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

powdr, my "opinion" is just reciting some mathematical relationships others have put forth. I never said you couldn't use the same powder to good effect!

Frankly, there is no such thing as a "3% slower powder." Such a statement means only that the optimum powder for the larger case has to be somewhat slower in gas generation ("burn rate") if you plan to fully fill the case. Filling the case is usually the route to maximum velocity, for it maximizes the chemical energy available to each grain of bullet mass. Whether you fully utilize that potential chemical energy depends on the burning characteristics of the powder, and burning characteristics simply cannot be reduced to a single number.

Keep in mind that 3% is a small change, less than the difference between 4831 and 4350. One might well be running the parent case nicely with a powder that's a wee bit slow for it; using it again in the blown out case might leave you with a powder a smidge too fast. Neither would be optimum, but neither would be bad. Further, it's quite possible there is no other powder that would do better in either case. Trying to distinguish such a fine difference without good quality pressure measurement tools is nearly impossible.

As for your observation, how exactly have you proven to yourself that you have the optimum powder for your blown out cases, or for the parent case, or that you've been operating each at the same peak pressure? As you point out, there is a mind numbing number of powders to try out. When it comes to optimum loads, "knowledge" is very difficult to establish.

Regardless, to get to tcp's specific question, you can bump the charge roughly 6%--but there's no guarantee. The various ways to estimate this are not in agreement. There is no "rule" for this.

2525, thank you so much for your response. I have on occasion in my younger years exhausted about every powder suitable for some of the Ackley's I've had built. Using chronograph and recoil as a means to let me know when enough has been reached or when to back off. I've always wondered if the load/s I've settled on is the best for velocity and pressure...I do not claim to have a crystal ball. For instance, for years I loaded RL19 in my 257Ackley and had good results. I recently though ran out in the powder shortage and thought I'd try something else since I have almost 30 cans of powder on my bench. I decided to try IMR 4831 on the advice of several of the fire members. It is a wonderful load and just as good if not better than the old RL19 load. The fact that the powders are not far apart on the burn chart also makes me feel OK with the change. Mr. Ackley felt like his cartridges could benefit from a 4-7% increase. I've always tried to stay around 3.5-4% to be on the safe side and have never gotten into any real trouble. I got lost in all of your math and percentages but after reading it about 10 times it made more sense each time I read it. It gave some validity to what I've been doing all of these years but left some questions in my mind as well. I'll be starting with Norma 201 in my new 338-08Ackley Improved and then maybe go to TAC or IMR 8208 or maybe even try AA 2495. I really don't know but I'll get it figured out. There's not any starting data that I can find anywhere but what there is for the 338Federal. Do you have any recommendations on powders that may work? I'd gladly appreciate any advice you may give me. I'll be the first to admit I don't have all the answers but will try and get this one going like my others. I want to thank you for your diatribe, it has caused me to do some rethinking on the subject. The real problem still is though... where does a man start...especially with a wildcat as unknown as mine. powdr

powdr, I'm sorry I have no (good) advice to give you. There is no way to calculate the optimum powder, and none of the cartridges which I load are similar to your .338-08. Were I in your shoes, I'd do pretty much as you propose. From .338 Federal load book data, pick the powder which looks best and work from that--and consider also trying a bit heavier charge of the next "slowest" powder shown in the .338 Federal data. For such a small difference in case capacity, I can't see being more complicated than that. Barsness' 4:1 rule is as good as any; use it and your chronograph.

You pointed out Ackley reckoned his blown out cases required 4 to 7 % more powder. My hunch is the upper end would be for single base stick powders, and the lower end for double base ball powders. Hodgdon has a large amount of data showing pressure for both maximum and starting loads. The trend I see is that the double base powders have a faster pressure rise with increasing charge. Like all else in internal ballistics, that's not a solid "rule;" I'm certain I can find exceptions with a few minutes of looking.

You also mentioned the numbers I tossed out "gave some validity to what I've been doing all of these years". The fact you have your fingers and your eyesight after years of wildcatting is more meaningful than my numbers --or maybe it's a testament to the quality of your guns. The numbers I gave are only coarse approximations.

Thank you 2525. I did not sleep very well thinking of the response I might receive. I will proceed w/great caution and intensity. On picking a powder for the 338 Federal as a starting point, in your opinion, which is better? A full case or the one that achieves the most velocity? Please don't say both. Yes, I have found that both seem to go hand in hand sometimes but have also found that for peak velocity many times a powder that does not fill the case such as BL-C2 will give the best velocity. It does seem like black magic sometimes and hand loading has been the most satisfying and mystifying thing I've ever done in my life. I haven't shot an animal w/a factory cartridge since about 1979. If you think of anything that might help me in my quest...please let me know. Thanks again, powdr

"... thinking of the response...": to borrow a line from Monty Python, pull the other one!

Yes, a dense packing, high energy double base ball can best a full case of single base stick, and I doubt there'd be enough air space left to make ignition inconsistent. But to compare apples to apples, there's likely a slightly "slower" ball which could fill the case and deliver a bit more speed. Or maybe there isn't: It depends on how progressive is the burn. Under test, BL-C2 can do wonders in some cases, eg. the .25-35 WCF (but now surpassed in it by CFE-223); and then there's Li'l Gun in the Hornet.

Thanks 25. I will keep you informed on my progress and what I find out. I hate like the dickens to buy any new powder since I have so much unused on my bench. I think I'll exhaust my bench supply before running out and buying new. Thanks again for your insight. powdr

No. The 25-303AI is the most improved. Ask me how I know.



Ted

How about a picture of the Improved version? powdr

I think I'll just use John's method on page 1 of the thread to load my new gun. powdr

2525,

I'm kind of interested in this part. I understand that the "burn rates" of these medium powders are close, (3031, CFE223, and BL-C2) but I also wonder about the expected pressures.

25-35 in a Mod 94, SAAMI 44,000 psi?

30-30 is 42,000 psi, I have read

223 is 55,000 psi?

I wonder if 25% more pressure changes the burning rate?

Do you find this to be an issue in the 25-35?

thanks,

Sycamore

Sycamore, "burning rate" is ambiguous. The instantaneous rate at which gases are being produced (the "rate of burn") is a function of deterrents, kernel geometry, and the current pressure. So, yes, pressure is a big factor in that sense. However, "burning rate" is more generally looked at as a relative measurement: Can you put more of this powder or that powder in the case and not exceed the pressure limit?

In this sense, deterrents and geometry are quite important. You want their combined effect to allow the afterburner to kick in, so to speak, once the bullet gets going. Once the bullet is moving along, it's leaving behind an ever increasing volume which must be filled with gases if one is to maintain maximum acceleration. The combination of case size, bullet inertia, and bullet engraving and friction all effect how fast gas production must be at any bullet position, and whether a particular powder will be optimum in its gas generation is not simple to predict. All of these factors combined affect the apparent "burning rate," and for this reason one can not construct a reliable rate table--the problem is far more complicated than a simple ordered list.

25 you're way too smart for me to be talking to. Geesh, this stuff is way over my head. powdr

powdr, don't assume too much, and don't sell yourself short.

I'm no expert on this stuff. I've read a few old books on the subject, some dating to WW-II. Looking at the various plots QuickLOAD offers helps visualize some of what's in those old books.

Sycamore, I might have an example to answer your question, but it's only one example and it might not be representative.

The .25-35-117 and the .308 Win with 168 gn spitzers are ballistic analogs. Each has the same bullet SD, and the two are quite close in bore capacity. The latter is the ratio of net case capacity to bore cross section, and it can be expressed as inches of the barrel's bore; it is to case capacity as SD is to bullet mass. If two cartridges have the same SD and bore capacity, they will, at the same pressure, ideally use the same powders, with the charges proportional.

In Hodgdon's data, ball powders are clearly the winner for the .25-35. The single base stick powders manage about 2200 fps (or a bit over), but BL-C2 gets over 2300 and CFE-223 gets a stunning 2400. Raise the pressure to .308 levels, and the sticks make a come-back, roughly equaling BL-C2, and percentage-wise they close the gap with CFE-223. At starting load pressures, the .308 data starts to look more like the order of the .25-35.

I can't emphasize enough this is only one example. Hodgdon's .308 data appears to have been shot in different barrels over the years and was likely shot by different technicians. The bullets are of different construction, and different primers were used. For such a comparison as I've made, one needs to look at the averages over a wide range of such data. I do think, though, the data would show a "burning rate" chart of slightly different order for these two pressure levels. For that matter, if you change the cartridge proportions (SD and bore capacity), you'd get another slightly different chart.

tcp, I couldn't think of two cartridges in Hodgdon's data that could be used to compare a change on the order of an AI job. Well, there's the .300 H&H and .300 Wea, but that data looks strange. Instead, I compared the .308 and the .30-06 using the same bullet, and looked at the single base stick powders. Here, the net case is about 29% more (well beyond the nominal 10% you mentioned), and I found the charge could increase about 14%. So, for each 10% of case, you get a bit less than 5% more charge. This is well below the 7.5% in the Lee "rules" and below the 6% suggested by my old copy of QuickLOAD. For two ball powders, I got more like 18% or about 6% per 10% of case increase --exactly the opposite of what I'd guessed, namely that single base stick powders would be closer to Lee's "rules" than the ball powders. Theory doesn't always match the real world! So, to answer your original question, each 10% of case will allow you about 5% more of the same powder, more or less. If you have some time and several good load books, you might compare the .300 H&H to the .300 Wea, as presented in one load book; none of my books have both in it.

2525- Thank you for your response and interest in my question . Prior to posting it, I did try to compare ratios of cartridges as you suggested - .308 vs.30'06, 30'06 vs. 300 H&H. I choose those examples because there was abundant data available and they could be compared using the same powders I plan to use in the 6mm AI round that I am loading for.

What I found was there was too much variability between maximum book loads, paricularly in the 30'06, for me to have any confidence. The difference in Sierra's max loads with a 30'06 and Hogdon's online data is is 2-4 grains of H4350 and this in turn would skew the ratio several percentage points.

AND in loading for my own 30'06's, rarely can I get to Hogdon's max without noticing the bolt lift is a little stiff, so their max does not appear to be watered down in my experience.

I appreciate all of the input thus far and, should anyone have good data for the 6mm Rem AI using H4831 and H4350 and the Nosler 95gr BT, I would appreciate it.

Thank you.

Yes, this is always true. The test equipment, plus differences in primers, bullet construction, powder lots, etc. all have an effect. In the Hodgdon data, one could at least get the same bullet in a few of the .308 / .30-06 comparisons. Being unable to control all the variables makes it difficult to extrapolate or interpolate from load book data.

For what it's worth, I seldom go much beyond the starting loads in the books, and I keep my measured speeds to those levels.