COL

Caboclo · November 7, 2009, 06:54 PM

Hi Folks. I'm new to reloading; I've read quite a bit, looked at data from many sources, done several searches on this forum. After putting it all together, I think I know how COL works, but I would like you guys to double check my conclusions.

1: Reducing COL raises pressure.

2. Increasing COL reduces pressure, unless the bullet is so far out it's touching the rifling, at which point the pressure increases.

3. Ergo, the ideal COL is the longest one which will reliable cycle through my gun, without resting on the rifling.

Does that sound about right?

RKG · November 7, 2009, 07:10 PM

This issue is far too complex for such generalizations.

In theory, and holding all other factors equal (which is eaiser said than done), shortening a cartridge's overall length reduces the free volume of the case, which should have an effect of increasing pressure. However, there are a bunch of other factors at work, and empirical tests have shown that sometimes it does and sometimes it doesn't (or does but to such a small degree as to be statistically insignificant).

In theory, adjusting a COAL so that the bullet touches the lands upon chambering should increase pressure, since the added force necessary to overcome both inertia and initial engraving delays the start of bullet travel (and, hence, combustion chamber volume expansion) to a later point in the burn curve. However, there are a bunch of other factors at work, and empirical tests have shown that sometimes it does and sometimes it doesn't (or does but to such a small degree as to be statistically insignificant).

Do you see the pattern here?

If one is loading for hunting or combat, the most significant attribute of COAL is reliable function, and rounds should be loaded with that in mind.

If one is loading for accuracy, there are a number of theories out there, and the only way to find out (a) if different COALs (and, therefore, different lengths of "jump" to the lands or "jam" onto the lands) will yield different sized groups and (b) if so, what is best, has to be determined empirically for any given rifle. Some like long throats; some like short ones.

Caboclo · November 7, 2009, 08:22 PM

Ok, so from a safety standpoint: the COAL listed in the data is the minimum which will produce safe pressure with that powder charge, right?

RKG · November 8, 2009, 05:42 AM

I don't think so. Rather, the significance of the COAL listed in a reloading table is (and is only) that this is the COAL that the author used when taking his pressure readings.

sourdough44 · November 8, 2009, 06:45 AM

Rifle col adjustments in my experience are more forgiving than with handgun. Meaning I will adjust COL with a rifle for all the usual, mag length, distance to rifling, accuracy. I'll go either way, within reason. With a handgun, especially higher pressure semi-auto(40 s&w etc) I keep things very close to book COL.

**Bud Helms** · November 8, 2009, 09:32 AM

Yes, the maximum OAL is that which limits or prevents cycling and chambering. Or until the bullet falls out of the neck ... most, but not all, well designed cartridges use one caliber as the minimum neck contact with a seated bullet. There are exceptions. Sometimes the entire neck is not a caliber long.

Another issue is reducing the COAL to the point that a powder charge is compressed when you don't want to do that. Compressed loads should be either avoided or used with great caution. I don't use them. Compressing a load can cause pressure to rise too quickly. Even compressed loads found in reloading manuals should be approached with caution. That's just my personal philosophy.

I read, with some humor, discussions about a cartridge overall length measured out to three places to the right of the decimal point: one thousandth of an inch. The ogive of the bullet, as well as its base-to-tip overall length determine overall length of the loaded cartridge. Of course, two different weight bullets in the same diameter (caliber) may result in different loaded cartridge OALs.

Simply changing from one design or style bullet to another of the same weight does not mean the OAL of one translates to the other, because of ogive differences. The point of the bullet does not make contact chambered. That only tells you the magazine limitation.

The ogive can be especially critical in rifle loads. Trying to find the lands and build a load that is right up against, or some few thousandths off, the lands is another game entirely.

1) Don't jam the bullet into the lands. Don't load a cartridge too long to cycle through the magazine. That is the long end of cartridge overall length limits.

2) Don't compress the powder (unless it is intentional). Don't load the bullet so far into the case that it won't feed. The case neck opening should be within the limits of the bullet bearing surface. Ideally, one calibre's worth of bearing surface should be in contact with the neck (there are exceptions). In the case where the neck length on a bottle necked cartridge is less than a caliber in length, I strive to make 100% contact between the neck and the bullet. There is also the distinct possibility that the heaviest bullet made in your caliber requires too much bullet below the neck (into the case cavity) to shoot well (accurately). These are the issues that should influence your choice of the short end of cartridge overall length limits.

These are true for each bullet, powder and action combination for which you load. Everything in between these limits is in the useable range of COAL. COAL is not a number three places to the right of the decimal point that you must achieve when loading. Simple differences measured in a purchased box of bullets may require a different COAL for some of the bullets in that box if you are measuring out to three places because of slight variations in ogive that are not discernable by the naked eye. What about a bullet nose that gets whacked during shipment? What about cast lead bullets? Don't get caught up picking the fly poop out of the pepper. It can be useful if you are loading for benchrest accuracy, but for hunting or even MOA accuracy, it is usually not. Powder charge may need downward adjustment as the bullet moves back into the case, even if it is not a compressed load.

Once you load 50 cartridges of your chosen load, measure their COAL. What happened? Why aren't they all x.xxx" COAL? Your dies. The bullets themselves. x.xx" may be good enough, especially in a carry or hunting handgun ... and most rifles.

It's pepper. There ain't no fly poop in the pepper.

Caboclo · November 8, 2009, 09:46 AM

Thanks everyone for all the info. Now I'm even more curious. Where can I get some piezo-electric equipment like the mfrs and actually measure what's going on? Chrono's are quite affordable, but they don't tell the whole story. I would love to have a full lab to play with.

SL1 · November 9, 2009, 07:25 AM

Try http://www.shootingsoftware.com/ .

SL1

Caboclo · November 9, 2009, 11:33 AM

SL1, thanks for the link. Does anyone have any experience with those strain gage systems? I've looked at them before; interesting theory, but the accuracy depends on proper installation of the gage and use of the correct algorithm. In short, it's just a guess, albeit a fairly educated one. In addition, the gage can't be attached to a semi-auto pistol, which is what I'm currently working with. Regarding use of factory ammo for calibration, will the big factories tell you what pressure their ammo is loaded to?

SL1 · November 10, 2009, 08:24 AM

Like you, I have looked at these things, but not (yet?) bought one. I think UncleNick has one (or more). I think I remember him saying that he has a .45 ACP chambered Contender barrel that he uses to presssure-test the ammo he uses in his .45 ACP auto-loader. So, he probably is getting a different pressure in his auto loader, but he has some RELATIVE comparisons that do have some value in keeping max loads in safe conditions. After all, the "pressure-tested" data that we see published in manuals was tested in a SAAMI-spec test barrel to get the pressure values, and then the velocities in the manual were (usually) obtained from a different, commercially available gun that probaby produced different pressures (and velocities) than the test barrel did. And, YOUR guns probably produces different pressure and velocity than any of those published values, too.

If UncleNick doesn't chime-in on this thread, maybe you could PM him.

SL1

RKG · November 10, 2009, 11:34 AM

The attached data, taken yesterday, will illustrate my prior post. For this rifle, using a 168 grain SMK, the bullet kisses the lands at a COAL of 2.805". The "long" rounds were loaded to 2.791" COAL (or about 15 thou off the lands), while the "short" rounds were loaded to 2.780" COAL (or about 25 thou off the lands). There is a small but distinct velocity difference, which implies a commensurate pressure difference.

November 7, 2009, 06:54 PM	#1
Caboclo Junior Member Join Date: November 2, 2009 Posts: 8	COL Hi Folks. I'm new to reloading; I've read quite a bit, looked at data from many sources, done several searches on this forum. After putting it all together, I think I know how COL works, but I would like you guys to double check my conclusions. 1: Reducing COL raises pressure. 2. Increasing COL reduces pressure, unless the bullet is so far out it's touching the rifling, at which point the pressure increases. 3. Ergo, the ideal COL is the longest one which will reliable cycle through my gun, without resting on the rifling. Does that sound about right?

November 8, 2009, 09:32 AM	#6
Bud Helms Staff Join Date: December 31, 1999 Location: Middle Georgia Posts: 10,102	Yes, the maximum OAL is that which limits or prevents cycling and chambering. Or until the bullet falls out of the neck ... most, but not all, well designed cartridges use one caliber as the minimum neck contact with a seated bullet. There are exceptions. Sometimes the entire neck is not a caliber long. Another issue is reducing the COAL to the point that a powder charge is compressed when you don't want to do that. Compressed loads should be either avoided or used with great caution. I don't use them. Compressing a load can cause pressure to rise too quickly. Even compressed loads found in reloading manuals should be approached with caution. That's just my personal philosophy. I read, with some humor, discussions about a cartridge overall length measured out to three places to the right of the decimal point: one thousandth of an inch. The ogive of the bullet, as well as its base-to-tip overall length determine overall length of the loaded cartridge. Of course, two different weight bullets in the same diameter (caliber) may result in different loaded cartridge OALs. Simply changing from one design or style bullet to another of the same weight does not mean the OAL of one translates to the other, because of ogive differences. The point of the bullet does not make contact chambered. That only tells you the magazine limitation. The ogive can be especially critical in rifle loads. Trying to find the lands and build a load that is right up against, or some few thousandths off, the lands is another game entirely. 1) Don't jam the bullet into the lands. Don't load a cartridge too long to cycle through the magazine. That is the long end of cartridge overall length limits. 2) Don't compress the powder (unless it is intentional). Don't load the bullet so far into the case that it won't feed. The case neck opening should be within the limits of the bullet bearing surface. Ideally, one calibre's worth of bearing surface should be in contact with the neck (there are exceptions). In the case where the neck length on a bottle necked cartridge is less than a caliber in length, I strive to make 100% contact between the neck and the bullet. There is also the distinct possibility that the heaviest bullet made in your caliber requires too much bullet below the neck (into the case cavity) to shoot well (accurately). These are the issues that should influence your choice of the short end of cartridge overall length limits. These are true for each bullet, powder and action combination for which you load. Everything in between these limits is in the useable range of COAL. COAL is not a number three places to the right of the decimal point that you must achieve when loading. Simple differences measured in a purchased box of bullets may require a different COAL for some of the bullets in that box if you are measuring out to three places because of slight variations in ogive that are not discernable by the naked eye. What about a bullet nose that gets whacked during shipment? What about cast lead bullets? Don't get caught up picking the fly poop out of the pepper. It can be useful if you are loading for benchrest accuracy, but for hunting or even MOA accuracy, it is usually not. Powder charge may need downward adjustment as the bullet moves back into the case, even if it is not a compressed load. Once you load 50 cartridges of your chosen load, measure their COAL. What happened? Why aren't they all x.xxx" COAL? Your dies. The bullets themselves. x.xx" may be good enough, especially in a carry or hunting handgun ... and most rifles. It's pepper. There ain't no fly poop in the pepper. __________________ "The irony of the Information Age is that it has given new respectability to uninformed opinion." - John Lawton, speaking to the American Association of Broadcast Journalists in 1995

November 9, 2009, 07:25 AM	#8
SL1 Senior Member Join Date: November 8, 2007 Posts: 947	Where to get pressure MEASURING equipment. Try http://www.shootingsoftware.com/ . SL1

November 10, 2009, 08:24 AM	#10
SL1 Senior Member Join Date: November 8, 2007 Posts: 947	Like you, I have looked at these things, but not (yet?) bought one. I think UncleNick has one (or more). I think I remember him saying that he has a .45 ACP chambered Contender barrel that he uses to presssure-test the ammo he uses in his .45 ACP auto-loader. So, he probably is getting a different pressure in his auto loader, but he has some RELATIVE comparisons that do have some value in keeping max loads in safe conditions. After all, the "pressure-tested" data that we see published in manuals was tested in a SAAMI-spec test barrel to get the pressure values, and then the velocities in the manual were (usually) obtained from a different, commercially available gun that probaby produced different pressures (and velocities) than the test barrel did. And, YOUR guns probably produces different pressure and velocity than any of those published values, too. If UncleNick doesn't chime-in on this thread, maybe you could PM him. SL1 Last edited by SL1; November 10, 2009 at 08:34 AM.

November 7, 2009, 07:10 PM	#2
RKG Member Join Date: June 18, 2009 Location: Boston Posts: 58	This issue is far too complex for such generalizations. In theory, and holding all other factors equal (which is eaiser said than done), shortening a cartridge's overall length reduces the free volume of the case, which should have an effect of increasing pressure. However, there are a bunch of other factors at work, and empirical tests have shown that sometimes it does and sometimes it doesn't (or does but to such a small degree as to be statistically insignificant). In theory, adjusting a COAL so that the bullet touches the lands upon chambering should increase pressure, since the added force necessary to overcome both inertia and initial engraving delays the start of bullet travel (and, hence, combustion chamber volume expansion) to a later point in the burn curve. However, there are a bunch of other factors at work, and empirical tests have shown that sometimes it does and sometimes it doesn't (or does but to such a small degree as to be statistically insignificant). Do you see the pattern here? If one is loading for hunting or combat, the most significant attribute of COAL is reliable function, and rounds should be loaded with that in mind. If one is loading for accuracy, there are a number of theories out there, and the only way to find out (a) if different COALs (and, therefore, different lengths of "jump" to the lands or "jam" onto the lands) will yield different sized groups and (b) if so, what is best, has to be determined empirically for any given rifle. Some like long throats; some like short ones.

November 7, 2009, 08:22 PM	#3
Caboclo Junior Member Join Date: November 2, 2009 Posts: 8	Ok, so from a safety standpoint: the COAL listed in the data is the minimum which will produce safe pressure with that powder charge, right?

November 8, 2009, 05:42 AM	#4
RKG Member Join Date: June 18, 2009 Location: Boston Posts: 58	I don't think so. Rather, the significance of the COAL listed in a reloading table is (and is only) that this is the COAL that the author used when taking his pressure readings.

November 8, 2009, 06:45 AM	#5
sourdough44 Senior Member Join Date: October 17, 2007 Location: WI Posts: 223	Rifle col adjustments in my experience are more forgiving than with handgun. Meaning I will adjust COL with a rifle for all the usual, mag length, distance to rifling, accuracy. I'll go either way, within reason. With a handgun, especially higher pressure semi-auto(40 s&w etc) I keep things very close to book COL.

November 8, 2009, 09:46 AM	#7
Caboclo Junior Member Join Date: November 2, 2009 Posts: 8	Thanks everyone for all the info. Now I'm even more curious. Where can I get some piezo-electric equipment like the mfrs and actually measure what's going on? Chrono's are quite affordable, but they don't tell the whole story. I would love to have a full lab to play with.

November 9, 2009, 11:33 AM	#9
Caboclo Junior Member Join Date: November 2, 2009 Posts: 8	SL1, thanks for the link. Does anyone have any experience with those strain gage systems? I've looked at them before; interesting theory, but the accuracy depends on proper installation of the gage and use of the correct algorithm. In short, it's just a guess, albeit a fairly educated one. In addition, the gage can't be attached to a semi-auto pistol, which is what I'm currently working with. Regarding use of factory ammo for calibration, will the big factories tell you what pressure their ammo is loaded to?