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Cartridge improvement limits
Alan Rushing
Member Posts: 9,002 ✭✭
What are the physical limits of "improving" (as with P.O Ackley) a case without suffering cartridge failures or worse.
With many of the 30-06 family of cartridges, the physical manipulations are not particularly extreme.
How far did he, or have others gone with "improving" without crossing or too closely approaching that point?
I have been tossing several things about mentally, and reading my ~ reference books, but haven't found anything that specifically addresses these questions!
If you have this information and experience, your sharing would be appreciated.
With many of the 30-06 family of cartridges, the physical manipulations are not particularly extreme.
How far did he, or have others gone with "improving" without crossing or too closely approaching that point?
I have been tossing several things about mentally, and reading my ~ reference books, but haven't found anything that specifically addresses these questions!
If you have this information and experience, your sharing would be appreciated.
Comments
Originally, mild shoulder angles and body taper worked to allow for easy extraction under heated conditions. These tapers are also conducive to easier, positive feeding also. In the example of the H&H cases, it also made for easier loading of the long strands of Cordite.
Rocky Gibbs did quite a bit of case modification for the .30-06 family of cartridges. The Gibbs cases generally featured a 35 degree shoulder and minimum body taper with a 0.455" shoulder diameter which is assumed to be the maximum for consistent extraction.
Best.
Well, regarding the example that I was using: the 7mm Rem Mag (or similar "more" modern magnums). It would seem at least at first glance that taking the case out to the minimum that you mention and pushing the shoulder out to 40. One could readily fire form with over the counter ammo and not be going anywhere near "going over the top". Would that seem so? It appears to me without knowing otherwise, that it'd be very straight forward.
I realize then, if that were so, the question then would be what if any benefit?
Who has done it, what were their results ... experiences, etc., correct?
I've a modest, but growing pile of books accumulating here in the study, living room and bedroom. Most recently to include:
Precision Shooting-Reloading Guide
Rifle Accuracy Facts
Accurizing the Factory Rifle
(I know that I have added a couple of others recently also, but haven't got them setting in front of me at the moment.)
I've in the past studied all of my reloading books and reviewed everything there regarding ballistics and the dynamics of chambers and bores.
I am attempting to get a much clearer and more definitive grasp of the physics, the causes and effects, and the relationships of one on the other. I am consuming Vaughn's work right now.
How are these for reference.
What further or additional referrals or references would you suggest?
If case capacity and bore size are constant, the potential velocity varies inversely at one half the percentage change of bullet weight.
If case capacity and bullet weight are constant, the potential velocity varies at one fourth the percentage change of the bore area.
If bore size and bullet weight are constant, the potential velocity varies at one fourth the percentage change of the case capacity.
If you look through a load manual with a calculator you see that these rules hold true. A larger cartridge in the same caliber, shooting the same bullet, will achieve a velocity one fourth the difference in case size. (If a case holds 12% more, it will get 3% more velocity.) The other two rules hold as well.
All three rules assume that the peak pressure also remains the same. That's the big key. Any time you read about a cartridge that is "improved" and suddenly gets waaaaay more velocity, there is no magic involved; the reloader is just loading to waaaaay above the original pressure levels. Ackley got most of his "improvement" that way: he just crammed in a lot more powder. He judged maximum loads by when his test guns exploded - and he blew them up by the truckload.
If you want more performance in a given bore size, buy a bigger cartridge. Unless you ignore safe pressure levels, so-called "improved" rounds get maybe 100 fps more, and most get only 50.
I know that it is the rare day that any of us picks up a "free lunch". And it is clear that the big bugaboo with comparisons of the performance of "improved" to the "un" is to do so with the chamber pressures matched ... to get a valid before and after evaluation.
Kind of like making sure when comparing vehicle performance or fuel consumption with different engines ... that the gross vehicle weight match up.
Have no means of measuring the pressures or speeds. May end up being nudged into changing that picture. Not certain what I will do regarding that ...
Do know that I will soon be doing exactly as you indicated ... calculator and reload manuals ... I can hardly wait, sure! Actually, when I get rolling with it and have the figuring underway, and the numbers start stacking up, it is gratifying. Just have to make certain that I get on task with several other things first. Plugging these numbers in though should give me a good idea of the case capacity gain and the potential for any end-benefits with velocity.
Do realize that if there were a change in accuracy, it would probably be due more to a higher quality barrel, bore and the chamber quality.
A big part of what I am looking for ... is the what? ... An idea of the actual physical parameters or limits of how much "improvement" the case, in effect, can accommodate ... before one has to actually "wildcat" rather than "improve" in order to keep things together ... literally.
I have an 7mm Rem that I will re barrel in the not too distant future. So I was looking at the prospect of making several barrel modifications during the process. Most of the costs would be the same either way. It would appear that the added costs would be the dies. (Maybe the reamers too.)
I was certain that someone has had to have done this previously ... it has been setting around too long for that not to have occurred. If so, there must be info available it would seem.
To me it appears off-the-top, that there would be little potential for much benefit with this. For way of example though I thought it staight forward.
Think that the writing is on the wall. You are both telling me "where to go"! And you have given me a useful number. Now I'd best get my schematics and the calculator and play around with it all a bit.
Appears that is the most obvious way to look into this quickly. Then I could check back to find out who might have gone in that direction and see if it ended up just being change for change sake ... a modification ... or an actual improvement.
A 308 holds 53 grains of water; a 30-06 holds 66. That's a 24% gain, so we'd predict a 6% gain in velocity according to the rules above. A 308 with a 150-grain bullet gets right at 2800 fps, so a 30-06 with the same bullet and pressure should get about 2970. Actual data shows almost exactly that. Rule three? Check.
A 308 has a bore (bullet base) area of about .0745 in? and a 7mm bore about .0633 in?- about 16% smaller. Rule two says if we shoot a smaller 7mm bullet of 150 grains weight (7-08 Rem) we should get one fourth or 4% less velocity or about 2688 fps. The manuals say the 7-08 gets just under 2700 fps with that bullet weight. Going the other way, a 308 launches a 180-gr bullet at about 2550. A 358 Win with 35% greater bullet base area ought to get 9% more or 2775. The manuals say it gets 2780. Rule Two? Check.
Using those same numbers, we'd expect the 308 changing from a 150-grain bullet at 2800 to a 180-grainer (which is 20% heavier) to get 10% less velocity. That computes to 2520, within 30 fps of the book numbers. Rule one? Check yet again.
Please check your e-mail.
Best.