Head space causing a huge difference in accuracy?

[matt2143]

I am shooting 178 BTHP over 2000MR and was running some tests. I found bumping the shoulder .003" from measured twice fired brass versus bumping back .007-8" cut my groups in half. Printed a 2" center to center 5 shot group at 560 yards without changing my POA for wind (about a 1.5" vertical dispersion).

I had been looking around and had not seen much on this variable in reloading other than over working the brass. I understand factory ammo is loaded to spec and may be fired from many different chambers but DAMN! My M118LR box ammo shoots pretty well but not 2" at 560, more like 1/2-3/4 MOA.

 

[0311@LAR]

Why were you bumping your shoulder .008? Seems pretty excessive. I don't know how much it will affect your groups but it would be hell on your brass I would think.

 

[matt2143]

It was an accident. I had measured some once fired brass incorrectly and thought I was going 0.004". Then I found out brass does not fully expand until the second or third firing so I started measuring the twice fired brass. Unfortunately the only way to bump the shoulder back out is to fire it

My RCBS die was giving me hell.