It is said that the 220 Swift is "over-bore", but the 22 Hornet is not, and the 22-250 is, "maybe". How do we decide?

What is the rate of diminishing returns? Are the new WSM's over-bore? Does it matter?

Thanks

In my opinion any speech by John Kerry or Teddy Kennedy is overly boring.
Jim

Overbore was more of a problem when there weren't as many available powder choices as there are now. before H4831, almost any big case was overbore. I don't think it matters, if you want to go fast as .220 Swift, you have to have a big case, barrel life is shorter. There's no magic that will change that. I've never heard of any specific idea as to where overbore occurs, except in the opinion of the shooter.

Arky has the Over Bore description down pat!!


"Does it matter" asked Terry? NOT on your life.
Nothing could matter less!

Cheers and Good shooting
seafire

There is no definiton of overbore. Everybody pretty much uses it as they want to, which makes it meaningless. There's a constant, rapidly declining return in velocity as the case capacity goes up, but where you draw the line depends on a lot of factors. Especially barrel life, which declines rapidly with big-for-bore case capacities and hot loads.

Ricochet is on this. Overbore is the point where you are putting alot more powder for little gain in vel. The 7mmRUM is overbore. You can get pretty close to the same vel. w/ a 7mmDakota & 10% or so less powder. That's really simplifying it but that's my understanding.

It is an overused phrase with no pat definition. Don't know for sure who started it, but I think Ackley was the fellow that made it popular. As an example he thought the .257 Roberts AI was nearly perfect for bore diameter due to case capacity. Well, when Mr. Ackley was exploring the world of wildcats and tamecats, the supply of powder varieties was extremely limited, and they were for the most part fast to medium speed powders by todays standards. He thought the .25-06 an overbore cartridge because it could not be loaded to achieve substantial improvement over the Bob-AI because there were no powders that could take advantage of the extra case capacity without generating MUCH HIGHER PRESSURES. We don't normally consider the .25-06 overbore today because of things like Reloader 22/25, or other slow powders applicable to other large volume cases.

Perhaps someday the powders used to make the "Heavy-Mag" and similar factory ammo will be available in canister form, and the likes of the Lazzeroni cartridges will not be the dreaded "overbore" cartridges thay are today. It's all relative.

Simply multiply the bore in inches times itself and divide by the usable case capacity in grains of water. When you get a result that is 1000 or more you are overbore.



This has a lot more to do with barrel erosion than anything else, since it also describes how much high pressure gas has to make its way through the throat for a given shot.



So far, the WSMs have been engineered to avoid this...the 270 WSM using 76 grains of capacity comes out to 990, and every thing else is better. The aforementioned 7MM RUM using 110 comes out to 1364. The 50 BMG using 230 grs of capacity is 920, and this is no accident.



There is another simple formula engineers use to estimate the appropriate barrel length for a round that is based on this "overbore" number times the muzzle velocity raised to the 0.22 power (half way between the 4th and 5th root).



Same thing for burn rate...the muzzle velocity divided by the overbore number is a burn rate number. So, the 50 BMG at 3000 fps/ 920 gives a 3.26 burn rate. For convenience, I often multiply this number by 10 and treat it like a highway speed sign. The BMG needs SLOW powder...32.6 MPH speed limit. The 7MM RUM at 3500 fps comes out to 25.7 MPH. These numbers are consistent with AA8700 and the like.



This compares with a 44 Mag in a rifle at 130 MPH. This is consistent with H110 and W296. Guess who is doing 55 MPH?...Yup, the 308 Winchester at 2850 fps and the 223 at 3070 fps. Sounds like W748 to me!



This method also works for handguns, but the velocity needs to be the velocity you would get in a barrel long enough to achieve rifle levels of efficiency, so you often can't use the revolver velocity from its shorter bbl.



Hope this helps!

Sabot, that's an interesting bit of info there, can you provide a reference(s)? Like to read up a bit if possible.

Sabot: i don't follow your math at all. correct me on this.

cartridge - .30-378 Wby.
case capacity - 140 grains
bore diameter in inches - 0.308

your calculation = (0.308 x 0.308) / 140
= 0.0949 / 140
= 0.0007

certainly most shooters consider the .30-378 Wby to be "overbore". the calculation shows it to be way the other direction.

where am i misunderstanding your explanation ???

I always thought it was an arbitrary definition of a cartridge that, with optimum powders did not use 100% of available case capacity

Bob: i see.......thank you

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Quote:

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Arky has the Over Bore description down pat!!"


Sure John! We in California have a Republican Governor who is anti gun. Forget the Republican Democrat BS and gives us an American to vote for. Some one who isn't bought and paid for to only serve his or her party Hierarcy and lobbyists. Sore spot!
HINT!
If Blue Dot were the slowest burning rifle powder everything over 22hornet capacity would be considered over bore capacity. We still however would enjoy shooting when not being brain washed by a politition. roger

Bill -

Thanks for catching my mistake...you have it right. Don't ever trust guys like me with equations...a touch of age and mild dislexia will screw things up not only good, but in a hurry!

I would also like to add that the notion that there is some particulay, exactpoint where something is "overbore" is hogwash...its more like the concept of heading in a direction, like "heading West".

Also, the energy one can extract from smokeless propellants for any fixed bore size and pressure varies very closely as a function of the ratio of capacities raised to the 0.6 power. This is a smooth curve depicting the diminishing returns one encounters when increaseing case capacity. Doubling it gives a 51.6 % increase, and you need to increase capacity 3.18 fold to DOUBLE the energy (the bullet's energy in ft-lbs). If one wishes to compare velocities for specific bullet weights, just change the exponent from 0.6 to 0.3. Hence, a 10% increase in capacity give us 1.10^0.6 = 1.0588 for energy, and 1010^0.3 = 1.0290 for velocity. the bullet will go 2.9% faster and deliver 5.88 % more energy for a 10% increase in capacity.

Now, this approach assumes that we are adjusting the powder burn rate as we change capacity to get the MAX potential muzzle energy.