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Old October 4, 2012, 06:57 AM   #20
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Join Date: November 8, 2007
Posts: 2,001
SAAMI drawings

show the groove diameter as 0.429" with a 0.004" positive tolerance and zero negative tolerance. So, your groove measurements appear to be normal.

A more significant measurement might be bore cross sectional area. That is tha actual open area in the barrel that the bullet must be squeezed into. SAAMI specifies 0.1405 square inches.

You can calculate it pretty well as the groove diameter squared, divided by 4 and multiplied by Pi then subtracting the area taken by the lands. That is the height of the lands (or the depth of the grooves) multiplied by the width of the lands times the number of lands.

The smaller the area, the higher the pressure and thus velocity.

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