Headspace measurement

[thewolf]

I decided to spend more money on sending rounds down range and collect the necessary tools as I can. With my last two neck sized batches of reloads I had a bit of “crush fit” when chambering. The real issue came with 5 rounds that I didn’t feel comfortable chambering, too much force. I have been getting away without proper headspace but it looks like I am at that point now.

So I went out and got an rcbs headspace mic but I am a little confused about which brass I should use.

Factory one fired = - 001
Reloaded = .002

So which brass measurement should I use?

I am assuming I would use the once fired, with the -.001 rebound it would indicate my chamber at 0. But I wanted to
make sure.

Thanks

 

[MitchAlsup]

Re: Headspace measurement

If you ere reloading for someone else, SAAMI specs would be in order. However, you are loading for yourself and your gun. Thus you want the ammo to optimally fit in your gun. The optimal fit in your gun is going to have the neck at least 0.005 from the throat cut in your chamber, and the shoulder be at least 0.001 shorter than the bolt face to chamber shoulder. Radially, you want 0.001 clearanace. This gives just enough slop so the the round is self centering in the chamber.

While the RCBS case mic is a fine tool, and manufactured with precision, you still have to figure out what your gun wants. The easiest way is to fine a set of 3 cases, one that you feel no pressure in closing the bolt, one that you almost can't close the bolt, and one where you can just feel the bolt drag--with the property that all 3 cases are within 0.001 on the RCBS csae mic. This gives you the measurement you need--so write it down and check it every 1000 rounds as it may slowly grow on you.

This just gives you the chamber dimensions, most of us just push the sholder back 0.002 and have done with it. A few try to push the shoulder back 0.001.

 

[thewolf]

Re: Headspace measurement

Thanks MitchAlsup for the response.

Here is what I got with just the bolt no firing pin
case 1. Almost can’t close. +.002 on the mic
case 2. Maybe 3-5 pounds. +.002
case 3. 1-2 pounds. +.001

So would saying chamber dimension is .002 correct?

Pushing back the .001 would be nice and tight for bench rest. And back .002 for when you’re rolling in the dirt. Is that the logic were going for?