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Overbore forumula
nemesisenforcer
Member Posts: 10,513 ✭✭✭
Been trying to develop a formula to determine if, and how much, a cartridge is "overbore." So far, I've come up with a crude, but workable formula as follows:
bore/bullet diameter in millimeters divided by cartridge capacity in cubic centimeters (all measurements from Modern Reloading, Second Edition by Richard Lee). The closer the result is to 1, the more overbore the cartridge is and the slower powder you need.
Let's take a look at some examples:
308 Winchester: 7.62/3.43=2.22 vs. 30-06: 7.62/4.38=1.74. As you can see an equivalent caliber in a larger case leads to more overbore results. No surprise there. Also, there's no surprise that Varget or RL15 or powders of a comparable burning rate are considered the best powders for most bullets in the 308 and 4350 (which is slower burning) for most in the '06.
223 Remington: 5.56/1.87=2.97, indicating a rather fast burning powder might be appropriate, something in the 4895 or RL12 range.
For my favorite, the 6.5 Swede, we get a result of 1.76, almost identical to the 30-06 and no surprise, powders of the 4350 variety work very well in the venerable Swede, though with heavier bullets something like 4831 might be better.
Like I said, the formula isn't perfect, but can give a thumbnail sketch of "overboreness" quickly and efficiently. It only works on rifle cartridges since overbore considerations are less relevant with pistol calibers.
bore/bullet diameter in millimeters divided by cartridge capacity in cubic centimeters (all measurements from Modern Reloading, Second Edition by Richard Lee). The closer the result is to 1, the more overbore the cartridge is and the slower powder you need.
Let's take a look at some examples:
308 Winchester: 7.62/3.43=2.22 vs. 30-06: 7.62/4.38=1.74. As you can see an equivalent caliber in a larger case leads to more overbore results. No surprise there. Also, there's no surprise that Varget or RL15 or powders of a comparable burning rate are considered the best powders for most bullets in the 308 and 4350 (which is slower burning) for most in the '06.
223 Remington: 5.56/1.87=2.97, indicating a rather fast burning powder might be appropriate, something in the 4895 or RL12 range.
For my favorite, the 6.5 Swede, we get a result of 1.76, almost identical to the 30-06 and no surprise, powders of the 4350 variety work very well in the venerable Swede, though with heavier bullets something like 4831 might be better.
Like I said, the formula isn't perfect, but can give a thumbnail sketch of "overboreness" quickly and efficiently. It only works on rifle cartridges since overbore considerations are less relevant with pistol calibers.
Comments
The 6mm Rem can be a real barrel eater, yet according to your formula it's not that much different than the 30-06. I'd say the 30-06 has a lot more barrel life potential than the 6mm Rem. It's also very flexible when it comes to powders.
Maybe look at a weight factor of the bullets? Weight/dia. vs. capacity?
It's an interesting start, putting a mathematical formula behind our reloading decisions.
My own formula is simpler: The bore diameter squared times 1,000 equals the largest case volume in grains of water to be efficient. Any larger case is "overbore" for that bullet diameter.
So, the largest reasonable case for a .22-caliber round holds about 50 grains of water (.224 x.224 x 1000 = 50.176). Note that the many .22 wildcats based on the .308 case (which holds 50 grains of water) are about the largest ones that work at all well.
You can easily calculate any larger or smaller bore size on your own.
The compromise in all calculations of powder space/charge/bore volume is; How much initial pressure are you willing to give up in exchange for secondary pressure. Put in simpler terms, as the powder is ignited and pressure begins to generate, the point of least resistance is for the bullet to move down the bore. Since pressure is a function of volume, as the bullet moves down the bore and area increases, trying to reduce the pressure. The result is upon ignition the pressure begins to build, reaching a peak as the bullet is gaining speed down the bore, and drops as the volume behind the bullet increases faster than the powder generates it.
The primary portion is from ignition to peak pressure, secondary pressure is from peak to bullet exiting the barrel.
The larger the case capacity, the slower the rise in primary pressure and the slower the initial accelleration of the bullet. This reduction in bullet performance may be offset by an increase in secondary pressure.
Two extreme examples- the 22 rimfire- very fast initial burn and pressure peak, followed by almost non-existant secondary pressure.
The 30/378 Weatherby- very slow initial rise in pressure followed by longlasting secondary pressure- provided the barrel is long enough to accommodate the secondary pressure.
So the term overbore is not something that can be generalized with a simple bore area= linear function. As noted, check the information needed for a Powley computation and you won't need to re-invent it.
The difference is due to bullet travel and total bore volume change, as you correctly point out. But in most bottleneck cartridges, the bore volume change is small compared to case volume. That's the only reason the peak can occur several inches down-bore. In small capacity or straight-wall cases, the pressure peak (and thus end of powder burn) can occur before the bullet clears the case mouth, or very shortly after.
There can be a small/insignificant amount of residual powder burn after the pressure peak, and even some powder that fails to burn at all. That's why we find powder kernels in the barrel. It is usually the result of a powder that is mis-matched for the pressure level of the load - using a powder too slow for that load combination.
Ray, your ideas have merit, but are based (I think) on the belief that the powder burns for much or all of the time the bullet transits the bore. And that is not true. The pressure peak is considered to mark the end of effective powder burn. As long as the powder burns, it does continue to produce more gas and thus more pressure, so the sudden cessation of pressure increase tells us that no additional gas is being produced. In most rifles, the pressure peak occurs from an inch to no more than about eight or nine inches down the bore.
The difference is due to bullet travel and total bore volume change, as you correctly point out. But in most bottleneck cartridges, the bore volume change is small compared to case volume. That's the only reason the peak can occur several inches down-bore. In small capacity or straight-wall cases, the pressure peak (and thus end of powder burn) can occur before the bullet clears the case mouth, or very shortly after.
There can be a small/insignificant amount of residual powder burn after the pressure peak, and even some powder that fails to burn at all. That's why we find powder kernels in the barrel. It is usually the result of a powder that is mis-matched for the pressure level of the load - using a powder too slow for that load combination.
Rocky,
I think we are close on this issue, but to some degree we still have differences. I agree that the majority of the 'push' is done in the first 1"-9". 9" may even be a little long. But what I see happening is the pressure peak is right at when the most volume of powder has been 'ignited' not 'burned'. To most that's a pretty slight difference. As the pressure wave goes down the barrel, there still is powder burning (and not burning, as you said, leaving kernels in the barrel as evidence) and therefore producing gas and still giving 'push' to the bullet going down the barrel.
I emphasize the fact a 77 gr. bullet in front of 15 gr. of RE-15 goes 3100 fps from a 26" barrel instead of 2690 from a 16" barrel.
In any case, I do agree with you in regards to most shooters in that when it takes a shovel full of powder to get minimal gains, it's time to move up. The other thing I call overbore is when shovel full after shovel full is thrown down the barrel and all you have left after 1000 rounds, or less, is a smooth-bore (for the first few inches anyway). Most shooters never experience this. Most competition shooters do. In our haste to proclaim what is and what isn't overbore, we also need to factor in how much we shoot before we say one cartridge or another burns barrels out. Another shovel-full of powder doesn't hurt if the rifle gets shot six times a year and lasts 15 before needing a new barrel. A small shovel-full needs to get even smaller if it goes through barrels quite quickly.
Of course, pressure is that ever present angel/devil on our shoulders telling us just how far we can push it...[;)][:D]
In short, the 6mm Rem AI is "overbore".
It isn't that the 6mm AI is overbore- it's that given a powder that will fill the case it needs a barrel to utilize the area under the pressure curve of that powder- which is to say: in your example the barrel is too short.
Ray,
Normally I would agree, But the barrel is 26" Trying some more powders would be the answer, as both powders I tried were double based. I could tweak a single base powder in there to get more volume/less pressure. FWIW, the two powders were RE-19 and RE-22.