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Newby question about OAL of 6.8mm cartridge. the Hodgdon Powder Co. data sheet list an OAL of 2.260", a purchased SSA measures 2.240". My loads are from 2.241 to 2.251". do I have a problem or is this acceptable ? The bullets are Sierra matchking and SSA brass
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Posted by: dillon
Posted on: 2010-08-18 at 07:30:21 AM
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I strongly suspect the Hodgdon data sheet is listing the SAAMI maximum cartridge length. If your SSA ammo is using the same projectile as what you are loading, then that should be fine. There are two parts to OAL variation, projectile-to-projectile variation, and machine variation. To minimize machine variation, adjust the seating die with a lubed, fired case in station 1 if loading on a progressive loader, to ensure consistant upward pressure on the toolhead. Additionally, tighten the seat die body lock ring with cases in all the stations, and tighten the seat stem lock ring with a loaded cartridge in the die. The upward pressure inside the die centers the die and seat stem in the threads, and makes sure everything is locked in place under the same load as will be there when you're actually loading ammunition.
You can measuure projectile-to - projectile variation by removing the bullet seating stem from the seat die, setting the nose of a bullet into it, and measuring the OAL of the seat stem + the bullet. Bullets are seated from whereever the seat stem contacts the ogive, or curved part of the bullet, down a set distance. If the diameter that the seat stem contacts the ogive moves up or down the bullet, then OAL will vary by this amount as well. It is normal to see .003" variation to as much as .025" variation. Most vary less than .010", well within manufacturing tolerances. This minor variation has no practical effect on accuracy, even at longer distances.
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Posted by: Hardball1911
Posted on: 2011-02-27 at 08:07:23 AM
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With 6.8, depending upon which bullet I use, I don't go by SAAMI each time for OAL. Those specs are generally put out so that the loaded cartridges will fit inside any manufacturer's magazine, as well as to minimize over pressures from excessive "empty" space between the bullet and powder charge.
Right now, my 6.8 AR likes Speer 100 gr Varmint bullets set fairly long on top of 24.8 grains of IMR 4198. I load three or so rounds and then make sure that I have about .03" between the bullet tip and the forward magazine wall. This is setup for MY rifle, and to achieve a desired spacing between bullet ogive and the beginning of the lands and grooves in MY barrel. I am able, with that load to run groups at 200+ yards that will fit inside the top of a soda can. My last target last Sunday was the lid of a Copenhagen can at 300 yards, which I put 10 rounds into before it fell off of the target board. The OAL on these rounds is right around 2.27- 2.29".
Right now, my 6.8 AR likes Speer 100 gr Varmint bullets set fairly long on top of 24.8 grains of IMR 4198. I load three or so rounds and then make sure that I have about .03" between the bullet tip and the forward magazine wall. This is setup for MY rifle, and to achieve a desired spacing between bullet ogive and the beginning of the lands and grooves in MY barrel. I am able, with that load to run groups at 200+ yards that will fit inside the top of a soda can. My last target last Sunday was the lid of a Copenhagen can at 300 yards, which I put 10 rounds into before it fell off of the target board. The OAL on these rounds is right around 2.27- 2.29".
