OAl Variation with Sierra 69gr Matchking


Pat M
March 1, 2010, 10:10 PM
A question for the experts: Last night I reloaded 100 rnds of .223 with 69 grain Sierra Match King bullets at what I thought was an overall length of 2.250 inches. I was measuring as I was going along, and I was getting a variance from 2.246 to 2.257. As far as I could tell, I was applying the same amount of pressure on each round. I am using a Lee bullet seating die in the Lee Challenger press. My 55 grain Hornady FMJ and my 53 grain Match Kings are near perfect, with a variance of .002. Am I doing something wrong, or could the ogive vary that much on a match bullet?

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March 1, 2010, 10:23 PM
I measured some bulk .308 168 OTMs while back. The extreme span for those was .01 inch. I am surprised that you would have that much with the .223 69 SMKs. Are you compressing the powder? Sometimes compressed powder pushes back. Perhaps you should pick 10 or more bullets out at random and measure just the bullet height, to get an idea if it is the bullet or the overall stackup.

March 2, 2010, 10:50 AM
I'm still working on a batch of a thousand 69grain SMK's that I bought a year or so ago and this batch is by far the least consistent I've seen. I measure my seating depth using Hornady bullet comparator and the difference has been similar to what you're seeing. It's the bullets and not your seating.

Pat M
March 2, 2010, 10:56 AM
I just measured 10 bullets randomly from the box, and I'm seeing lengths of .990 to .997. At this point in my competitive high power career it probably won't make much of a difference, but I need all the help I can get in terms of accuracy.

March 2, 2010, 11:49 AM
Measuring just the bullets, variances up to .015" aren`t out of the ordinary on the MKs.

March 2, 2010, 01:03 PM
Are you measuring from the tip or the ogive?

March 2, 2010, 04:04 PM
I've seen some aux. tooling in the trade mags, that one can use to help true up the truncated meplat. Some of the "bulk" OTMs I have definitely show a distorted meplat. Anyway, if your bullet height (base to meplat) is ES = .015, then your COAL will vary that amount and then some due to other variances.

March 2, 2010, 07:42 PM
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 (the Lee Factory Crimp die is good for this) and/or trim cases to a uniform length.

The amount of force required to cycle a progressive press varies with the number of cases in the shell plate. When the shell plate is full, it is harder to lower the lever than when there are one or two cases present. This can lead to variation in cartridge overall length because there are different loads placed on the working parts of the press. When the shell plate is full, seating depth will be slightly long, because the load is higher and all of the clearances are taken up. With the shell plate nearly empty, the load is not great enough to squeeze out these clearances, and the seating depth is short. http://www.leeprecision.com/cgi/faq/index.cgi

March 2, 2010, 09:44 PM
I have never seen that much variation with the 69gr SMKs and I have loaded 1000s of them. Redding and RCBS seater dies.

March 2, 2010, 10:09 PM
You are measuring something that has no effect on group size.

Seat them so all will fit in the magazine and forget about it.

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