Will this overload the head or make it run twice as long going from 1x123 battery to 2x.

It will overload it. The M300 head is designed to run on 3v, the M600 is 6v. I've heard of 3v Surefire LEDs being burned out just by using rechargeable RCR-123s that are 4.2v fully charged.

Thank you.

Ok..sorry again with the questions.

If you were to reverse that and put an M600 IR head on a M300 body, will that be an issue?
It's the KM2 head.

Hmm, that I'm not sure about. It would either run it at reduced lumens, or more likely, not work at all.

You could always buy a conversion head that uses something like a CREE Q5 or XM-L; those are usually rated for up to 18v (you'd need to double check the specific one you're buying). It might not be as perfect as a Surefire bulb, but it depends on which you buy. The CREE 1000 lumen XM-L bulbs I've gotten have been absolutely flawless, with no shadows or beam imperfections, and a great mix of throw and spill. Some of the 350 lumen CREE Q5 bulbs I've gotten had shadows/imperfections in the center of the beam. With a conversion head, you could put an M300 head onto an M600, and most likely an M600 onto an M300. Another benefit to conversion heads is that you can use rechargeable RCR-123s, or a single cell 16650 to replace 2 CR-123s, since those bulbs are rated for much higher voltages than a standard SF LED.

What are you trying to accomplish, exactly? I don't mean that antagonistically, just a solution is a lot easier to advise if the goal is clear.

Just tried this on my M300 and M600.

The M600 head on the M300 body didn't even light up.

So...in trying to be a cheap ass--I've been buying individual parts of the lights (opposed to whole kits).

I bought the larue mounts both separate (got great EE deals)
I bought a M300 Body & M600 Body separate (EE great deals)
I bought a KM2 & M300 Head separately (EE great deals)

I'm now attempting to mix and match all the parts together to make (2) fully functioning lights.
I'd like the KM2 on my KAC 10.5, but it works out better with the KM2 head for IR/NODS stuff.
There's also the issue of tan & black parts mixing and how to make that work.

It's mostly just a question and my own knowledge also. I didn't want to plug it in and fry it..
Thanks for input.

I tried it and it did fine, but it wasn't very bright. Definitely dimmer than the M300 head on the M300 body.