I have often though about this, and it might make a interesting little wildcat. A 6x45 mm cartridge is merely a .223 case with a 6mm (.243) bullet if you are not familar with it. I read about it in my Sierra reloading manual, and it talked quite favorably about it. For a small sacrifice in velocity (for equal weight bullets, the velocities were usually better), you had noticeably more energy. After hearing all the rumurs about how the 223 doesn't do much damage to enemy soliders, I figure this would be a good solution to this problem. What do you guys think?

I too am intrigued by this loading.  Ballistics are actually somewhat better than the 5.56 loadings.

But what I keep going back to is my original attraction to the AR is cheap, available ammo.

In the 6 X 45, by the time you buy forming die, trim die, and loading dies, you are already out over 100 bucks.  Then you have to acquire brass (preferably virgin if you are going to load heavy), bullets, primers, and powder.  By the time you are done, you will be hard pressed to get close to what you can readily buy commercial .223 for.

6 X 45 will still be too small for deer, too big for squirrel and devastating on rabbit.  If it was available for close to 20 cents a round, I would have one today, though.

A wildcat I've been meaning to try is based on the 225 winchester chamber.

Basically, cut the chamber to fit the M16 mag - about the same case length as a 223, and you can use a 7.62 x 39 bolt.  You should be able to use a 223 bolt if you have small rifle primed cases and you turn the cartridge base to fit.  About a hours worth of lathe work for a batch of 50 cases.

The pluses to this are a standard reamer and dies (trimming 1/2" off the base), a maximum sized cartridge that fits the magazine, and cheap basic brass.

Stuff your magazine length 69/77 grain bullets and load them for speed.

Don't want to pee in your porridge, but you are hardly 'wildcatting' anything here!! Oly offered barrels chambered for this round YEARS ago!! I ordered my first AR that way, have since sold the barrel. Oly might still offer it?  As you are only necking up .020, you don't need a 'forming' die. You can load the same brass you have been using. Sizing up to 6mm shortens the case just a tad, no need for a trim die. I bought my barrel in this chambering because, at that time, there were a lot more choices of .243/6mm bullets than .223. With the current availibility of fast twist barrels and heavier .224 bullets, I don't think you can do much with the 6X45 that you can't do with the .224.  

Why not wait a little longer for the .243 WSSM from Winchester?  Oly has developed an upper for it already, although there's some sort of delay waiting for SAAMI approval or something.  It appears to have energies equivalent to or better than the old .243 Winchester, and supposedly the factory rounds are somewhat lightly loaded, so handloading ought to improve on that.  I suspect this round was designed specifically for the AR and varminting crowds--the cartridge appears to be the absolute maximum volume that will fit down the mag well of the AR with a pointed bullet.

From what I've read on the 243 wssm, it won't fit the AR15 system.

Case head of .555", length 1.675".  Nice idea, trying to wring out the maximum possible cartridge from the standard magazine and bolt components.

It's already been done.  Check it out:

http://www.olyarms.com/pcr8mag.html

They're using a proprietary upper to make everything fit, but apparently it'll fit on a standard lower.

They don't say what they'll use for magazines, but based on measuring a standard 30-rd, I believe it will work.  Although it might be necessary to modify the "lips" at the top, depending on how the feed works in the .243WSSM upper.

I had an opportunity to buy a 6mm PPC barrel for an AR-15 once.  Didn't do it.

Kinda half assed wondered if I should of.

Bought an AR10 instead.  Easier ammo acquisition all the way around and over comes that pesky "not enough umph" issue.

I built an AR in 6mm PPC... the problem I found while researching the 6x45 vs. the 6mm PPC was the overall length of the 6x45 loaded prohibits using the heavier bullets without seating them deeply and wasting powder space.  So I went with the 6mm PPC... went through all the hassle of building the thing (ok, that was fun!)... fireforming 6mm PPC brass from .220 russian (expensive!)...  and got a very accurate, sub 1/2 moa rifle... so, you punch a few tiny groups on some paper... gets boring fast...  you're better off with a .223 or pistol caliber carbine, cheap surplus ammo, and blaze away!

Or get a big bore like a .50 Beowulf... a friend of mine has one... awesome! (but expensive) to shoot.

Scot