Given currently available powder, bullets, etc. what cartridges would you consider close to perfect bore for their respective calibers? I haven't loaded all calibers, but of those I have, I have the following comments regarding "bore" for the following cartridges:

22 BR - mildly over bore
22-250 - clearly over bore
6mm PPC - under bore
6mm BR - close to perfect bore
257 Roberts - under bore
257 AI - perfect bore
25-06 - over bore
25-06 AI - way over bore
280 - mildly under bore
280 AI - about perfect bore
7mm RemMag - over bore
308 - way under bore
30-06 Springfield - under bore
30-06 AI - under bore
30-338 (308 Norma Mag) - perfect bore
300 WinMag - mildly over bore
300 WybMag - overb ore
300 RUM - way over bore
338 WinMag - under bore
338 RUM - about perfect bore
375 WinMag - under bore

I recommend both the 22 BR and 6mm BR, 22-250, 257 AI, 280 AI, 30-338, 300 WinMag, and 338 RUM.

Regards and safe/happy shooting, AIU

You missed two that come close 260 and 7/08.

If I understand your question correctly, I would like to add:

240 Weatherby - overbore
264 Winchester - way overbore
270 Winchester - PERFECT bore

and change

300 Winchester - perfect bore


Without guns we are subjects, with guns we are citizens

Do you think .243 win is overbore?

I believe P.O Ackley himself liked the 7X57. In his reloading books he mentions that that was a cartridge he didn't feel needed improvement.

I'm not sure what the terms over bore and under bore really mean or what significance they have anyway. Is there any benefit or gain once we baptise a cartridge OVER BORE?

The over bore term seems to be received as a negative cognotation. Why??? roger

As I understand it, overbore is a loose unscientific definition of a case having more capacity than available powders can use "efficiently" for that particular caliber. But it ain't scientific. In general, I think most of us think of it as that point where adding capacity to the case gives almost no or only marginal increases in velocity.

I'm certain the topic "what is under/over bore" will create varied opinions. I've noticed as you load some cartridges you can't take full advantage of modern high-energy (HE) slow-burning powders (e.g., Re22, Vn560, Re25, Retumbo, 7828, etc.), because there isn't enough case capacity - these I would consider under-bore cartridges.

A good example is the .308 or .30-06 (even the 30-06 AI); but, the 30-338 has just enough case capacity to exploit these newer HE slow-burning powders. Yet, by the time you get to 300 WinMag, the gain in case capacity and powder consumption doesn't seem to produce much increase in performance over the 30-338. This inefficiency is clearly exaggerated by the 300 RUM and 30-378 Wby Mag - these massively over bore cartridges become "powder" guzzlers with markedly increased unpleasant kick and blast. Remember, all that powder creates kick too!

PO Ackley was essentially limited to early slow-burning powders - that is, 4350 and 4831. His opinions regarding "bore" would change, I believe, if he had access to modern powders. Nonetheless, some of his opinions still stand. For example, he thought the 25-06 AI was not much of an improvement - he was right. I can essentially duplicate anything the 25-06 AI can do with my 257 AI and with less powder and blast.

Also, "under/over bore" depends upon bullet weight as well. For example, the 30-06 AI is underbore with 200 grain bullets, but less underbore with 110 gr. bullets.

For me, I want to get top-notch performance with the minimum blast and kick. I enjoy shooting my 30-06 AI with a 180 NBT with 70 grs. of Re25 (~3100 fps in a 26" barrel), but shooting my 300 WM with a 180 NBT with 88 grs. of Retumbo (~3230 fps in a 26" barrel) becomes unpleasant, because of noticeably much more kick and blast. This effect really gets irritating when you start loading 100s of grs. of 50BMG-type powders, which I believe have less energy content.

Regards and safe/happy shooting, AIU

I think the concept of "overbore" is a very useful one. It simply means that the case is large enough in proportion to volume of the bore, that efficiency suffers.

As powder technology develops, the efficiency of overbore cartridges can improve.

Ackley asserted that the 270 is actually slightly overbore. As evidence, he shows that a 270 bullet on a 308 case will perform very nearly as well as the regular 270. (Awfully close to a 7mm08!)

So, is the 270 a bad round? You'd have a hard time selling that idea. It is obviously an extremely effective round, even though it is not as efficient at converting the chemical energy of the powder into kinetic energy of the bullet.

quote:

Originally posted by Wink:
As I understand it, overbore is a loose unscientific definition of a case having more capacity than available powders can use "efficiently" for that particular caliber. But it ain't scientific. In general, I think most of us think of it as that point where adding capacity to the case gives almost no or only marginal increases in velocity.

Wink I think what you are saying is correct.But the word efficiency here has little meaning. Case in point;I designed and built a 6mm on a full length 270 with a modest neck length and a blown out body.When I used the than inexpensive 4831 by your definition it was over bore. When I used a full case of 5010 it was another story.

If "safisticatsed" rifle men who converse and meaningfully use the term there is no problem.

I guess I just feel like an orphan because I find no meaning and am unable to participate. roger

Some I consider "balanced"

222
375 Weatherby

W/ the vast assortment of powders, the vast amount of bullets in different weights, shapes, etc., and the multitude of combinations of barrel length & twist, the terms "Overbore" and "Underbore" are not factual terms as far as Factory produced cartridges are concerned. They are merely Opinions. Now, w/ some of the amazing wildcats being produced, I can see where "Overbore" could definitely be factual.

Reloader

When you are cleanly burning 90+ grs of retumbo in a 300RUM with a 216-240gr pill in the 90%+ density range, I find it hard to call that overbore. I see it as a case made to use todays super-magnum propellants. That is my opinion, but as clean burning as these powders are today, I think "overbore" needs a definition re-vamp.

Overbore,...is overrated. I like burning lots of powder,...hell it's cheap.

Overbore is a term most applicable to discussions that are more boring than they sould be...

The relationship of Case capacity to bore cross sectional area IS interesting, however, as big cases in small bore burn out bbls more quickly than those that force less gas down the tube at high pressure and heat. They will also heat up the barrel faster and possibly change POI as the heated steel changes the vibration characteristics of the bbl.

Also, there are some POWDERS which happen to have burn rates that more perfectly match a given case capacity for a given bore, as long as the barrel length is allowed to vary enough to make good use of that powder.

These burn characteristics drive everything as far as efficiency is concerned. For example, if you look at published reloading data, and find a given powder in a given caliber which delivers the highest velocity for a given bullet weight, its probably the most efficient for that case and bullet as long as the test bbl length is adequate.

Has anyone tried calculating the ratio of the barrel volume to the case volume? Seems that for 'overbore' cartridges the ratio would be smaller than for an 'underbore' cartridge.

Using the original list posted by AIU, it might even be possible to define a range of ratios for which a cartridge/ barrel length combination are 'perfect', 'overbore' or 'underbore'.

Washougal Chris

quote:

I believe P.O Ackley himself liked the 7X57. In his reloading books he mentions that that was a cartridge he didn't feel needed improvement.

He may have had the sentiment somewhere along the way, but the 7x57AI provides velocities well above the parent cartridge.

Overbore is a transitory and unscientific state of being, mostly related to available powder burn rates and/or quickness, and somewhat subjective opinion. Ackley thought the 25-06 overbore in his day, it certainly isn't today.

And that is my opinion.

In big bores, straight and long does it for me.Like 9000 ft lbs in my 458HE. Or 20,000
in my 700.Perfection.And can't beat the effiency of straight cases,with necked cases,when 135 gr of powder gets 9000 ft lbs....Ed.

quote:

Has anyone tried calculating the ratio of the barrel volume to the case volume? Seems that for 'overbore' cartridges the ratio would be smaller than for an 'underbore' cartridge.

Bingo!

Whatever reasoning led you down that path is absolutely correct. It's called expansion ratio, the number of times the gas will expand before reaching the muzzle. High ratios give long barrel life. Small ratios don't.

A rifle is a thermodynamic engine, just like a gasoline engine. The more compressed the gas is just before the bullet exits, the hotter the gas is, the more energy goes to waste as hot gas is vented into the air, and the less efficient the engine is.

Yes and bigger bores relative to case capacity
gives bigger expansion ratio, and better
effiency.

And when it comes to comparing underbore to perfect bore(whatever that is)
to overbore you are comparing shapes and lengths
of what are in reality combustion chambers.
And each one has it own perfect powder, bullet
wt and charge combinations.So it is hard to
compare trying to use same stuff in every
chamber length and shape, for a caliber.
Because one may be perfect with what you are using and others aren't, so you label others as bad, when they just need better matched components.Ed.

quote:

High ratios give long barrel life. Small ratios don't.

Well ... I really don't think that increasing the barrel length on a .257 Wea from 24" to 32" will decrease throat erosion unless you're willing to keep muzzle fps the same with the longer barrel. If so, you can run at lower pressures, and lower pressures mean lower temperatures, and that should increase barrel life.

I believe "overbore" has best been defined as a case and bullet combination such that no powder is available that can safely (ie. within pressure limits) be loaded for a full case. For a given pressure, a full case will generally give you the best performance since you've stuffed as much chemical energy as possible into it.

Expansion ratio is indeed important to barrel life because with greater ER, you can get a given fps at lower pressure. Conversely, decreasing ER for a given pressure and fps means you need a "fatter" pressure peak, and this in turn means the throat is exposed to the highest temperatures (and pressures) for a longer pulse. This is done by using a bigger case with slower powders.

Where can one access a list of expansion ratios for various rounds?

Denton wins !!!!!

The answer indeed lies in expansion ratios.

Many may not have pondered the fact that the earliest internal combustion motor is in fact the gun chamber and bore with a bullet as piston

The "ideal cartridge" in our current ballistics model and line up has the case capacity of the 338 lapua and a bore diameter of around .366 inches. ie roughly a 8.5 to 9.3mm caliber on a 338 Lapua case.

Now if you have something on either side of ideal you can fiddle about with propellant burn rates etc.

There is a caveat to in this though, for each bore diameter there is a ideal case volume so if you go from smallest to large each caliber will have an ideal case capacity to match. Again anything falling on ieither side needs to be compensated for.

I have posted this at least 3 times before on different occasions, but here we go again:

Divide the usable case capacity, in grs water, by the bullet diameter squared. This is one of the most useful indicies in all of the internal ballistics kingdom.

EXAMPLE: 308 Win with 48 grs case capacity

48/(.308X.308) = 506 rounded.

1. This number divided into the velocity that the 308 bullet leaves the bbl yeild a number that indicates the best burn rate for the powder. 5.64 = 748, R15, Varget, 2520, etc.

2. This number times the velocity raised to the 0.22 power gives the optimal bbl length in inches. (506X3850)^0.22 = 22.6 inches

3. Barrel life diminishes rapidly as this number approaches and exceeds a value of 1000.

EXAMPLE: 264 win Mag 79 grs case capacity

79/(.264X.264) = 1133
optimal bbl length: 27.5 inches

Within the range of available propellant burn rates "efficiency" as viewed by a ballistician, does not change, because the graph of muzzle energy vs bore cross sectional area times case capacity is a smooth curve driven by the exponent 0.6 - it is not a straight line. All of the points on this curve are the "expected" ME for any rifle cartridge at a stated max pressure.

This leaves one in a quandry in defining "overbore" as everything is a point on one smooth curve or another with no obvious jumps or breaks. If your bbl is shorter than the optimal predicted, you could say that your rifle is overbore. Or, you could say that as you approach or exceed the value of 1000, you are BECOMING overbore, IF your concern is bbl life.

I personally like cartridges that yeild a value of 700 to 800 for rifles with a 24 inch bbl, and from 800 to 900 for rifles with a 26 inch bbl. There are not many offerings that have optimal bbl lengths exceeding 26 inches that do not also have bbl life problems.

quote:

Where can one access a list of expansion ratios for various rounds?

I'm not aware of such a list. The problem is the ER varies with case, bullet length (somewhat), and barrel length. ER is not a function of case alone unless you fix barrel length. I placed a Powley Computer I wrote for myself on the internet, and it computes things like ER along its way.

I think you should add the 375H&H to the "perfect" list.

I'm in the camp that finds "expansion ratio" irrelevant. It's bore diameter that is important. In standard length rifle barrels, there is not difference, but it is spurious to argue that changing the barrel length has any difference on barrel life. It just ain't so.

Expansion ratio has an effect on efficiency, of course. The longer the barrel, the lower the muzzle pressure, the more efficient the "engine". One of the reasons that I like "big bore" cartridges. Their muzzle pressure (i.e. blast) is so much lower. A 350 Rem mag, for example has a muzzle pressure in the 7,000 PSI range. My 223 WSSM has muzzle pressure in the 18,000 PSI range, my 7mag in the 17,000 psi range......

For hunting, without ear-plugs, that's a pretty substantial consideration. FWIW, Dutch.

You seem to have omitted the .358 bore. My perfect bore is the .35 whelen.

I'm surprised you list the 25-06 as overbore and 257AI as perfect bore since they are so close in capacity. I would argue that the 250 Savage AI is the perfect bore in .257...

In the real world, such considerations don't mean anything, not when there are bigger and much more important cartridge considerations to contend with, such as practical logistics.

In other words, you can design the perfect wildcat that is neither under-bore, nor over-bore -- in other words, theoretically just right. But if cartridges for same are only avialable on your loading bench back home and your ammo is seperated from you on the other side of the world, never to be seen again, guess what you have left?

A cartridge that is, for practical purposes, a "no-bore".........

AD

Sabot...

That's a very interesting post. You may have posted it before, but this is the first I've seen it. Thank you. Is there an assumption about bullet weight in the optimum barrel length calculation? I've never seen a burn rate chart that gives a number... just rankings. Where can we get the numbers?

Dutch...

This whole conversation continues to make me think... painful, but fun. What I think at this point is this: Expansion ratio governs efficiency. Overbore relates strictly to the efficiency of converting the chemical energy to kinetic energy, i.e., useful energy out divided by total energy in. Overbore systems are less efficient. A gun may be overbore because the barrel is short, or because the bore is small, compared with case capacity. Lengthening the barrel obviously increases efficiency, up to a point.

Taken that way, overbore predicts efficiency, not barrel life. Your statement of case capacity vs. bore diameter (or bore diameter squared) would be the correct predictor for barrel life.

Denton, as you probably know, there is a technical definition for "over bore capacity". Which, of course, is nearly universally ignored in the popular press and on the internet boards.

The technical definition of an "over bore capacity" cartridge is a cartridge for which a maximum charge of the slowest powder available does not fill the case.

In the vernacular, over-bore has become a synonym for "inefficient" and "barrel eater", and is probably losing it's technical meaning. FWIW, Dutch.

quote:

The technical definition of an "over bore capacity" cartridge is a cartridge for which a maximum charge of the slowest powder available does not fill the case.

I like this definition, but bullet weight must be included in some way. For example, with my 300 WM and 180 gr. bullet, I'll use 83 grs. of Re25, because I'm at 65000 psi, although I could pack more in - possibly up to 88 grs of Re25. I've hyper-packed as much as 100 grs. of H870 behind a 180 bullet and still not achieved 65,000 psi - and, the blast and recoil was unbearable. In this example the 300 WM is over bore for Re25 and under bore for H870. With a 150 gr. bullet the 300 WM might be "perfect bore" with Re25, but massively underbore with H870.

As the case gets bigger-and-bigger and the caliber smaller-and-smaller the cartridge becomes less-and-less efficient and more-and-more over bore. But, where the line exists between under and over bore, seems to vary with the individual shooter's preferences, especially as we develop slower burning powders.

I don't think there's an easy or clear end-point to this discussion.

quote:

The technical definition of an "over bore capacity" cartridge is a cartridge for which a maximum charge of the slowest powder available does not fill the case.

Hmmmm... not sure I agree with that definition at all. In my mind, the 264 Win Mag is overbore, if anything is. Yet, I'm pretty sure I could find a slow milsurp powder that would safely fill the case.

On the other hand, maybe we all have it completely wrong... perhaps it's just a shortened name for an OVERbearing person, who tends to BORE you. Politicians come to mind as an example.

quote:

Originally posted by Dutch:
...The technical definition of an "over bore capacity" cartridge is a cartridge for which a maximum charge of the slowest powder available does not fill the case. ...

Yes indeed, Dutch is 100% correct.

Hey AIU, The Bullet Weight doesn't matter because (as Dutch said) you are looking at a "max charge"(MAX Pressure) irregardless of the Bullet Weight you are using.
---

I've always thought of Perfect Bore Cartridges as those that work extremely well with a multitude of both Powders and Bullets. And I see a lot of them listed by various previous posts, plus a few that only work well with a couple of powders.

Cartridges like the 223Rem, 7x57Mau, 308Win, 358Win, 30-06, 300WinMag, 444Mar, 45-70 and cartridges similar to them in the number of Bullets and Powders that work extremely well is what comes to mind for your original question. Though the 7mmRemMag is one of my personal favorites, I'd keep it off the list due to the limited number of Powders that work extremely well in it.

.30-30
.30-06
.318 Wesley Richards
9.3x66
.375 H&H

Within their class all will kill game beyond what one should expect based solely on ballistic tables and the like. All perfect bore.

Been having a little fun, rummaging around on the web, finding what some notable folks have to say about the subject. It's a hoot! There is no more agreement "out there", than there is "in here". So for everyone's amusement, here's what I found:

quote:

Ackley:
Bore capacity is a measure of efficient powder burning in a restricted space. This burning is affected by six different factors as follows:
1. Type of powder
2. Bore-groove diameter
3. Pitch of rifling
4. Sectional density of bullet
5. Allowable pressure
6. Case shape.

That's the definition I've been using.

quote:

Hodgdon
Overbore Capacity: A common but unscientific term referring to a cartridge case that has too much case volume for its bore volume. Technically, every case can be over its bore capacity with some powder. Generally used when a case has a volume so large in relation to the bore diameter that only the very slow burning powders will give satisfactory performance.

quote:

Rabbi Mermelstein

An "overbore" cartridge is thought to be one that contains a larger quantity of propellant than can be completely consumed in the rifle's barrel before the projectile exits the muzzle. It is true that as the quantity of propellant increases beyond a given volume it will not impart additional velocity to the projectile, as the added energy generated by the greater volume of fuel will be used to move its own mass. There is a law of diminishing returns. "Overbore", therefore, would more correctly be described as "less efficient". That is, beyond a certain quantity of propellant more is needed to gain a small, disproportionate increase in projectile velocity. As a quick example, the .300 Winchester Magnum requires 31% more propellant than the .30-06 to give a 180 gr. bullet only 9.4% more velocity. These figures are averaged from loading data for the .30-06 and .300 Win. Magnum from an older edition of the Speer reloading manual than the one currently in print. There are other variables involved, especially the factors of bore diameter and barrel length, but that's the basic concept.

This definition involves unburned powder exiting the muzzle, and I don't buy that, at least in rifles.

quote:

SAAMI

OVER-BORE
A shotgun whose barrel bore diameter is greater than the SAAMI maximum for that gauge.
OVER-BORE CAPACITY
A firearm chambered for a cartridge which contains more powder than can normally be burned in that bore diameter and volume.

quote:

Ken Howell

Bore capacity is easy enough to understand with a little thinking. The term refers to the capacity of the loaded case, with a certain weight of bullet. If a caseful of Powder A would produce excessively high pressure with the lightest bullet in that cartridge, that cartridge is over bore capacity for that powder. The cartridges that were traditonally labeled "over bore capacity" could not safely use a caseful of the slowest powders that were commonly available at the time -- notably IMR-4350 and H-4831.

Very close to what Dutch was saying.

Ya pays yer money, and ya takes yer choice...

Expansion ratio, I understand. Efficiency, I understand. Those have good, generally accepted definitions. But I give up on getting a really good definition of overbore. I think it means whatever the person using the term thinks it means.

Denton, there is nothing to "get" I am afraid. You either accept the definition, and use the term correctly, or you don't.

Powley's definition (as quoted by Howell) is also described by your SAAMI quote.

The Ackley quote describes "bore capacity", not "over bore capacity". He essentially invents a new term. Bully for him, but the new term has nothing to do with the already established term "over bore capacity".

The Hodgdon definition is two fold: the same term as described by Saami and Powley (though remarkably obtusely put), and a "common use" term. Which is hardly useful. Further, their description "common but unscientific" is simply wrong. The term "over bore copacity" is scientific, the common use is unscientific. Improper use does not prejudice the scientific term.

I'll stick with the scientific terminology. The "popular writers" may be able to write a good story, but I think I'll refrain from commenting on their technical and scientific contributions. If so, I would have to try and explain "knock out factors" and such. FWIW, Dutch.

I think the Hodgdon, SAAMI, and Howell definitions have important differences.

The SAAMI definition seems to involve the idea of unburned powder exiting the muzzle, "more powder than can normally be burned in [within??] that bore diameter and volume."

Howell's definition involves including bullet size in the calculation, and none of the others explicitly do. That makes "overbore" a function of bullet, not just of case and bore volumes.

Back in the deep, dark recesses of my memory, there is an article that describes what's known as "the delta", which is the point past which peak pressure accelerates rapidly, and muzzle speed rises less rapidly. IIRC, bores to the right of the delta are overbore. Been looking for the article, without success so far.

It does seem that SAAMI, Howell, and Hodgdon are closer together than Ackley, but I don't think they agree particularly well.

What input variables do you need to establish "overbore"? Just the case volume and bore diameter? Or do we include bore length to get bore volume? Do you need bullet weight or powder speed? If so, whether you're overbore depends on more than just your gun dimensions. None of the definitions unambiguously answer all those questions, which I think you have to do to get a good definition.

Oh PLEEEAZE explain "knock out factor"!

No, no, no, no! Cases are not over bore capacity, CARTRIDGES are over bore capacity! Cartridges are a combination of powder, case and bullets (and a primer).

If a case full of the slowest burning powder available "cannot be burnt" in the cartridge (the system), it is over-bore capacity.

The definition of over bore capacity accounts for the bullet. I wholeheartedly agree that the Saami definition is ambiguous. As I read it, "cannot be burnt" should be qualified with "within pressure limits" or some such.

No dice on the knock-out factor! Or hydrostatic shock, for that matter. Nor "bullet hang time" (don't even ask). LOL! Dutch.

All the military Mauser cartridges were very well designed and just about "perfect bore". The people running Mauser before the turn of the last century really knew what they were doing.

I'd say the 7 x 57 mm is the best of the group, but the 6.5 x 55 and 8 x 57 are excellent too - and the 7.65 x 53 is better than the 308 but still not as good as the others.