May 1, 2008, 08:03 AM | #1 |
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Case Overall Length??
I'm fairly new to reloading, and I have many questions, but the one that gets me the most is the C.O.L.. In my reloading manual it says that if I seat my 22-250 bullet anywhere from .010 to .030 off the lands I might gain some accuracy. Now, it also says that there is a maximum overall length for this cartridge, so here's where I get confused, if I set the bullet say .020 off the lands wont that surpass the maximum overall lenght? Or am I looking at this the wrong way?
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May 1, 2008, 09:04 AM | #2 |
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There are several things here that need to be put into their proper relationships:
The Small Arms and Ammunition Manufacturers' Institute (SAAMI) sets standards for manufacture of guns and ammo in the U.S. They have a maximum cartridge length standard, which mainly has to do with how magazines (clips) are constructed to make sure that all commercial cartridges will fit into them. SAAMI has minimum chamber dimensions to make sure (among other things) that commercial cartridges will not be so long that bullet will be jammed into the rifling and raise pressures when fired. But, manufacturers can and do make cartridges that are shorter and chamber throats that are longer than SAAMI specifications. That may not be the most accurate setup, so handloaders often make cartridges with the lengths adjusted to give the best accuracy, even though those lengths may exceed SAAMI standards and even may not fit in the magazines of their guns (so they would need to single-load the rounds in that situation). Another factor is the cartridge overall length (COAL) that the handloading data publisher used when constructing the ammo that was tested for pressure. That normally is provided with the data. The reason is that the pressure depends IN PART on the amount of space inside the cartridge for the powder. Making the cartridge longer by seating the bullet out further tends to make more space for the powder and lower the pressure when the cartridge is fired IF ALL OTHER FACTORS ARE THE SAME. But, seating the bullet out further must change at least one other factor in a particular gun, and that is the distance the bullet has to travel to reach the rifling. Moving the bullet closer to the rifling raises pressure, so that effect works in the opposite direction of the pressure decreas from the increased powder space. Which effect is more important depends on the gun. In rifles with big bottleneck cases, the percentage change in volume is typically small, while the percentage change in distance to the rifling is typically large, so pressure tends to be increased by seating the bullet closer to the rifling. In revolvers, the opposite is typically true, so seating bullets further out tends to decrease pressure. It would be nice if the published handloading data gave the bullet jump in the test rifle as well as the COAL. It is one of the reasons that max velocities vary so much from manual to manual. Remember, the maximum CHARGE of powder is found with a special pressure test barrel that is constructed to MINIMUM SAAMI specs, but the published velocities are typically from firing the same loads in a sporting firearm that may have a much different throat length (and other chamber dimensional differences). In some cases, it has been alleged that rather hot loads published in some manuals are the result of the special pressure testing barrels becoming eroded to the point that bullet jump has been increased substantially beyond SAAMI specs. I hope this all helps. Ask more questions if you need clarification. SL1 |
May 1, 2008, 10:17 AM | #3 |
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That's a very good and in-depth explanation.
IMO, in your case, if you are fairly new to the game, you should be loading to a COAL that's within SAAMI published spec, or more to the point, you should load to the COAL that is published with your loading data. Once you've safely fired those rounds, looking for all your typical signs of pressure, check the accuracy you are seeing and ask yourself if you are getting what you want or need out of that load. If you ARE getting the accuracy you desire, at a speed and "power" you want, without showing signs of excess pressure, then you've got it-- the holy grail! A pet load! Go make three thousand of them! If you find that you'd like better accuracy, it's time to start adjusting the easy things that affect accuracy, like brass quality, inspection and preparation, bullet quality, and most obviously, the choice of powder and your charge weight. Also, your shooting skills, etc. Playing with COAL in an effort to seat bullets closer to the lands of your rifle is well forward on the checklist of things to complete and there are many easier and more pressing concerns to tackle before it. There are so many things to work on before worrying about seating bullets closer to the lands.
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May 1, 2008, 10:19 AM | #4 |
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SL1 said it very well, However if you are new to reloading it might have caused you some additional confusion.
COL = Cartridge overall length. Seating your bullet .020 away from the lands is used by those who load for bench rest shooting. Your reloading manual is giving a COL that should work in just about every rifle chambered for that cartridge. Take my .270win, If I seat my bullet close to the lands, I can't feed it from the magazine. Therefore I wouldn't load like that for hunting. When shooting from the bench we (most of us) will load directly into the chamber. Who would want to go through all the steps to make a sub MOA cartridge and then chance distorting it by cycling it through the magazine? Yes I found out the hard way my new loads were to long for the magazine when I was loading one opening morning and they wouldn't fit.
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May 2, 2008, 02:19 AM | #5 | ||
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Quote:
One thing I might add that when seating to longer than the max OAL listed in your manual you need to make sure that the bullet is still into the case about as much as the bullet diameter but I agree with the quote from Sevens below. Quote:
SAAMI is the abbreviation for Sporting Arms and Ammunition Manufacturers' Institute. I get this wrong all the time and I just looked it up again. straz Here is a link to SAAMI’s glossary. I find it a good place to see the correct definitions of many terms used in reloading. http://www.saami.org/Glossary/index.cfm .
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