I am looking to build another AR15.  I build my own lowers and have a gunsmith friend who is well trained on AR's who will build my uppers.  Ineed info on the 6mm/223.

1) Optimum barrel length and gas tube length and twist rate
2) Any charateristic problem or issues with the round?
3) Can I use my 5.56 mags?
4) I plan on using the rifle for coyote and, hopefully, deer.  Is the round really capable of clean kills on deer at that range?  Duh!  I meant to say a 200 yard deer rifle.
5) what is the heaviest bullet that will fit in the AR15 magazine?

I have one in 18" and if I were to do another, which I really do want to, would also be in 18"... I suggest 18" or 20" with rifle gas length.  I've had no problems with feeding/functioning using Speer 80gr Spitzers and have taken a few deer with the load.

Yes... standard mags work fine.

As stated above, great hunting round.

Check out BWH barrels...

BHW

BHW 6x45 forum

Also known as a 6x45
If you buy a couple of tools you can bolt together the upper.

1. Depends but IMO 20" and 24" if you are shooting really long distance, Rifle length, 8 twist for me.
2. Dont know of any
3. Yes
4. What range?/Probably Yes
5. you can load a pretty long bullet in the case. I would be loading the 80 gr TTSX personally. With the reduced powder volume eventually longer bullets will lose their advantage.

You might want to read through this on longer bullets in the 6x45.
http://www.ar15.com/archive/topic.html?b=3&f=121&t=505629

Thumbs -up on the BHW barrels recently built a 16" 6x45 with a BHW barrel and really enjoying shooting  it