Overbore, a working definition...

[IdahoSharpshooter]

A member named John L has an interesting theory on the front page about overbore. Well thought out, and well presented.

That said, I have to disagree. Thirty years ago I did some research on the topic. My results, although limited to three or four cartridges I worked with. By his definition all were way overbore.
My results, however, contradicted some of his theories.

A better working definition has to take into account the purpose of the wildcat case.

Example 1: I built a 22-378 Wby AI. I was friends with Jimmy Knox, founder of JLK Bullets. I conned him into making the 80gr VLD that Sierra copied. I was able to shoot them close to 4500fps with a lot, a LOT of RVO-72.

I also built a 1:8 twist 22-284. Shot the same bullets 3600fps with a lot of RVO-62 (iirc, it's been 25 or 26 years now).

I am going to build another 22-284 based on the new Sierra MK.

My definition, for what it's worth: overbore is a cartridge you can't find a slow enough powder to shoot heavy for caliber bullets faster than anything else out there.

works for me...

Rich

 

[ShootDots]

Here is how I define "overbore".. I will use some examples: Take a .458 Win., it is basically a "straight walled case" with no shoulder. Even though it shoots a 500 grain bullet, it does so with faster burning powders. It would be the OPPOSITE of an overbore. Start putting shoulders on cartridges with the cartridge bodies wider than the case neck, and you start down the road of "overbore"..Example: A .460 Weatherby shoots the same bullet as the .458 Win. but it's body is wider than the neck and it carries a shoulder. It shoots considerably slower powders than the Win. Does than make the .460 an "overbore"? I don't know, but it is MORE of an overbore than the Win. is, if you would consider the Win. an overbore at all>>>>which I would not.. Take a .260 Rem., it uses slow"ish" powders. Now make it into an Ackley, it CAN use a touch slower powders than the "non" Ackley version, and burn them efficiently. Does that mean it is MORE "overbore" than the regular .260? I don't know. But I believe ANY case that has a "noticeably" smaller neck than the body and has at least say a 28 degree shoulder, would be, in MY opinion an overbore cartridge. If you go W-A-Y overbore, like the cartridges you are talking about or even a 6mm x 284, which is VERY overbore, uses VERY slow powders to drive "heavy for caliber" bullets at extreme velocity for the bullet weight, these would be considered classic examples of overbore. So I tend to use the term as "more" or "less" overbore than a similar cartridge and will use appropriate powders for that application.

 

[JRS]

The relationship between powder capacity and barrel bore diameter.

 

[fredo]

Overbore, schmoverbore...
While many shooters may consider that label a deterrent to trying a particular cartridge, others might think of it as a prerequisite...

Hallelujah & pass the Retumbo!!!

On a serious note, I agree with Shootsdots assessment of "overbore" being used as a comparative, rather than a hard set definition...

 

[riflewoman]

If you think of a gun as a single action piston engine it may be easier to put into perspective.

Expansion ratio is analogous to compression ratio. Most gearheads know that engine efficiency is better with higher compression ratio. And when compression ratios lower than 6, thermal efficiency is very poor. So overbore is 6 or below expansion ratio.

 

[IdahoSharpshooter]

well said riflewoman and Fredo!

 

[ebb]

Who cares what you call it! Would you rather drive a Porsche or a Yugo. Even if a Yugo would last forever it is still a Yugo. Rebarrell my 6.5-284 or my 7RUM as a 308 I don't think so. I sooner shoot a savage with the worst crap trigger they can be had with, oh that's all you can get on a savage is a crap trigger.

 

[Dusty Stevens]

Is a 22/338lm overbore? How bout a 17 cheytac? 375 h&h or a 22/375h&h?

 

[mikecr]

This, with respect to best LR bullets in cal, and of course shifted by barrel length and powders available:

 

[MrMajestic]

Fredo, that Retumbo is pretty fast. Pass the Kingsford!

 

[M-61]

If you think of a gun as a single action piston engine it may be easier to put into perspective.

Single stroke engine. One direction for the piston (bullet) out. Hopefully not returning.

 

[snert]

A member named John L has an interesting theory on the front page about overbore. Well thought out, and well presented.

That said, I have to disagree. Thirty years ago I did some research on the topic. My results, although limited to three or four cartridges I worked with. By his definition all were way overbore.
My results, however, contradicted some of his theories.

A better working definition has to take into account the purpose of the wildcat case.

Example 1: I built a 22-378 Wby AI. I was friends with Jimmy Knox, founder of JLK Bullets. I conned him into making the 80gr VLD that Sierra copied. I was able to shoot them close to 4500fps with a lot, a LOT of RVO-72.

I also built a 1:8 twist 22-284. Shot the same bullets 3600fps with a lot of RVO-62 (iirc, it's been 25 or 26 years now).

I am going to build another 22-284 based on the new Sierra MK.

My definition, for what it's worth: overbore is a cartridge you can't find a slow enough powder to shoot heavy for caliber bullets faster than anything else out there.

works for me...

Rich

I KNEW Fredo would like that post!!!

 

[Laurie]

True over bore-capacity is where the increase in case volume and powder charge gives no increase in velocity. or even as can happen, a decrease.

However, more practical considerations say it's when the ratio of case capacity / charge weight is such that it produces a grossly inefficient return in terms of ME per 1.0gn of charge weight, and/or barrel life is so reduced as to make the rifle barely usable.

The problem comes in deciding where the boundaries lie, as firstly propellant technology keeps changing and improving and in recent years far, far slower burning grades that still behave well have become available making cartridges viable today whose characteristics are such that nobody would have designed them a generation ago. P.O. Ackley says somewhere in his two volume handbook that IMR-4350 is the slowest powder available and that there will never be a need for anything slower burning. He goes on to say that it is in fact so slow burning that there is hardly any use for it. (A comment that surprised me a lot when I read it as surplus H4831 must have been available when Ackley was writing this and there were already many quite commonly found cartridges around that could use IMR-4350 efficiently, .30-06 for one with any bullet of 180gn and heavier. I think many in Ackley's generation saw IMR-4895 as the norm and as .30-06 and similar could produce excellent results with nearly every bullet weight with IMR-4064 / 4895 / 4320, they simply considered heavier charges of a slower burning powder as wasteful.)

Rather later than Ackley, Dr. Geoff Kolbe in his book on ballistics looks at this subject in his section on internal ballistics. I can't remember the methodology now, but there is a sloping graph line that is based on the 7mm Remington Magnum as being the embodiment of the ratio where over bore capacity kicks and it is placed on a central position on the line. Everything else is on either side as below or above depending on their size and how much they diverge from the 7mm RM's ratio in either direction. This was the 1980s - I imagine few would define the 7mm RM as particularly over bore-capacity today.

So where there is a powder that can produce an MV/ME increase for a particular bullet in a particular calibre seated in increasingly high volume cases cases, you can argue that if there are sufficient people around who are prepared to pay for that increase in powder costs, recoil increase, and barrel life reduction, then it is not over that bore's capacity. It becomes a personal choice - I might reject it as 'stupidly inefficient' but my friend might take the opposite view.

In terms of efficiency, the problem is that whilst it is easy to calculate ME in terms of ft/lbs per grain of powder, all this tells you is that the .22 Short is the most efficient cartridge 'out there', and that as any increase in case to bore ratio reduces efficiency, we don't need anything bigger - but that rather overlooks the purpose of firearms to hit things potentially a long way away and/or kill rather large animals (and people when you get into military design) and that a minimum set of external ballistics capabilities is needed to fulfill those roles. That's how the military work when designing a new weapons system (or should but they've tended to lose the plot ever since NATO appeared). The users define the capabilities (ie maximum effective range and retained terminal energy), the designers choose a calibre / bullet weight and put together a minimum internal ballistics requirement to get there and design a cartridge to suit. The rifle designer then builds the weapon around it. Simples! Only it's not because of all the incompatibilities and required compromises! Nevertheless, a sensible approach produces the smallest cartridge that does the job and that simplifies the weapon designer's job as it reduces size, weight, barrel heat, and recoil forces to be catered for. If that had been done in the late 40s / early 50s, NATO would have adopted the 7X43mm (280/30) British as it had been designed in just such a fashion and not the 7.62 NATO that was too big and powerful.

When you get into recreational firearms, it's probably only those cartridges designed for the big (dangerous) African four, and especially elephant, that use that approach. However, read Terry Wieland's books on Dangerous Game Rifles and Cartridges and he is very critical of modern US cartridge design - it has gone to too small a case / charge and high-pressure formula in his view risking temperature induced pressure increases and related problems such as hard extraction. Traditional English dangerous quarry cartridges deliberately employed over-large cases with low fill-ratios and peak pressures for those very reasons and if they needed longer actions to accommodate them than a Winchester Model 70, so be it.

So far as everything else goes, it comes back to the user's (ie buyer's) perceptions. If a huge cased cartridge that doubles the size of the powder charge to get an extra two or three hundred FPS MV (eg 260 Nosler v 260 Remington) turns enough examples of him or her on and they are prepared in large enough numbers to pay the extra for a barrel burner that kicks like a mule, then that's what the market will provide whether it's an efficient way of doing things or not. (I say he / she in this, but I bet there are few she's around who lean this way - it'll be a male purchase decision 99% of the time.)

What it also does is to create an endless source of fascinating discussion among shooters, whether field or target, as to 'the ideal calibre / cartridge' for whatever purpose. Look at the variations you see in club F-Class matches, everything from 6mm BR with 105s to .300 RUM with 230s in some matches I've shot in. We have to make a personal decision on the best mix.

 

[mikecockcroft]

That was a 2 beer post

 

[Tommie]

A member named John L has an interesting theory on the front page about overbore. Well thought out, and well presented.

That said, I have to disagree. Thirty years ago I did some research on the topic. My results, although limited to three or four cartridges I worked with. By his definition all were way overbore.
My results, however, contradicted some of his theories.

A better working definition has to take into account the purpose of the wildcat case.

Example 1: I built a 22-378 Wby AI. I was friends with Jimmy Knox, founder of JLK Bullets. I conned him into making the 80gr VLD that Sierra copied. I was able to shoot them close to 4500fps with a lot, a LOT of RVO-72.

I also built a 1:8 twist 22-284. Shot the same bullets 3600fps with a lot of RVO-62 (iirc, it's been 25 or 26 years now).

I am going to build another 22-284 based on the new Sierra MK.

My definition, for what it's worth: overbore is a cartridge you can't find a slow enough powder to shoot heavy for caliber bullets faster than anything else out there.

works for me...

Rich

This is it EXACTLY. Everything else is a discussion of relative efficiency.

 

[fredo]

I can appreciate the 'case efficiency' discussion, but sometimes a shooter just wants to 'blow the doors off' somethin'...

Case in point, a comparison of 'efficiency':

Which one ya think is more fun to drive?

Like I said, it's not that I can't appreciate an 'efficient' cartridge design, heck I shoot a few. They have their place. It's just that sometimes shooters consider, and then outright dismiss, 'efficiency' in favor of a level of performance that can otherwise not be achieved...

It's easy to marginalize a couple hundred FPS on the top end. But with today's high b.c. bullets, those extra few hundred FPS can transform "ho-hum" performance into "ho-Lee-chit!" performance. If that kinda top end performance it's the goal, to heck with barrel life and powder burn! In other words, being "stupidly inefficient" translates directly to a stupidly high level of enjoyment...

Just think if you were driving that Kia, and that P1 rolls up next to ya at a stop light. What, if anything, would cross your mind???

1. "Boy, that crazy car must burn a lot of fuel, sure glad I have this 'efficient' car to drive!"

Or...

2. "Hole-E-chit, that thing is a badazZ, wonder how fast it goes!?!"

If you find yourself asking #1, you might want to take some time to re-evaluate that lump a coal your trying to cheek-squeeze into a diamond...just sayin!

All that said, and as mentioned above, the advent of super slow burn powders has opened the door for experimenting with sparkin' off lotsa powder down little holes, while still retaining some semblance of 'acceptable' barrel life...

Think from this perspective. Even if a varmint hunter shot 100+ woodchucks and/or coyotes a year, an 'overbore' cartridge with only a (ridiculously short) 600rd barrel life would still provide ~5 years worth of fun, give of take. Figure a barrel/chamber job is a $500 investment, that's comes to about $100/year to "drive" the rifle equivelant of a 'McLaren P1' when hunting!

Now again, consider that a super slow burn powder charge at a somewhat sane case pressure might just double that 600rd barrel life to ~1200rds. That'd allow the same varmint hunter a decade's worth of fun!

If that logic sounds "stupidly inefficient", then put me on the stool in the corner! But know this, there'll be a big chit-eatin' grin under my dunce cap...

Have fun!!!

 

[riflewoman]

I don’t mind inefficiency, and I have a few rifles that stretch that....here is what I drive...

 

[ShootDots]

View attachment 1036397 I don’t mind inefficiency, and I have a few rifles that stretch that....here is what I drive...

If we only bought what we NEEDED, we would be a most miserable people! We buy what we WANT! Most of what we want is "inefficient" per se! However, it is the "wants" that drive innovation and performance, literally in every arena of living! I LOVE INEFICIENCY!!!

 

[JLT]

Just think about what has happened with internal combustion engines in the last couple of decades. Specific output per liter has skyrocketed to the point where engines under two liters in displacement make enough power to put a smile on your face and make you glad that you're not a clutch. Bump the displacement up a bit and things can get really exciting. Some motorcycle engines can exceed specific output of 200 hp/liter without going nuts.

Much of the technology that has made this possible has it's roots in 'inefficient' (overbore?) racing engines. Perhaps today's overbore is tomorrow's mainstream. Anyway, I'll happily sign up for this kind of research. Gasoline or smokeless powder, either one. Not sure about nitro methane . . .

 

[hogpatrol]

The only time I've been overbored is when the discussion turns to the quintessential attributes of the 6.5 Creedmoor.