Resizing setup: use fired case or press-fit case in RCBS Precision Mic?

[IMtheNRA]

Rifle: Colt HBAR 5.56 1/7 circa-1994
Brass: Winchester and Lapua
Die: RCBS full length with Lyman carbide expander
Press: Dillon 1050
Lube: RCBS Case Slick
Measuring tool: RCBS Precision Mic

For years, I've loaded for my .223 AR with the usual method of setting back the shoulder by about .003 from the fired case, as measured with my RCBS Precision Mic tool. I've never had a feed malfunction, nor was I ever unable to easily eject an unfired cartridge.

Recently, I experimented with chambering dummy cartridges made with cases that were deliberately not resized enough. To my surprise, the cases of the dummy cartridges came out with shoulders pushed back quite a bit more than I normally set them back while resizing. Here are the measurements using the RCBS Precision Mic tool:

Fired picked up cases read 0 or -1 on the tool
My "as-usual" resized cases are mostly -3 or -4 after exiting the resizing die
Experimental dummy cartridge case, not resized down enough reads -1
Same case after chambering from magazine and dropping the AR bolt on it is now -7 !!!

What's going on? Have I been setting back my case shoulders insufficiently all these years, and having the rifle do the rest of the job by press-fitting the cartridges to the chamber?

Should I stick with my tried and true shoulder set-back numbers or should I readjust my resizing die to push the shoulders back a few thousands from this -7 reading of press-fit dummy cartridge cases? If I readjust the die, it would be a huge change from how far I normally push back the case shoulders.

 

[troy fairweather]

i could be from the cases hitting the chamber. this happens when a round enters the chamber fast. the shoulder hits the chamber shoulder and sets the brass back. this could be what happend. there is a name for it . u can read about it in Hatcher's Notebook

 

[IMtheNRA]

this happens when a round enters the chamber fast. the shoulder hits the chamber shoulder and sets the brass back. this could be what happend.

I have no doubt that's exactly what happened during my test. I'm wondering if the unfired, but press-fit case is a better representation of my chamber and if I should reset my sizing die to push the shoulders back even more than I currently do.

I do intend to look up Hatcher's Notebook to see if he was wondering the same thing and what conclusion he reached.

 

[troy fairweather]

I have no doubt that's exactly what happened during my test. I'm wondering if the unfired, but press-fit case is a better representation of my chamber and if I should reset my sizing die to push the shoulders back even more than I currently do.

I do intend to look up Hatcher's Notebook to see if he was wondering the same thing and what conclusion he reached.

try it see what happens. the book is a good read u can find the download for free, dont remember offhand.

 

[IMtheNRA]

If anyone knows how the book ended, let me know.

 

[troy fairweather]

https://www.tngun.com/wp-content/uploads/Hatchers-Notebook.pdf

 

[Blue68f100]

The bolts on AR's have enough energy to bump the shoulder back when released from full open. This is a well know fact. If you want to see if chambers without sizing, do not let the bolt fly home, let it down in a controlled method. My AR-10 has a very strong spring. When testing ammo I always use a bob-sled to single feed the rounds. Once I have my load worked up I test both ways to see if my group changes.

 

[Chuck R.]

You could treat it like your checking headspace and remove the extractor and ejector. That would give you a decent "feel" when going into battery by hand.