Home Forums Hunting & Shooting Ask The Gunwriters case capacity question

If one increases the case capacity by 10% in a given bore size, say 257 Roberts to 257 Roberts AI for example, what is the rule of thumb for the increase in powder charge? I suspect that making the case 10% bigger does not just let you use 10% more 4350.

I know Mr. Barsness has a rule of thumb for increasing case capacity relative to velocity gain of 4 to 1, so I recognize that velocity gains for the 10% case size increase should be on the order of 2.5%.

Just wondering if there is a guideline that lets me predict appropriate powder charges if the increase in case capacity is known.

Thank you.

What cartridges do you gain 10% by AI?

257ai is most improved, no?

The biggest gain on a .257ai would be the higher pressure limits.

I have read that 6mm Rem AI is increased 9% to 11% in capacity over the parent case. It should be similar for the 257 Roberts.

300 H&H and 250 Savage blow out a good bit going AI.

I would expect that moving to an AI in anything 25-30 caliber, 30-30 to 06 size case would yield a max increase no more than 2.5-5%...
Just guessing.
But AI doesn't invalidate physics.
There is NO WAY to determine the actual pressure of your load without presure testing equipment...... flattened primers, stiff bolt lift, etc ARE NOT reliable indicators of safe pressure!!!

tcp,

Here's what I do when working with an "improved" case where there's no pressure-tested loading data:

First, I measure the case capacity of the standard round. This is most accurately and easily done with a fired but unsized case. Weight the case empty, then fill it full of water and insert a bullet in the neck, causing the excess water to overflow. A cannelured bullet is easiest, because you simply insert it to the cannelure.

After removing the bullet, wipe any water off the outside of the case and re-weigh it. (The water has enough tension to stay in the case, if you don't try to knock it out.) The difference between the empty case and the water-filled case is the "functional" case capacity with that bullet.

Next, divide the capacity of the improved case by the capacity of the standard case. Let's use the .257 Ackley as an example, since you already mentioned it. Usually the capacity of the improved case with a 100-grain bullet will be around 57 grains, and the capacity of the standard case around 52. Divide 57 by 52 and we get 1.096.

Now we us the 4-to-1 Formula for calculating the potential extra velocity. Dividing .096 by 4 results in .024, which means the improved case will get approximately 2.4% more velocity when loaded TO THE SAME PRESSURE.

If we look at +P data for the .257 (not the old-fashioned low-pressure wimp data, since nobody has any idea why it's still used, or ever was), we find the top velocity loads for 100-grain bullets get around 3100-3200 fps. This means our Ackley Improved rifle is capable of around 3175-3275 fps AT THE SAME PRESSURE.

Now we can start loading ammo. Maximum loads for the standard cartridge will be perfectly safe, since the larger improved chamber reduces pressures. We start with one of these, using one of the powder producing the highest velocity in the standard case, and note the average velocity.

Hornady's latest manual, for instance, lists 49.7 grains if Superformance powder as producing 3200 fps with the 100-grain Interlock Spire Point in the .257. This is from a 22" barrel.

When we fire this load in our .257 Ackley Improved rifle (let's say it also has a 22" barrel) it won't get that much velocity, because of the larger chamber. Let's say it gets 3100 fps.

So we add more powder, a half-grain at a time, watching the chronograph to check the velocity. Eventually the chronograph will probably read around 3275 WITHOUT ANY "PRESSURE SIGNS"." I know this from using this technique with several rifles chambered for improved cartridges.

Now, we probably could continue to add powder without seeing any pressure signs, but that doesn't mean pressure isn't really high, since often they don't show up until 70,000 PSI or even more. And 3275 is plenty of velocity with a 100-grain bullet anyway.

Or, if you do have a .257 Ackley Improved and don't want to go to all this trouble, you could look up the data in any of several current sources that include pressure-tested .257 Ackley data. But the technique does work for improved rounds.

But somebody already commented that the big jump in velocities with improved rounds is due to more pressure. This is true, since most handloaders using improved rounds work up loads using "pressure signs." Since most factory cartridges are limited to 60-65,000 PSI, adding another 5,000-10,000 PSI results in considerably higher velocity.

If the factory cartridge has an even lower pressure limit (and many do) the difference between the velocity of the standard round and Improved ammo loaded with "pressure signs" is even greater. This is exactly why the .250 Savage AI gets so much more velocity than the factory ammo, since the SAAMI pressure limit is very low, and even +P .257 Roberts data is only 58,000 PSI.

But the biggie is that you can start with maximum powder charges for the standard round without any worries.

Without getting into pressure problems?

Thanks for the detailed explanation John!

Of course, we know none of this applies to the 223AI which, with a very slight increase in capacity, turns a 223 into a 22-250; if not a Swift. GD

Yes ShootinNurse. Thank you John. I've gone up to 4-5% w/o using your formula on Ackley's book recommendation and never run into any problems but my loads are right on the verge or just a smidge higher than what you have calculated. powdr

Thank you for the detailed response. I am not particularly interested in the increased velocity, I recognize that those gains are pretty trivial.

I did want to know how to extrapolate when pressure tested book data cannot be found for the powder you want to use or have on hand. Both Sierra and Nosler have good manuals with respect to the AI rounds I am loading for, but the powders they choose to publish are not as complete as those listed for the parent rounds.

John, I was specifically referring to cartridges such as the .257 Roberts and .280 Rem that have reduced pressure standards due to chambering in non-bolt action rifles. Just loading these rounds to 65K PSI in a modern bolt action rifle provides a significant jump over SAMMI. The extra powder capacity is just icing on the cake.

And I was agreeing with you!

Good information!

Thanks John.

How does Quickload compare to your mathematical calculations?

If you increase net case capacity by 10%, you can put in about 10% more powder. The trick is knowing which powder will keep you within the pressure limit. The only thing known is the new burn rate must be somewhat "slower."

If for some odd reason you want to keep the same powder in both the parent and the blown out case, that's a different problem. Lee's first edition reloading book had a table of approximations one could use to estimate this, at least for single base powders -- I don't have my notes at hand; sorry. QuickLOAD will give you an approximation, too. You might compare the maximums in a load book for the .300 H&H and the .300 Wea for a given powder and bullet weight. Hodgdon's data isn't helpful, though: It shows the smaller case outperforming the bigger one when using 220 gn bullets.

I dusted off my old PC and found Lee's rules of thumb for internal ballistics. He said they came from an older VihtaVouri data book (one he noted was notorious for hot loads). The two relationships are: 1) increasing the next case capacity by 10% while holding the charge constant drops peak pressure 13% and velocity 3%; and 2) increasing the charge by 10% in a given case will raise pressure 20% and velocity 8%. (Based on the latter relationship, I'll hazard a guess the data was taken with single base powders and used crushers for pressure measurement.)

So, for your problem (where you wish to use the same powder in the parent and the blown out case), blow out the case 10% and your pressure will drop to 87%. To get your pressure back, you need to increase the charge enough to raise the pressure 1.00/0.87, or a 15% rise. Your charge increase is, then, 10% * (15%/20%) = 7.5%.

Going through the math for velocity, your 10% bigger case will give you 2.8% more velocity. This compares well to Barsness' 4:1 rule, which corresponds to 2.5% more velocity.

Going to QuickLOAD (and my copy is 10 years old), it finds that single base powders for a .257 Roberts size case will need about 6.3% more charge to maintain pressure when the case is blown out 10%. Velocity will increase about 1.3% --well below the rule's 2.5%.

Rules of thumb and simulators are just that. It's hard to beat pressure tested data!

Lastly, the old Powley computer will let you quickly estimate new powder charges. In this case, it predicts you'll need 10% more charge but of a powder about 3% slower in burn rate. The velocity increase will be about 2.6%.

2525, Thank you for the response. This was the information I was hoping to sort out.

I've never gone to a slower powder after forming to Ackley and I now have five(5) of them. The powder in the former case has always worked well in the Improved case. I believe manuals will bear me out on this. For instance Norma 201 works well in the small cases w/the heavier than normal for caliber bullets like the 338F, 9X57, and 358W and I think it will work very well w/the new 338-08Ackley Improved that I'm having built. Besides that, how's a guy to know what powder is 3% slower now days w/so many listed so close together on the burn rate chart as well as moving up and down on that chart constantly. 2525 may be correct in his opinion but it is not born out in my experience after loading Ackley's for over 30 years. powdr