Thread: Relationship Between Case Capacity and Muzzle Velocity

Greetings All,
One of my favourite authors for things gunny is John Barsness who writes for Handloader, Rifle and other magazines. Around 20 years ago John did some analysis of pressure tested load data and developed a rule which states:

Potential muzzle velocity increases (or decreases) at one fourth (25%) of case capacity in cases of the same bore diameter.

I have previously used this rule to predict possible velocity for the 6.5 x 55 at full pressure based on data for the 6.5 Creedmoor and 6.5 - 284. The long running thread on the 25-06 recently wound up talking about the .250 Savage and inevitably the Ackley Improved version was mentioned together with some of the claims of fabulous improvements in performance. I thought it might be useful to apply John's rule to see what it tells us.
The .250 Savage was introduced over 100 years ago claiming 3,000 feet per second for it's 87 grain projectile. A 100 grain projectile was considered but could not achieve the magic velocity with the powders then available. Pressure was kept at 45,000 CUP due the Model 99 action. Fast forward to today and modern powders allow velocity close to 3,000 fps at this pressure. The Ackley Improved increases the case capacity by about 14% based on what I have read which works out to an increase little over 100 fps or 3.5% of 3000 fps. So what about these claims of stupendous velocity increases you ask? For a start wildcatters don't use lever action rifles so in strong bolt action rifles and good cases pressure can be increased to about 50,000 CUP which allows an additional 100 fps or a little more. This gives us a possible 24 inch barrel velocity of approaching 3,200 fps. The wildcatters of yester year clearly went well past 50,000 CUP assuming they chronographed their loads at all.
Regards Grandpamac.

as a 99 shooter I do just have to point out that when the 99 action was modified to take the 308 win family of cartridges it was lengthened and upgraded to the nominal 50,000Cu or 60/62,000psi that was the SAAMI spec for the 308 group. Vs the 45,000 Cu of the 300 Sav family.

nb. I have seen in NZ for sale 300 Sav that had been reamed out to 308. You just can't use the longer big bullet OAL 308 cartridges as they won't fit in the mag. (Lets just ignore the minor over pressure issues for now)
Z

So what happens if you use the same load in the AI case as in the original? Do you have a reduction in pressure and speed and does the sharper angle of the AI case shoulder mean less bolt thrust?

Would brass quality not be a major factor/consideration?

I know that the top brass makers (Lapua and more gucci brands like Atlas Development Group) advertise their brass as being something a reloader would buy.

Don't think Winchester or Remington would give you the time of day if you complained their brass case web/head let go on the 3rd firing, locking your rifle up or worse...

Greetings All,
To ZQLewis. I was aware that the 99 had been upgraded for the 08 group of cartridges but the action is still not the equal in strength of a modern front locking bolt action. If I had a 99, which I regrettably do not, I would likely work up to approaching factory ballistics for decent case life and trouble free operation.

To Nor-west. Firing an original load in an AI chamber produces much less velocity and also less pressure. The relationship between charge and velocity in the same chamber is quite linear. If you graph charge and velocity for a single cartridge and projectile weight it produces a straight line. A slightly larger case will produce a line that is parallel to the first but at lower velocity. Much of the extra charge in an improved case just gets you back to where you started. All sorts of wonderful things were promised from the AI case shoulder most of which have been proven to be myths. I don't think that there is any reduction in bolt thrust as the case just stretches between the head and the body and this is determined by the brass strength and thickness at this point.

To Caberslash. Yes brass quality will have an effect but the same pressure will still be there and the stress on the action will be the same. I prefer to keep substantial safety margins and only go past published data where pressures are known to be low as with much of the data for the 6.5 x 55.
Regards Grandpamac.

based on my own experience with 280ai (REM700 action, 24in bbl 1:85 twist) I think 75 - 100 fps gain is about right for the 7mm/280 comps. The exact number (or size of gain) I've found to be more dependent on selection of powder than brass. I'm getting 2980 fps with the old 162 AMAX running N560. With 2217 it was maxing out at 2920 - 2930 fps before hitting pressure signs. I've also seen claims of 280ai driving the 160 -162 pills at well north of 3000 fps .... I guess some chronies must have a feature to report "optimistic" values.

The shoulder angle has nothing to do with bolt thrust. What determines bolt thrust is the “grippieness” of the case wall. Obviously there will be a small section of the cartridge base that isn't in the chamber and doesn't grip the chamber wall.

Ackleys have a very small angle on the case wall with respect of the base. An exaggerated analogy would be to compare an Ackley case with its almost parallel case wall to a cartridge shaped more like a triangle (again. exaggerated). The Ackley will grip quicker and better than the sharper angled triangular shaped case. In fact, the flatter the triangular shape may lead to no gripping at all - obviously never a practical shape.

What the 40 degree shoulder does is minimize brass flow. After 5 firings I have yet to trim my 6.5 x 55 AI brass. There are also theories around improved gas turbulence with the 40 degree shoulder. That, I couldn’t verify.

i remember a kiwi rifle builder was running a very short barrelled ( not exactly sure on length but less than 12 inches ) .308 with 125grn. bullets at what i thought BS velocity . . sorry about the vague memory but it was a long time ago.
Anyhoo, i dropped him a line asking how he was getting these fantastic speeds . . he wouldnt tell me the powder or any other details except it totally destroying the case in 1 firing, the pressure must have been massive. Which just goes to show that some people will run on the dangerious side of insanity trying to get higher than normal perfomance.

Originally Posted by chainsaw

based on my own experience with 280ai (REM700 action, 24in bbl 1:85 twist) I think 75 - 100 fps gain is about right for the 7mm/280 comps. The exact number (or size of gain) I've found to be more dependent on selection of powder than brass. I'm getting 2980 fps with the old 162 AMAX running N560. With 2217 it was maxing out at 2920 - 2930 fps before hitting pressure signs. I've also seen claims of 280ai driving the 160 -162 pills at well north of 3000 fps .... I guess some chronies must have a feature to report "optimistic" values.

Greetings Chainsaw,
The increase in velocity using John's rule is about 50 fps in the .280AI. Handloading data from Hodgdons gives more but once again the pressure is higher so you could get half the gain by loading the .280 to the higher pressure. Wouldn't look as cool or be as trendy though. And there we have the main attraction.
Regards Grandpamac.

I've read the Ackley case lifts safe operating pressure to 67000 psi. If true, may explain why AI performs better than expected over standard case??

It does and it doesn't. Whilst bolt thrust is better controlled the limiting factor is still the brass case. In my Swede for example that's just over 55000 psi. The outcome of hot rodding is loose primer pockets after only a few firings. Plus I'm firing mine in an extremely strong action with top notch gas blow handling. Something that is variable, even in modern designs.
AIs perform better basically because the powder capacity (the size of the pressure vessel) is bigger than the standard case they are based on.
So, no free lunch.

so if you use the issue BACKWARDS..... you can run a 30/06 or 308 and 7.62x39mm or 30/30 velocities at very low pressures EASILY.... you cant go the other way.
a friend was trying to get 3000fps with .308 winchester and 150grn projectiles....I loaded up 5 different lots of powder for him at levels I WAS HAPPY WITH...but refused to load what he wanted me to do as no data to support it....suggeted he get a 30/06 if wanted to achieve goal...."nah dont want one they recoil too much" was reply......
this fella now uses a 300 magnum and reckons its the ducks nuts.....

@Wingman will come along soon with his tiny wee .224 loadings and give real world figures.

Originally Posted by zimmer

It does and it doesn't. Whilst bolt thrust is better controlled the limiting factor is still the brass case. In my Swede for example that's just over 55000 psi. The outcome of hot rodding is loose primer pockets after only a few firings. Plus I'm firing mine in an extremely strong action with top notch gas blow handling. Something that is variable, even in modern designs.
AIs perform better basically because the powder capacity (the size of the pressure vessel) is bigger than the standard case they are based on.
So, no free lunch.

The last four words say it all.
Grandpamac.

Originally Posted by Micky Duck

so if you use the issue BACKWARDS..... you can run a 30/06 or 308 and 7.62x39mm or 30/30 velocities at very low pressures EASILY.... you cant go the other way.
a friend was trying to get 3000fps with .308 winchester and 150grn projectiles....I loaded up 5 different lots of powder for him at levels I WAS HAPPY WITH...but refused to load what he wanted me to do as no data to support it....suggeted he get a 30/06 if wanted to achieve goal...."nah dont want one they recoil too much" was reply......
this fella now uses a 300 magnum and reckons its the ducks nuts.....

Which clearly shows that careful reasoning has no place whatsoever in cartridge selection. May it always be so. Expiring from boredom can be staved of a little longer.
Grandpamac.