I have a side-by-side 12 gauge with full choke barrels (no removable choke tubes), and I have a 12 gauge Over/Under with 5 removable chokes (one is for Cylander bore, and the other 5 a designated by stars).

I bought both of these guns for skeet/pheasants, but now I am wondering if I could use these guns for defense if I had to. Could I use say .00 buck shot in either of these guns?

I think you could use buck, although it will act a lot like a slug. Why? Thats only two rounds to get it right. Don't you have anything better?

I have a bunch of other guns that would be better, I am just wondering if my double barrels can be used for defense in a pinch.

The quesiton is more academic than  anything else.

In a pinch it would beat surrender. If you have a choice, not a good one to pick. I was wondering if perhaps you were worried about the legal ramifications of shooting someone with a military weapon or handgun in MI.

Nope. I have a CPL and carry a handgun for defense at all times.

With enough practice, a side by side with selective ejectors, and some handy way of keeping extra ammo handy, you just might be very surprised at what can be done with a good short barreled double. Learn to hold two extra rounds between the fingers of the support hand and load with it...I have seen money lost and won on who could get four rounds off fastest...the guy with a pump or auto plugged to three round capacity (no Browning A5s or Double Autos, please) or the guy with a double. The guy with a double if he is good, will win every time! Beretta actually made a very sexy little 18" OU for a few years for defense wirk. (Wish I'd bought one at $300 back then) but they kicked like Hell!

Told ya mike new everything.

This thread reminds me of my first shotgun... a .410 single shot, with an ejector I carried from 7 years old until I got my 12 at age 12, so I guess I got 5 seasons out of that thing.

I carried 2 rounds in my left hand (between my hand & forend).  I could run those 3 rounds through pretty quick.  My glory moment of all time was dropping 2 quail from the same jump.  I've never done that again w/ a pump, auto, or otherwise.