Years ago I snatched up an RCBS micrometer seating stem for .270 Winchester dies and by the time I got home it had morphed into a .270 Precision Mic. Either that, or I didn�t read the box label close enough. The gun show where I purchased it had packed up and moved on, so I tossed the set into the black hole we all have under the reloading bench.
Due to a fluctuation in gravity at my house, the Precision Mic was spit out on my bench yesterday for me to fool around with. Either that, or I happened to find it while looking for something else.
The only .270 Winchester in the safe is a Browning-you-guessed-it-A-Bolt, and the only loaded rounds and fired brass are from the same rifle. I have no idea how accurate this rifle is except to say that I can manage MOA with it however, my dog could probably out-shoot me if he had a trigger finger.
According to the RCBS directions, the chamber headspace for a .270 Winchester should range from 2.0487 to 2.0587 inches. Once-fired brass from the A-Bolt measures dead on the minimum.
Fully-resized cases measure slightly longer at +0.001 to +0.003. This answers my question about why some factory and fully-resized cases chambered with (very) slightly more effort in this rifle than neck-sized only. So the rifle has a short chamber � what does that mean to me?
The Precision Mic set also includes a doodad for determining the best bullet seating depth, in the form of friction-fit dummy cartridge. One adjusts the dummy �too long� and then chambers it in the rifle. The nose of the dummy is stopped by the barrel lands and pressure from the closing bolt sizes the body. The dummy is carefully extracted and measured using the freebore tool included with the set. As near as I can tell, the dummy can�t be used with an A-Bolt or any bolt with a plunger ejector. The friction surfaces of the dummy don�t have enough backbone to compress the plunger spring, at least not consistently. The freebore measurement ends up waaay short.
