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.

[Unclenick]

If you read Hatcher's Notebook, he observed something similar; about -0.006" setback on 30-06 chambered rapidly in a 1917 Enfield. Inertia slams the cartridge into the chamber, and if it hits the chamber shoulder, the chamber is wider than a resizing die, so it lets the brass get enough fatter to get that short. A sizing die constrains the sides, so it doesn't happen there.

[IMtheNRA]

No, I'm not familiar with that book, but I see that he beat me to this observation by about a hundred years.

So, which case best represents my chamber and should I increase the amount of shoulder set-back that my resizing die provides? I've never had a problem, and I don't want to ruin my brass any faster than necessary.

[mehavey]

`Does make`ya wonder about all this uber-precision-1-2thou-just-enuf shoulder bump on gas guns, don't it ?

[LineStretcher]

There's an old saying, "don't fix what isn't broke".

[cw308]

Have you sized the case to the fired length to see if it chambered ? .003 set back is what you want on a semi , chambers don't change . Your .003 using the mic before is fine you just want to make sure it's not really longer or shorter for that matter . Did you start your chamber testing with a longer sized case if so your fine , that bolt slams pretty hard at lock up without any forward assist . Colt HBar 5.56 chrome 1/7 twist 20" barrel is very nice , gave mine to my Son-in-law .

[Unclenick]

Originally Posted by IMtheNRA

So, which case best represents my chamber…

The one that comes out of the chamber as-fired comes closest to mirroring the chamber dimensions. The dummy would have to somehow be fired to bring it back to chamber length.

That said, there is probably about -0.001" of spring-back in the as-fired case, making it likely to be about that much shorter than your actual chamber interior. So if you size to get a shoulder about -0.001 to -0.002" back from the as-fired shape, that is probably enough. I think Glen Zediker has used -0.002" for a long time.

If you want to get a case really close to actual case size, you need to neck size it only and load and then chamber it gently and fire again. Remeasure it afterward. Repeat until it gets reluctant to chamber, then use the last reading. It should be very close.

One reason for using SLEDs or magazines for single-loading is to slow the bolt down on the cartridges way into the chamber. Doing so should help extend case life by not having the casehead need to blow back as far to find the bolt face. Not using an overpowered recoil spring also helps.