My favorite caliber is 2506,it is said to be overbore.What does this mean?

My definition of overbore: A cartridge large enough that the slowest available powder reaches pressure limits, well before the case is full. With the slower powders available today, the 25-06 is no longer overbore.

"Overbore" is a somewhat nebulous concept and T_bob's definition is as good as any. The smaller the case capacity in relation to the bore, the greater the efficiency (energy/grain of powder), and likewise, the larger the case, the less efficient. "Overbore" generally means the point at which greater case capacity results in rapidly diminishing returns in terms of powder efficiency. As T_bob says, with today's powders and a barrel of reasonable length, most modern shooter would not consider the case capacity of a .25-06 to be particularly "overbore".

As was already mentioned, over-bore capacity is a term used in reference to the canister grade powder(s) available which can provide a charge that fills the case without raising pressures above the safe recommended levels (ie 65,000 PSI). With the available powders of today, the 25-06 can be: at, under and over-bore capacity. For example, if you were to use Alliant Unique in the 25-06, you would have a situation where the case is dangerously over-bore capacity for the powder.

If you select powders like IMR-4831 or R-19, the 25-06 case falls into the �at� bore capacity. At the same time, if you choose a powder like 50 BMG, you would be at the extreme opposite end of the spectrum, with a case that is Under bore capacity. Offhand, I can�t think of any cartridges today that are �over-bore� capacity for any of the powders available on the market.

Most of the time when people talk about a case being overbore, they use the term incorrectly, or have no idea what the term is referring to.

Turok

Wolfer: The definition of overbore as I've always been told occurs when, for example, you add 10% more powder to gain only say 5% in velocity. It's the law of diminishing returns. You're right, the .25-06 is considered overbore. A .270 Win. is considered by some people to be over bore (although I love it) the smaller cal. Weatherby magnums are overbore as is the .300 Win. Mag.
Probably ruffle some feathers of some on the board with this one but that's the way I see it.
Bear in Fairbanks

Thanks.

If you add 10% more powder and get 5% more velocity, what you end up with is around 10% more energy. Which is as it should be.

Turok speaks the truth, I handload for the 25/06 and use RL22 w/100gr. bullets and it screams 3500 fps out of 24" barrel. This is why I had a 264 Winchester Mag. built, never a better time in history to have an overbore in the rifle rack with all the new slow burn powders out there.

This is the way I interpret the term overbore. It is entirely relitive, as in there are a million shades of grey between black and white.
7.62 x 39 is overbore compared to 30 carbine.
308 is overbore compared to 7.62 x 39
30-06 is overbore compared to 308
300 Win mag is overbore compared to 30-06
and the 300 RUM is overbore compared to 300 Win mag.
With each of the above mentioned cartridges, it is possible to achieve dangerously over pressure loads by using a powder which is faster than should be used in that cartridge. I sure would not want to pull the trigger on a 30 carbine stoked with 17 gr of unique.
Some might say, "If you can not match the 22LR for muzzle energy produced per grain of powder burned, your cartridge is overbore."
Each of us will have to determine how much black it takes before our grey gets to dark.
I draw the line for cartridge efficeincy (or more accurately inefficeincy) at the 264 Win mag, the 7mm STW, and maybe someday an 8mm RUM. But that is just my opinion.

Idaho Shooter

You can find your answer and a whole lot more if you read P.O. Ackleys two volume set of books. I enjoyed the hell out of them.

Elmo

Overbore for me means a lott of power burnt and getting only a little more fps out of a cartridge, that is magnums calibers, the 270 win is ok, the 270 Weatherby needs a lott more power to get a little more velocity

P. O. Ackley's Handbook For Shooters and Reloaders is the best source for answers I have seen, as mentioned above. Mr. Ackley suggests that "overbore" is a relative term that is best explained in terms of diminishing returns in velocity from added powder capacity and reduced barrel life.

A cartridge that erodes the barrel throat to the point of diminishing accuracy in 500 rounds is more overbore than one which does it in 1000 rounds, though both might be considered too overbore for practical use. In extreme cases, the barrel may be worn out before a good load can be developed.

He says the 25-06 is "probably inferior" to the Improved .257 Roberts, which is "just about maximum capcity for best results in the .25 bore".

In his data, the Improved Roberts will push a 125 grain bullet to 3050 fps with 48/4350, while it takes 57/4831 to get 3101 from a 120 grain bullet in the 25-06. Little is gained from the additional powder capacity, thus it is relatively "overbore" and can be expected to wear out a barrel sooner than a more efficient cartridge.

PO's books aren't bad, but they are rather dated. Many of the cartridges he calls overbore, were, but they are no longer due to the fact that we can get canister grade powders that fill the case while producing the accepted pressure limits.

His concepts concerning bore erosion have been seriously challeneged as well. For example, Ken Howell has stated on numerous accounts that pressure has more to do with erosion than does the amount of powder running down the tube. This is the premise behind his whole line of cartridges. Use a large cartridge, with a large charge of slow burning powder, BUT keep the pressures low, and not only do you increase your personal margin of safety, your barrel life is greatly increased while maintaining the same velocities as a smaller cartridge. Here, peak pressure is the problem, as it was with the large cases mentioned in P.O's book. People were pushing them as hard as they could in order to get as many fps out of them to wow the crowd.

Turok

[ 02-12-2003, 02:52: Message edited by: Turok ]

Turok, you are right about pressure being the
main factor in erosion.Tested from a cold barrel
my 458 wildcat cartridge with a temp sensor, and
it would take 10 loads that were reduced from max
10% to get the same raise in barrel temp as the
5 max loads.More pressure, more heat, hence more wear.Ed.

Ackley would be the first to agree that better powders improve efficiency and that reduced pressure increases barrel life. That was just as true then as it is now.

He said "overbore" is a relative term, and that remains entirely true. Loaded to the same pressure, a larger case is going to be less efficient and harder on barrels than a smaller one, regardless of the powder used, for a given bore size. Until an entirely new propulsion principle is invented, that will always be true, and Ackley's works will be the best source of understanding of the principles involved.

No matter how hard the new crowd tries to out-do the old masters, there is nothing new under the sun, and it is unlikely that anyone will ever equal P. O. Ackley's understanding of internal ballistics.

I agree that overbore is a relative term, i.e. using Unique in the 25-06 compared to using a more apt powder such as R22. But I don�t agree with the larger cases being harder on the barrels than a smaller case when loaded to the same pressures. Why? Because 65,000 PSI is still 65,000 PSI regardless of how you look at it. Ken Howell agrees on this point, and so does Homer Powelly. Neither of which are considered the �new crowd�, and both of which had a better understanding of internal ballistics than P.O. ever had.

It all comes down to the critical pressures, whereby the pressures hit a point that melts a thin layer of steel and in the process washes it out at the shot. This can happen with ANY cartridge. A great example of this is the relatively small 243. It�s considered a real barrel burner by many due to the higher pressures that erode the throat, but it doesn�t have the powder capacity to have the �sand blast� effect that P.O speaks of. On the other side, has anyone ever heard of a barrel being burned up by squib loads in the larger Mag cases? Reduce the pressures, and reduce the barrel ware.

While Ackley was fairly good at what he did, those that actually knew him, considering him far from being the foremost authority on internal ballistics. Virtually everyone today has access to the tools that exceed anything P.O. had access to, but with your point of view, we will never understand more than P.O. He has long since been surpassed. So will the others, that�s just the evolution of the game.

Turok