My son is interested in getting an AR with boltface/magazine etc dimensioned for the .223 (rather than the 30 AR or 308) but would be at least 6mm so that he could use it for deer (Washington & Oregon require minimum 6mm bore for "big game" such as deer. I'm aware of various wildcats and military development cartridges, but is there anything that approaches factory production with a .223 basic case and a bullet diameter of at least 6mm that would be suitable for deer to 150 yards?

I'm only a novice when it comes to AR platform rifles but what quickly comes to mind is the 300 blackout.
From what I understand it is becoming more common, ammo available and runs through a standard AR15 configuration just fine.
.308 diameter bullets in a .223 case is basically what it is all about.
A 6x45 would be high on my list but I'm not sure of anyone producing barrels for that cartridge so it may be a gunsmithed barrel.
Have fun with your AR!

300 Blackout is probably closest to factory. You can go more custom with 6mm, 7mm or this
https://en.wikipedia.org/wiki/6.8mm_Remington_SPC
Depends on how much you want to get custom.

6x45
Barrels and ammo are available from Les Baer last time I checked.
Will feed and function with no other modifications.
85gr Sierra Game King is my go to bullet

Also 6.5 Grendel/.264 Les Baer, and the same cartridge necked down to 6mm, called 6mm AR, I believe.

ClaMar

For the least miss and fuss go with a 300 Blackout. Shoot bullets from 110 to 220 grains. No problem to get a load good for 150 yds.

quote:

Originally posted by ClaMar:
Also 6.5 Grendel/.264 Les Baer, and the same cartridge necked down to 6mm, called 6mm AR, I believe.

ClaMar

I believe those use a larger boltface than a .223.

I just checked and Krieger makes AR barrels in 6x45

What you need is a wildcat
Rebate the rim of any benchrest cart especially the 6.5BR and 30BR to .380" the same size as the 223 rim. Problem will be reliable feeding and what mags to use.

I picked up a S.O.T.A. Arms upper for my AR15 in 7.62x39. It is totally reliable. I haven't thoroughly tested it for accuracy yet but initially it seems to pass muster. I'm using SOTA Arms magazines in it, not a bobble so far.

Ray, I like the 6.8 SPC myself but you say you are looking for something on the standard case.

For a good while before buying the 6.8 SPC I was seriously considering a 6x45/6mm-223. Then I had the opportunity to shoot some game with a .243 Winchester, including one of our blacktails. I was disappointed in the way it performed and the 6x45 doesn't even approach .243 ballistics. That changed my mind and left me convinced that a bigger diameter bullet would be better.

If I was looking for a necked up .223 caliber I'd get a rifle in .25-45 Sharps ( http://srcarms.com ). As with any cartridge based on an opened up .223 the .25-45 uses the lighter bullets available for the caliber. However, factory 87gr .25-45 ammunition duplicates original .250-3000 Savage ballistics and should be just as effective on game. Sharps also offers 70gr "BlitzKing" and 100gr "Swine Smasher" factory loads. I imagine reloading the cartridge is a piece of cake. Components are widely available - especially brass!

Here is a video of several hogs getting shot with the 100gr bullets: https://www.youtube.com/watch?v=KFrYklOUXgc

Grenadier is smart and wise. I am shocked that the 25-45 Sharps is not more popular.

My 300 Blackout is by SOTA Arms and I'm happy with it.

I've shot white tail and pronghorn with the 6X47 aka 6-.222 Rem Mag. I've not found it lacking in anyway that would make me think a .014 larger diameter bullet is going to offer any real advantages. Even if the .25-45 Sharps can hit 3000 fps with the 87 grain Speer at 200 yards it's essentially the same as a 85 grain 6mm bullet at 2800 fps out of a 6X45. I'm guessing on the velocity of the 85 grain 6mm bullet though, as I've reached the high 2900's with the 85 grain Sierra HPBT out of my 20" barrel.

Unless you are going to build from an existing AR15, as already suggested I would get a 6.5 Grendel upper. Put it on and shoot 120 grain bullets with great accuracy. This will match any 6.5 caliber round on an AR platform.

I have no AR rifles, but I have had a 6x45 Sako on a L-461 action, 18 inch LW barrel, and custom stock..Its my little English rifle!!

As to its performance on deer and antelope and some African plainsgame of which I have shot many and used it in a number of cull operations, I would say its as effective as a 250-3000 with 87 gr. bullets or a 243 wih 85 gr. bullets and that's high praise IMO..

My favorite bullet is the 75 gr. GSC or one of the old discontinued 75 gr. Barnes X HPs at 2900 FPS in my short tube. I use H-335 mostly but 4198 works well..For pure accuracy its hard to beat H-322, but you will lose about 100 FPS, but that ain't much.