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[ThunderDownUnder]

The 90 grain Sierra MKs can reach an approx max of 2900fps in my long barrel 223R. I really like this caliber and have plenty of cases, etc and am about to build a 223AI.

I'll screw a 32", 7 twist Krieger barrel onto my Barnard action and see if I can make a 6mm BR competitor out of a 223AI shooting the high BC 90gr bullets. On paper it would appear that I should reach approx 2950fps with the 90 grainers and at that velocity if the accuracy follows then I could almost match it with the 6mm BR guns. I'm guessing the 80gr Amax bullets that I currently use in my 223R would easily reach 3000+fps in the 223AI. Its going to be interesting to see what pushing the 223 caliber to its safe limit will bring about.

I would shoot the 223AI over the shooter ranges (up to 500yds) in the F-Open class and my 6.5x47L at the longer ranges. Has anyone built up a 223AI with enough freebore to shoot the longer 90 grain bullets? If you have tried this combo out I would like to hear about how the combination of 223AI and 90 grain bullets performs.

[fivering]

A couple of years ago I built an AR with a 6.5 twist Krieger barrel in .223AI, with the idea of shooting the NM course with 77's loaded to mag length and single loading the 90's at the 600.
It works very well, with the following caveats:
With the 6.5 twist, which is recommended, the Sierra's regularly came apart above 2650 fps.
The JLK 90's, which are made with their 6mm jackets, have yet to fail me at above 2800.
I find that the 80 Bergers pushed at the higher velocities obtainable with the AI case shoot right with the 90's at the lesser velocity.
My best results came with H-4350 and RE-17 and Small Rifle Magnum primers.

[ThunderDownUnder]

Thanks fivering!
I really thought that the Sierra 90s coming apart at medium velocity was just a rare event but after reading your post I contacted Sierra. I contacted their Tech Support and got the following reply from Sierra's Duane Siercks...

"We developed the bullet for the 223 with a max velocity of around 2650 fps. We recommend not exceeding that velocity by much. Pointing might help, but we have not tested that as yet. Thanks"

It looks like I have escaped problems shooting them at 2800fps in my 8 twist barrel probably because I have been pointing them. This means a re-think is necessary and like you suggest maybe the 80 grainers shot at high velocity is a more reliable solution. I wont be buying anymore Sierra 90s as 2650fps is way too low to justify the cost of the 90s over Amax 80s which shoot superbly.

After giving this some thought and looking thru Berger's on-line catalog it seems their .224", 90gr VLD has a higher BC than any of the 6mm bullets they make. Looks like I will change over to the Bergers and see what velocity I can get in a 223AI chamber. If I can get my predicted 2950fps with the Berger 90s then the 6mm boys wont have much if any advantage over the humble 224AI.

[acloco]

Don't stop at the 223 AI.  I just built a 222 Mag AI (204 ruger brass, necked up, and then fireformed)....using 80 gr AMax's at 3150.