So a .410" diam. bullet fired from 50 BMG brass is "Over Bore".

What is the minimum diam. bullet that would NOT be "Over Bore"

from 50 BMG brass? Is there a a mathematical formula? I'd like

to know what diam. bullet crosses the line, and thus is categorized

"over bore", in most every piece of brass "out there". And what

is the correct definition of "over bore", and what is bad about be-
ing "over bore"? Thanks guys.

"Overbore just means that you haven't found a slow enough burning powder...".

courtesy one Parker Oliphant Ackley about 1979 at his shop in SLC.

Rich
DRSS

Can you say Plasma Torch? Thats what Over bore really means.

Not machining Stellite yet Rob ...??

I'm no ballistics expert, but I have always understood over bore to mean that the case capacity to bore volume ratio is out of whack.

The result of this is that the case can hold far more powder (even slow burning powder) than the cartridge can efficiently convert into bullet velocity and energy.

The extra powder (if it is burned at all) is instead converted mostly into heat - which is a useless by-product and will burn out the bore of the rifle very quickly.

Put another way, a cartridge is over bore if a smaller case for the same caliber would be as capable (or nearly as capable) of producing comparable bullet velocity and energy - i.e., would be more efficient and practical.

As slower burning powders have been developed and become available, formerly over bore cartridges have begun to come into their own.

But there are still plenty of over bore cartridges out there.

Michael,

I had a 22-378 Wbee with an Ackley shoulder.
150 rounds per barrel. That's my definition of overbore.

Rich
DRSS

So Using the 300 RUM brass, which is 2.85" tall and .550" at the base,
at what bullet diameter do we hit the "OVERBORE" mark please?

Less that .510.-Rob

quote:

Originally posted by Robgunbuilder:
Less that .510.-Rob

I beg that you are just having sport with me...

quote:

And what is the correct definition of "over bore", and what is bad about be-
ing "over bore"?

Case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives an index number, anything over 1000 is Overbore and will tend to be hard on barrels. A 50 BMG has a grain capacity of 300 with a bore diameter of .508, giving an index of 1480.1 which is very hard on barrels. Compare this to a 458 Win which has a grain capacity of 94 with a bore diameter of .458, giving an index of 570.6 which is very easy on barrels.

Index numbers for some common cartridges are 751.6 for the 308 Win, 915.4 for the 30-06, 1091.1 for the 22-250, and 1272.3 for the 220 swift. Your .410/50 BMG would have an Overbore Index of 2,272.3 and would probably vaporize the projectile!

Best

GaryVA,

where did you get that opinion?

Rich
DRSS

It came from a comparative index that was developed and published years back based on analyses of a wide range of cartridges to show the relationship between powder capacity and barrel bore diameter. It is used to help predict barrel life.

quote:

Originally posted by GaryVA:

quote:

And what is the correct definition of "over bore", and what is bad about be-
ing "over bore"?

Case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives an index number, anything over 1000 is Overbore and will tend to be hard on barrels. A 50 BMG has a grain capacity of 300 with a bore diameter of .508, giving an index of 1480.1 which is very hard on barrels. Compare this to a 458 Win which has a grain capacity of 94 with a bore diameter of .458, giving an index of 570.6 which is very easy on barrels.

Index numbers for some common cartridges are 751.6 for the 308 Win, 915.4 for the 30-06, 1091.1 for the 22-250, and 1272.3 for the 220 swift. Your .410/50 BMG would have an Overbore Index of 2,272.3 and would probably vaporize the projectile!

Best

I have seen that formula - I believe it was originally developed by Barsness - but I think it's too inclusive with a threshold of 1,000.

With the powders we have today, I would argue that somewhere around 1,300 or so should be the threshold.

B5J - Getting the drift yet?

There is no truly scientific, or even generally agreed, definition.

You could argue for a threshold of 1,300...but the Empirical data supports the 1,000 threshold as the 22-250, 243Win, and 220Swift all fall under 1,300 yet tend to be harder on barrels with a shorter barrel life compared to a 223Rem as example. Lots of factors go into excessive barrel wear and therefore such an index is only used as a comparative guide to help make predictions.

I'd place more emphasis on barrel heat.

Your point on the 50 BMG does not hold water when you use the FCSA rifles. Hundreds of rounds in most cases.

If you substitute "inefficient" for "overbore" it has a more realistic value.

A serious benchrest shooter will go thru two 6PPC barrels in a good year. 30 grain water capacity and a 6mm bore. What does that compute to?

Rich

Rich,

When it comes to heat:

Absent other damage, barrels lose accuracy to throat erosion which is markedly increased with excessive heat from continuous fire. This applies to any barrel and is a factor of abuse no different than if you cleaned a barrel improperly damaging the crown and rifling causing an accelerated degrading of accuracy. This is much different than the purpose of a comparative index. As example, the 308 Win, with its index of 751.6, and the 243 Win, with its index of 1164.4, share the same case. I’m sure it would be possible to damage and abuse a 308 Win while using normal care on a 243 Win, and see the 243 Win end up with a longer “accurate” barrel life. But in reality with normal care, the 308 Win will have a longer barrel life.

When it comes to 1000 yard benchrest:

As Dan Lilja proclaims, there is “Accurate” barrel life and there is “Absolute” barrel life. When shooting a 1000 yard benchrest match, group size is the measure of success. A competitor will have a very low tolerance for any degrading of accuracy in terms of precision accuracy. Some will not tolerate having accuracy that degrades from .250 inches to .300 inches at 100 yards, and would consider the barrel done. Flip side, a varmint shooter in the field would probably be oblivious to a change in accuracy from a quarter-MOA to a half-MOA while shooting varmints.

Same thing applies when you look at FCSA. For someone like Lee Rasmussen who shoots sub-2″ (1.9557″ 5-shot groups) at 1000 yards with the 50-Cal and holds ten FCSA World Records, he will have a much lower tolerance for any degrading in accuracy and will retire his FCSA barrel much quicker than the average shooter.

The 6mm PPC has a grain capacity of 33 with a bore diameter of .243, giving an index of 711.6 which is similar to the 308 Win. Outside extreme precision requirements and abuse as noted above, barrel accuracy starts to drop off with .22 and 6mm PPC cartridges at around 3000 rounds.

Best

overbore can mean a few things as it applies to the bullet and gun combo generally relating to velocity efficiency case capacity and barrel life, but the diameter of a given bullet in a specific bore is not part of it ie it has nothing to do with shooting a smaller bullet through a bigger bore ie the .410 bullet through a .50 bore, which would not yield anything that i would imagine a shooter wanting to do.
a gun of caliber combo that is overbore is generally faster less efficient per grain of powder and has less barrel life and more case capacity. a 300 rum is very overbore relative to a 308.

hope this helps
d

Excellent response and rejoin, GaryVa.

I would also submit that the cut-off area could be a small curved range rather than a point, that would include the latest powders and still be valid and not change the definition materially...although, no matter how it is designated it will take a while for any ruffled feathers to get smoothed out.

There will be "Lumpers" and "Splitters" when it comes to ANY kind of definition, however.

I think the definition is much more definitive and clearer than all the other gobbledy-gook definitions I've read in the past.

Now if the rank and file will think about it and accept it, it will further the understanding of ballistics...for some anyway.

Luck

Thanks for the clarification GaryVA. This makes more sense. I think the 6PPC guys wear the barrel out cleaning more than shooting.

I put the formula to use on several of my rifles, it seems to work well; based on thirty years of personal experience.

Regards,

Rich
DRSS

Excellent response and rejoin, GaryVa.

I would also submit that the cut-off area could be a small curved range rather than a point, that would include the latest powders and still be valid and not change the definition materially...although, no matter how it is designated it will take a while for any ruffled feathers to get smoothed out.

There will be "Lumpers" and "Splitters" when it comes to ANY kind of definition, however.

I think the definition is much more definitive and clearer than all the other gobbledy-gook definitions I've read in the past.

Now if the rank and file will think about it and accept it, it will further the understanding of ballistics...for some anyway.

CC in H20/Bore X Pi X radius X radius...Case Capacity in water divided by the Area of a Circle formula...seems simple enough and works for my 6mm-284 at 1400 and 22-243 at 1270...both of which eat barrels for breakfast, lunch AND dinner.

Luck

quote:

Originally posted by GaryVA:
You could argue for a threshold of 1,300...but the Empirical data supports the 1,000 threshold as the 22-250, 243Win, and 220Swift all fall under 1,300 yet tend to be harder on barrels with a shorter barrel life compared to a 223Rem as example. Lots of factors go into excessive barrel wear and therefore such an index is only used as a comparative guide to help make predictions.

I should have been more specific.

I meant to suggest that the definition is too inclusive for hunting purposes.

The standard is and must be different for target shooting.

Target shooters, not to mention bench rest target shooters, routinely throw away barrels that most hunters would be thrilled to have on their rifles!

I greatly appreciate the info posted by you
guys here. Thank you very much!

Seems a Rem 300 SAUM is close to perfect (1000) without going over;
and can be made as a very light rifle too!

And the 375 RUM also comes really close to 1000!!!!!

quote:

Target shooters, not to mention bench rest target shooters, routinely throw away barrels that most hunters would be thrilled to have on their rifles!

It's exactly for that reason, that at some point I plan to build a 6mm ppc with one of those "wore out" .30 barrels. I'm sure the coyotes a p-dogs won't be able to tell the difference between a .25 and a.30 barrel.

With the tendency to burn up barrel throats,
does the shorter and fatter case tend to cause
damage less rapidly than the longer thinner
case if they both have the same capacity in
grains of water?

powder type may be as big a factor as anything else. Stick powders are notorious for erosion, where ball powders of equivalent burn rate are said to "scour" the throat as opposed to burning it. Uneven burn rate of the "stick" granules compared to a ball or sphere.

Rich
DRSS

Sphere has (in general) a larger and more uniform surface area (by weight).

Surface area is "where" the burn occurs.

As I understand it--

In general, the slower the burn rate the grater the throat burn/erosion.

There are exceptions to almost every rule--but--

if you achieve approximately the same velocity with differing loads--

the load with the faster powder has less throat
burn/erosion.

This was shown in study done with the 300 WinMag

(it's been so long I cannot remember source )

quote:

Originally posted by Idaho Sharpshooter:
-- This makes more sense. I think the 6PPC guys wear the barrel out cleaning more than shooting.
---

Rich
DRSS

Without a doubt