Thread: OAL = COL ?
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Old September 3, 2012, 10:08 AM   #24
SL1
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Join Date: November 8, 2007
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Today, MOST reloading manuals provide data that was produced using TEST barrels to determine PRESSURE. Some simply report the velocities achieved in those test barrels, while others fire the same loads in an "example" or "test" firearm that is commercially available. The rationale is that the test barrels is carefully made to SAAMI minimum dimension specifications, so it will produce as-high or higher pressure than more sloppy commercial barrels. That helps ensure that the loads developed will not cause excessive pressures in handloaders' guns. But, the more sloppy chambers may produce less pressure, and therefore less velocity. SO, many manuals also report the velocities from the example commercial guns (which may also have different barrel lengths than the standard SAAMI test barrels). That helps handloaders understand what vleocities they can realistically expect to achieve. But, it does provide a half-apples-half-oranges type of data fruit salad for the armchair reloader to try to reconcile in his head from manual to manual. With all the other varibles involved, figuring-out what chamber pressure should equal what velocity in your own gun is generally just not doable without a computer program like QuickLOAD or some acutal pressure testing equipment like NECO sells.

With that in mind, the COL listed in a manual still is VERY important when developing handloads for small, high-pressure cartridges like the 9mm and 40 S&W. If you shorten the COL WITH THE SAME BULLET as used in the data, you will increase the pressure. If you use a DIFFERENT bullet with the same COL, you may change the pressure up or down if the bullet is not the same length as the bullet used to develop the data. That is because the real variable of interest is SEATING DEPTH, which is the sum of bullet length and case length minus the COL. That (and the case's internal volume) is what determines the space remaining for the powder, which is what is needed to determine the pressure (in conjunction with a lot of other variables including the powder, bullet weight, primer, etc).

When you change the seating depth by 0.01" or more from some loading manual data for a specified bullet weight, either by changing the COL or by using a different bullet that has a different length, then you should be addressing the change in pressure. Laboratory tests have shown that changing the seating depth in a 9mm by something like 0.030" CAN change the peak pressure by tens of thousands of psi with particularly sensitive loads. Typically, pressure differences are much smaller than that, but can still be quite significant, especially if you are pushing the max.

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