house burns down, gets flooded, gets blown off the foundation by a cyclone, etc.  IIRC from when I bought my house, the term used was the purchaser is the 'constructive owner during transfer'.

MHO, YMMV, etc.

If you paid money you own it and it is appropriate to add to your homeowner's insurance.

It's your property when you pay the seller for it. You have ownership at that point.

If you want to complicate things , there are basically 3 questions when it comes to control of NFA firearms:
1. Who is the owner?
2. Who has possession?
3. Who is it registered to?

These things all change during a transfer, but not at the same time.