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Posted: 5/7/2010 7:49:45 PM EDT
| I'm using RCBS dies in a Rock Chucker and with everything set up and tightened down, my cartridge overall length varies as much as 0.01" between the high and low in a batch of rounds. Am I doing something wrong? Should I not worry about it? I'm using LC brass and mostly Hornady 68gr BTHP, but I've seen it with other projectiles. |
| As mentioned, you need a tool called a comparator that allows you to measure off the oglive of the bullet. Bullets vary in length and since the bullet seater seats off the oglive of the bullet, a comparator which measures off that datum point will give you a more consistant reading. The OAL length still matters and you need to make sure all the rounds don't exceed that maximum listed measurement. |
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That one round might be the short bullet in the bunch and then your other rounds will be long. I'de measure a small sample of your bullets and find the longest one and set the seater die for that one, then the rest should be ok. +1, measure all your bullets and check the manufacture specs, typically I find the bullets will not be longer than what the manufacture has specified. Using the longest bullets (bullets at the spec) I set up my seating die. This has worked very well for me, YMMV! |
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That one round might be the short bullet in the bunch and then your other rounds will be long. I'de measure a small sample of your bullets and find the longest one and set the seater die for that one, then the rest should be ok. Once you find the longest bullets (don't look at every bullet, a sample batch of 20 or so will be sufficient), set the COAL to that bullet, then seat all of the bullets with that setting of the seater die. Take the seater stem out of your die. Look at how it engages the bullet by pressing against the ogive, not the tip. (If the seater mashes the tip, then either the die is defective or the bullet is unusual. Some really old stems in the used market weren't deep enough for the new VLD and high BC bullets coming out and had to be drilled out, but that would be a defect now.) By pushing on the ogive, the distance from the base of the bullet to the contact datum is identical for every bullet no matter it's length. This also means every bullet is seated the same depth into the case - this is good, it provides consistent internal case volume and uniform engagement length of the bullet's bearing surface with the case neck. |
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a .001" inconsistency is going to kill you. In fact, when reloading on a progressive press like a Dillon, the Dillon tech people will even tell you that it's simply the nature of the beast. The Sierra meplats aren't all exactly the same, neither are Hornady, Berger, Norma, Lapua etc. etc. - Hence the arrival of the meplat trimmer tools. One thing you can check, to try and make your OAL 's a little more consistent, is to check the case lengths. If they aren't exactly the same, neither will the COAL's.
I know you've said you're running a Rock Chucker press - one thing that has caused inconsistent COAL's for me in the past, has been compressed charges. If you're in the 24gr. range, and you're using an extruded powder, and setting the COAL to 2.250", you're likely compressing powder. This will certainly give you some inconsistencies. I've used my COAL of 2.260" per the Lyman 49th ed. and never run into compressed charges. As soon as I used the Hornady 68gr. BTHP, and used their recommended COAL of 2.250", I started compressing powder, and the lengths were all over the place. I bumped the COAL back out to 2.255", and the problem went away. From practical experience, you can load a LC case with 24gr. of Varget, and 4895, with a COAL of 2.260", and it still fits in the magazine just fine. Hope this helps. -tc |
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You can get a basic comparator set pretty cheap, I just recently purchased a OAL gage along with a basic set from Hornady and I think I spent $75, I found that even SMK's varied by .005-.007" not sure if I will start trimming Meplat's yet A comparator is completely useless for loading to magazine length. However, where loading close to the start of the rifling, they come in handy for understanding the length to touch with a particular bullet, and then the clearance. |
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One thing you can check, to try and make your OAL 's a little more consistent, is to check the case lengths. If they aren't exactly the same, neither will the COAL's. That is not a true statement. While I agree that having consitent case length is important for a few reasons, it does not make your COALs different. |
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http://www.leeprecision.com/cgi/faq/index.cgi "Seating depth variations
There are a number of possible causes for overall length variation. One is the way it is measured. If you measure overall length from the tip of the bullet to the base of the case, remember to subtract the variation due to bullet length tolerance. The bullets will vary in length due to manufacturing tolerances (bullets with exposed lead noses are the worst in this regard) and this will add to the overall cartridge length variation. Remember that the bullet seater plug does not (or shouldn't) contact the tip of the bullet when seating, but contacts farther down the ogive. For a more accurate seating depth measurement, take the seater plug out of the bullet seating die, place it on top of the cartridge and measure from the base of the case to the top of the seater plug. Another possible cause for bullet seating depth variation is seating and crimping at the same time when trying to apply a firm crimp to untrimmed cases. Variation in case length also causes variation in the amount of crimp applied. Long cases get a heavier crimp than short ones. When seating and crimping at the same time, the crimp is formed as the bullet is seated into the case. The crimp will form sooner on a long case, and therefore the bullet will not be seated as deeply. The solution is to seat and crimp in a separate step and/or trim cases to a uniform length." |
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You can get a basic comparator set pretty cheap, I just recently purchased a OAL gage along with a basic set from Hornady and I think I spent $75, I found that even SMK's varied by .005-.007" not sure if I will start trimming Meplat's yet A comparator is completely useless for loading to magazine length. However, where loading close to the start of the rifling, they come in handy for understanding the length to touch with a particular bullet, and then the clearance. Actually my Savage will hold rounds in the box loaded 2.240" from the ogive which puts the total OAL at slightly over 2.840" |
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Check some Hornady match rounds or SMK's. OAL can vary by as much as .005".
Focusing on COAL will drive you mad. So will runout, but that's another thread. I rarely check COAL length anymore. It's just not that important for the rifle ammo I load. With one exception. When I load for .223, I'm either seating in the cannelure or loading the heavy bullets to mag length. OAL isn't relevant. When loading for my 300 Weatherby, I am limited by the length of the Howaby mag box so once again I'm loading to mag length. With my M700 270, I can seat out to the rifling. I have a tool to help calculate the OAL and load to that length using the comparator. With the SMLE, both 308 and 303 British it's mag length. The Mausers have long magazines. I usually seat to the cannelure or the COAL recommended by the manual. I use the comparator to make sure there is uniformity. Loading 300 Savage for the M99 Savage the COAL is critical, so I pay attention. Make the round too long and it won't load into the magazine. |
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By pushing on the ogive, the distance from the base of the bullet to the contact datum is identical for every bullet no matter it's length. This also means every bullet is seated the same depth into the case - this is good, it provides consistent internal case volume and uniform engagement length of the bullet's bearing surface with the case neck. Is it a manufacturing process that can create a consistant length from base to ogive, but not ogive to tip? Do polymer tip bullets perform better in this regard? |
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You can get a basic comparator set pretty cheap, I just recently purchased a OAL gage along with a basic set from Hornady and I think I spent $75, I found that even SMK's varied by .005-.007" not sure if I will start trimming Meplat's yet A comparator is completely useless for loading to magazine length. However, where loading close to the start of the rifling, they come in handy for understanding the length to touch with a particular bullet, and then the clearance. Actually my Savage will hold rounds in the box loaded 2.240" from the ogive which puts the total OAL at slightly over 2.840" The ammunition still has to be loaded to fit the magazine; the COAL is the principal dimension for fit in the magazine, the length to the ogive is peripheral and not important. |
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Quoted:
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By pushing on the ogive, the distance from the base of the bullet to the contact datum is identical for every bullet no matter it's length. This also means every bullet is seated the same depth into the case - this is good, it provides consistent internal case volume and uniform engagement length of the bullet's bearing surface with the case neck. Is it a manufacturing process that can create a consistant length from base to ogive, but not ogive to tip? Do polymer tip bullets perform better in this regard? Yes. Hollow point match bullets are manufactured by filling a jacket with a lead core, then pointing up the meplat. Meplat trimmers are available, but aren't really necessary for shooting at distances inside 600 yards. The uniformed meplat causes the bullets to fly with identical but slightly reduced BC for better control of vertical dispersion at long ranges. The BC can be preserved or improved by using a die that points up the bullets, too. |
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