Full - Length Sizing of Brass, Semiautomatic Rifles

[Edsel]

This is a repost from another online Forum.

From what I've read online:

1. For a bolt - action rifle, set the shoulder of the case back 0.0015 - 0.002", as the casing very closely approximates chamber dimensions upon extraction.

2. For a semiautomatic rifle, bump the shoulder back by 0.004" - 0.005", as extraction begins while the case is still hot and under pressure, and the brass typically expands more during the extraction process.

Questions:

1. Running any rifle which permits setting the gas system OFF, and using manually extracted cases as my "fireformed brass," would it do me better to bump the shoulder back 0.0015 - 0.002" and use the resulting case dimensions as my reference for full - length sizing?

2. In rifles which can only be run with the gas system ON, will manually loading a previously fired case from the same rifle into the chamber and allowing the bolt to slam shut on it size the brass close to chamber dimensions, and allow me to bump the shoulder back 0.0015 - 0.002" instead of 0.004" - 0.005"?

 

[maggitas]

Sounds right. Bumping the shoulders back (For a semiautomatic rifle, bump the shoulder back by 0.004" - 0.005", as extraction begins while the case is still hot and under pressure, and the brass typically expands more during the extraction process) is for function. It allows the gun to cycle and strip the next round from the magazine.

 

[Edsel]

Sounds right. Bumping the shoulders back (For a semiautomatic rifle, bump the shoulder back by 0.004" - 0.005", as extraction begins while the case is still hot and under pressure, and the brass typically expands more during the extraction process) is for function. It allows the gun to cycle and strip the next round from the magazine.

What are your thoughts on Questions 1 & 2?

 

[MitchAlsup]

Sometimes is is simply easier to measure the shoulder position.
1) If the shoulder position is in the right place, all you need do is to size the neck to hold another bullet.
2) If the shoulder is not in the right position, you size the shoulder so that it is. This might be 0.001 or as much as 0.004 depending on the case in hand not depending so much on the gun at hand.
3) oversizing the shoulder can result in hang-fires and no fires.

 

[maggitas]

If you want a Semi auto to extract reliably you need to bump the shoulders back to the recommended settings.

 

[JMGlasgow]

Sometimes you may need small base dies for a semi auto as well.

 

[Edsel]

Again, this is a repost from another Forum...

Still looking for opinions, and answers to the prior questions.

The main query concerns proper headspacing measurements for a gas - operated rifle, using baseline measurements ("fireformed brass") from casings fired with the gas port off.

I'm merely practicing on my 5.56x45mm AR rifle while waiting for the dies for my .308 Winchester AR to arrive, so let's treat it as a "Precision AR Platform."

I was playing around with some measurements using the Hornady Lock-N-Load Headspace Comparator and tried sizing some 5.56x45mm NATO and .223 Remington range trash. I replaced the brass thumbscrews with plasti*... cough, cough polymer thumbscrews to prevent gouging my fancy calipers.

I took some baseline measurements on these rounds,

...as well as these. Please note the inclusion of the utterly irrelevant, phallic surrogate.

 

[Edsel]

Both batches were surprisingly consistent at 1.4585", even the el cheapo ammunition.

Now, measuring once - fired brass from my rifle was tricky... These casings weren't as easy to center and required some "spinning." These casings were fired with the gas system off, and were extracted manually after each shot. I'm presuming that this would be equivalent to rounds fired from a bolt - action rifle with really sloppy headspacing. Measurements were more or less consistent at 1.4610".

Please note the subconscious suggestion of an oppressive father figure. Since fancy man - jewelry is warranted and I didn't have any at hand...

...I suspect this Breitling will suffice. Look. It's a Chronoslide! Oodles of utter awesomeness!!!

Instead of performing Full - Length Resizing and bumping the shoulders back to 1.4585", Partial Full - Length Resizing was done - with the shoulders bumped back 1.4610" - 1.4595" = 0.0015" (the recommended case shrinkage for bolt - actions). It was difficult getting a consistent measurement of 1.4595" at first... Until I realized how the carbide expander ball pulled the shoulders up when extracting the cases. Removed the carbide expander ball because of this.

Look. Another knife, but not quite phallic - since slicing, unlike stabbing and poking, doesn't really connotate such... Not quite sure what it implies. But it's likewise awesome because it's forged from genuine German steel. Rostfrei!

Testing how the bolt behaved using both unfired FGMM ammunition and Partial Full - Length resized brass (shoulders bumped back to 1.4595", or by 0.0015") as detailed above...

Riding the bolt gently (instead of letting it slam shut) prevented full closure with both cases (unfired FGMM, as well as resized cases).

A very gentle push on the bolt assist facilitated full closure for both. Best that I could tell from what I could "feel," the cases were chambered with little issue. Extraction and ejection were likewise trouble - free.

 

[Edsel]

Regard the 5.56x45mm AR as a "Precision Rifle" for a moment, as this is merely a practice run.

The questions are (reiterating some):

1. Running any rifle which permits setting the gas system OFF, and using manually extracted cases as my "fireformed brass," would it do me better to bump the shoulder back 0.0015 - 0.002" instead of 0.004" - 0.005" and use the resulting case dimensions as my reference for full - length sizing?

2. In rifles which can only be run with the gas system ON, will manually loading a previously fired case from the same rifle into the chamber and allowing the bolt to slam shut on it size the brass close to chamber dimensions, and allow me to bump the shoulder back 0.0015 - 0.002" instead of 0.004" - 0.005"?

3. If I resized my casings using the measurements as detailed above (1.4610" - 1.4595" = 0.0015") for my 5.56x45mm AR only, would I be doing anything horribly unsafe?

 

[maggitas]

Edsel,

The only way to know is to try. The smart money, through plenty of trial and error, is that you will have a case stuck in the chamber. You will constantly have to use your bore rod to remove the spent case or worse the expansion of the fired casing will seize the gun in battery. It kind of defeats the purpose of using a semi gun. If you want that level of accuracy you should buy a Bolt action rifle and call it a day.

 

[mdmp5]

For bolt guns, shoulder setback of 0 to .001 does just fine for me. For gas guns, .005 is a bit too much. I have inadvertently oversized brass that much in the past and have run into extreme pressure issues because of the diminished volume in the case. A safe load of 47 grains was very hot in these oversized cases. In my opinion, .003 is all you need in gas guns.

 

[Edsel]

Edsel,

The only way to know is to try. The smart money, through plenty of trial and error, is that you will have a case stuck in the chamber. You will constantly have to use your bore rod to remove the spent case or worse the expansion of the fired casing will seize the gun in batter. It kind of defeats the purpose of using a semi gun. If you want that level of accuracy you should buy a Bolt action rifle and call it a day.

Ok, will do.

 

[No1]

FWIW, I set the shoulder .002" back for both of my AR10s. No feeding issues. Both chambers are as near identical as you can get so same ammo works in both.