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Using NOE Moulds => Casting and Reloading => Topic started by: Guy_From_Michigan on July 27, 2020, 07:45:53 PM

Title: Headspace in 308
Post by: Guy_From_Michigan on July 27, 2020, 07:45:53 PM
I have a question about head-space in a 308. I am loading Starline brass for my AR-10 and will be loading for my M1A soon. My children purchased me this AT-10 (Sig 718 I think) and its a beautiful rifle and shoots great. I do not want to do anything to hurt it.

I am working with a batch of 20 pieces of Starline brass (new when I started) I have fired them 4 times so far. My fired cases show me 1.628 and the M1A show me 1.631 head-space. From what I read I do not want a lot of extra head-space and I am using RCBS Small Base Dies for semi autos. When I size my brass I clean off the lube and measure it and the head-space is all over the place. Most are around 1.625 which is fine but some are as low as 1.620 (threw that one away) and as high as 1.627.

Is this normal? am I just worrying to much? After this firing I annealed them and cut 2 of them open to check the condition of the cases and they were fine no case head separation (Looked bran new).

I know I can not come up to the head-space that the brass is at after firing but how low can it go safely? I want my brass to last but I really do not want to blow up the rifles (or me).

If anyone has the time to educate me I would be thankful I have received some good advice on what bullet to use with what gun over the years and I have read enough post here to think there are some real wise people here.
Title: Re: Headspace in 308
Post by: Mike B. on July 28, 2020, 08:48:36 AM
Ok Lets see where we can start to answer your questions. First of all you noted that you are using a SMALL BASE DIE SET. The Small base die sets are basically designed to size the cartridge case down to insure that the loaded cartridge case will fully chamber into any Semi Auto. That is most probably a whole lot more sizing on the individual cartridge case each time that you use that particular set of sizing dies than might be needed for your individual rifle. They in itself are in addition to resizing the base of the cartridge are most probably moving the shoulder of the fired cartridge case back more than is needed to insure that the loaded cartridge case will fully chamber in any Semi Auto.

SO… How much is needed to insure that the cartridge case is sufficiently sized to insure that your cartridge will basically chamber in any 308 Winchester in the world? For that you can go to the SAAMI specifications and download the pdf of the Rifle Cartridges. You will I believe find the 308 Win Cartridge and Chamber specs on page 122 of that document.
Here is the link to the SAAMI website :   

The 308 Win head spaces on the cartridge shoulder which according to the SAAMI specifications is listed as 1.634-007. From that SAAMI dimension drawing one would assume that one has a range from (1.634 to 1.627).

For your Convenience I have attached the scan of page 122 from that document. That should be enough for now to answer the portion of your question on cartridge sizing without going into a doctoral dissertation on the whole loading process…
Title: Re: Headspace in 308
Post by: Guy_From_Michigan on July 28, 2020, 07:35:30 PM
Thank You for taking the time to reply with all this good information and even links to the drawings. I printed it and saved it. I have a set of Lee Dies I may try them and see what I get. I really do not understand why each case I resize does not have the same head space its all over the place. I just watched a video from a man called the Hornady Reloader and he said the Hornady Comparator is just that its not exact head space figures it just lets you compare it to something else. I have fired over 150 reloads in this rifle so far and no problems but I need to understand what I am doing and not just be lucky.

Thank You again for your great information and your time !
Title: Re: Headspace in 308
Post by: Mike B. on July 28, 2020, 09:40:41 PM
In resizing your 308 Win brass you will I believe want to move the shoulder as little as possible in resizing the cartridge case. Most all of the reloading die instructions state that one must run the sizing die down until the press just cams over the ram. That will give you the maximum amount of sizing that the particular sizing can deliver.

One of the ways to adjust the resizing settings is to back the sizing die off just a bit so that the cartridge case is resized a tiny bit less and then see if the resized cartridge case will chamber. A standard business card is around .005 in thickness.  If you back the sizing die off by inserting a business card in between the shell holder & the bottom of the sizing die gently… you will be sizing and moving the cartridge case shoulder that much less. 

SAAMI says that the 308 Win has a .007 tolerance window on the shoulder dimension, and your fired but unsized case dimensions apparently measure within the SAAMI tolerance window… as per your original query statement dimensions.

Doing this type of a Go -- No Go resizing check in a bolt gun it is easier than in a semi auto. You might have to sacrifice a case or two by making up a dummy cartridge (sans primer) to see if the action will truly go into battery with slightly less sizing.  Unfortunately this is the case neck on a resized (Partially resized case… In this instance) may hang up when moving from the magazine in to the chamber on the empty cartridge case neck. (Without a bullet to help guide it into the chamber).

It’s also basically why a lot of the serious accuracy shooters only tend to neck size… and leave the cartridge case shoulder as close as possible to where it is for that particular rifle & dedicate the use of that reload to that rifle only… 

That is the down side to most neck sizing applications… Those shooters are also normally shooting their neck sized reloaded cartridges in bolt guns which have a greater cam over force when closing the bolt to put the rifle into battery.

Semi Autos unfortunately only have the force of the carrier slamming back forward to strip the cartridge from the magazine and to fully chamber the round by going into battery.

One way to do this is to size a case the case just enough to allow the reloads to chamber in both rifles…
You will be resizing the cartridge case a bit more… but you won’t necessarily have to dedicate your limited amount of cartridge cases to one rifle or the other.

What you also have to being to look at is what the loads are doing to the individual cartridge cases. A hotter load will tend to stretch the case more than a milder one. Also the bullet weight comes into play.

Do you know which of the cartridge cases were fired with what load combination?
Also what was the bullet weight that was used in each of those four sets of reloads…?

One might surmise that the original case whose measured shoulder was shorter after sizing might possibly have been fired with a milder load or possibly a lighter bullet in doing your shooting events.

Also in an AR & the M1A action, I believe that you may find that the action may actually begin to unlock and begin to remove the fired cartridge from the chamber before the bullet actually leaves the barrel… depending on where the gas port is on the barrel and the powder power curve at the instant that the fired bullet passes the gas port and a portion of that gas is bled off to cycle the action either by direct impingement of the gas into the rifle chamber or by piston cycling… depending on your individual firearm.  ARs were originally direct impingement and lately come in a piston version also. The M1A is a piston driven action...

In shooting different loads in a Semi Auto you have to basically also take into account that you probably want the rifle to cycle and load the next round… automatically… Unless you are satisfied with shooting the rifle as a single shot to try to get a bit more accuracy & then recharging the rifle chamber by cycling the carrier by hand to extract and load the next round. Thus you have a much narrower range of loads that will cycle the firearm…

Also did you use more than one type of powder in those four sets of reloads? Or worked up accuracy loads with incrementally larger charges? All this Intel is necessary to try to properly & completely give you a more quantitative answer your original question.

Finally… You have to in your mind decide if the round in question besides being somewhat accurate is meant to go bang each and every time in a Semi Auto… Factory Loads, & or possibly limit your reloading spectrum to reloads that tend to duplicate the factory loads.  They will be the safest ones that have been tested so as not to damage your firearms. The major US powder manufacturers each have on their websites links to their load data…

I would recommend using that load data Intel, & bookmarking those websites… the websites also allow one to print your selections out for later reference. Anything else… Please check the Intel against the published reloading manuals that are out there. One can never have enough of those manual on hand to use a reference material.

Last but not least… Annealing your cartridge cases will tend to greatly lengthen the life of your cartridge cases.
Doing that properly is another lesson in cartridge reloading…. Starting to sound at all… Like a Doctoral Dissertation yet?

Take Care & Stay Safe…