Thread: Anyone played with 223AI?

Greetings All,
I did a bit of work a few years back developing a decent load for the 129 grain projectile in my standard 6.5x55 so repeated it for the 140 grain. The Hodgdon data includes pressure in CUP (Copper Units of Pressure). The pressure is kept down to around 46,000 CUP due to some of the weaker actions that the round is chambered in so what would the velocity be an a more normal maximum pressure of say 52,000 CUP? Using the increase in pressure form the start to the max load and extending that to the higher pressure produced a velocity of about 2,750 fps. No surprises here as this is what factory Norma 140 grain loads chronographed about 30 years ago. Next question was how much additional velocity should we expect from the AI case. A quick search on the internet produced a capacity of 57 grains of water for the standard case and 62 grains for the AI. I don't know if these are total case capacity to the mouth or capacity with projectile seated which is more accurate but it will do for our purposes. This amounts to an increase of about 9% but the increase in velocity is only proportional to 25% of the increase in case capacity or 2.25%. This was worked out a few years ago by John Barsness aka Mule Deer. This explains why we don't get 4,000 fps from the .300 Win Mag which has 50% more capacity than the .308. Actual velocity increase for the AI case, at the same pressure is only about 50 fps bringing us to 2,800 fps.
So how do handloaders get 3,000 fps form the 6.5 AI? The answer is more pressure and lots of it. Going back to our pressure calculations at the start pressure is now approaching 60,000 CUP using the soft Speer projectile from the Hodgdon data. Well above the hottest current cartridges. Some may choose to go there but not me. I often hear no pressure signs mentioned and wonder what is being talked about as to me there is a big fat pressure sign sitting on the screen of the chronograph.
Regards Grandpamac

Originally Posted by grandpamac

Greetings All,
I did a bit of work a few years back developing a decent load for the 129 grain projectile in my standard 6.5x55 so repeated it for the 140 grain. The Hodgdon data includes pressure in CUP (Copper Units of Pressure). The pressure is kept down to around 46,000 CUP due to some of the weaker actions that the round is chambered in so what would the velocity be an a more normal maximum pressure of say 52,000 CUP? Using the increase in pressure form the start to the max load and extending that to the higher pressure produced a velocity of about 2,750 fps. No surprises here as this is what factory Norma 140 grain loads chronographed about 30 years ago. Next question was how much additional velocity should we expect from the AI case. A quick search on the internet produced a capacity of 57 grains of water for the standard case and 62 grains for the AI. I don't know if these are total case capacity to the mouth or capacity with projectile seated which is more accurate but it will do for our purposes. This amounts to an increase of about 9% but the increase in velocity is only proportional to 25% of the increase in case capacity or 2.25%. This was worked out a few years ago by John Barsness aka Mule Deer. This explains why we don't get 4,000 fps from the .300 Win Mag which has 50% more capacity than the .308. Actual velocity increase for the AI case, at the same pressure is only about 50 fps bringing us to 2,800 fps.
So how do handloaders get 3,000 fps form the 6.5 AI? The answer is more pressure and lots of it. Going back to our pressure calculations at the start pressure is now approaching 60,000 CUP using the soft Speer projectile from the Hodgdon data. Well above the hottest current cartridges. Some may choose to go there but not me. I often hear no pressure signs mentioned and wonder what is being talked about as to me there is a big fat pressure sign sitting on the screen of the chronograph.
Regards Grandpamac

Some things to think about here, firstly stating velocity/pressure without referencing barrel length and powder burn rate is not particularly useful. Pressure in internal ballistics is not linear, you can reach higher velocities with lower peak pressure by flattening the pressure curve or "increasing area under curve", usually by using a slower powder or/and reducing start pressure. Some of the new double based powders give benefits here. Or by simply using a longer barrel.
Looking at your conclusion of 60,000 CUP to gain 3000fps in a 6.5x55ai equates to around 73000psi. I would expect to see substantial case head expansion at those levels making the case unusable in a couple of cycles. If the users running their 6.5s at 3000 are getting good case life then I would suspect the pressure would be considerably lower than 60000 cup.

Originally Posted by sneeze

Some things to think about here, firstly stating velocity/pressure without referencing barrel length and powder burn rate is not particularly useful. Pressure in internal ballistics is not linear, you can reach higher velocities with lower peak pressure by flattening the pressure curve or "increasing area under curve", usually by using a slower powder or/and reducing start pressure. Some of the new double based powders give benefits here. Or by simply using a longer barrel.
Looking at your conclusion of 60,000 CUP to gain 3000fps in a 6.5x55ai equates to around 73000psi. I would expect to see substantial case head expansion at those levels making the case unusable in a couple of cycles. If the users running their 6.5s at 3000 are getting good case life then I would suspect the pressure would be considerably lower than 60000 cup.

Greetings @sneeze,
Thanks for your reply. I don't dispute any of the points you make on the newer powders, barrel length etc. My point was that most of the gains in velocity could be made with the parent case. I also said approaching 60,000 CUP so likely closer to 70,000 PSI. John Barsness also suggested in his articles that 70,000 PSI was around the level of pressure that hot shot handloaders worked up to by looking for pressure signs. I have loaded some pretty hot loads in the 6.5x55 over the years, initially through ignorance and recently almost caught out by the significant increase in speed of AR2209 about 20 years ago. There have also been some learnings about significant changes in velocity and likely pressure between different projectile of the same weight including projectiles of the same manufacture and cat no. Now I prefer to keep a decent safety margin rather than trying for that last erosive foot per second.
Regards Grandpamac.