Head Space

[purple bikerr]

Hey all!! First time poster here. I am an experienced reloader, been doing it for 28 yrs. Rifle and hand gun. But this will be my first rodeo loading for a semi auto rifle. I just bought my 1st AR. What is the proper headspace on a .556? Yes I am sure it will safely shoot .556 ammo. It's a Spikes ST-15.

I found on the 'net that max is 1.464, but the Winnie factory ammo I had measured 1.458.

So what do I set my sizing die to?

I know that I cannot "bump" the shoulder back .002" like I do for my bolt guns. .002" is not enough to reliably cycle the bolt (action) on a semi auto. Is this gonna be a "trial and error" exercise to find the proper headspace for "my" gun?

This is the first question I have concerning AR's. Many more to come.

Edit: I just measured my fired brass and they head spaced at 1.462". Reading this post again I think I answered my own question. Seems 1.458 is the magic number.

[fedupflyer]

Easiest way to do it is to fire 5 or so factory rounds and measure them with a Hornady headspace comparator.
For gas guns, bump it back between .003 and .005.

Drop in the .223 case gauge to verify headspace.

[purple bikerr]

You must have been replying while I was measuring my cases and editing my OP. Your suggestion to bump 'er back .003 - .005 falls right in line with my thinking.

[T2C]

I neck size bolt action match brass and bump the shoulder back 0.002" for general bolt action ammunition. I bump the shoulder back 0.004" for semi-automatic reloads.

Depending on what brand cases you use, you may have some spring back after cycling through the resizing die. If you are using mixed headstamps watch your brass headspace closely to ensure you are getting consistent 0.004" set back.

[noylj]

I just always sized until the cases chambered easily and went from there. Never heard of the shoulder dimension affecting whether the bolt cycles--thought that was chamber pressure.
PS: 0.556 is a large caliber. I think you moved the decimal point and meant 5.56...

[purple bikerr]

You are correct about the shoulder not affecting he bolt cycle. I meant that the round would not chamber. Good catch!!

Yea, the decimal is kinda important to put in its proper place. Wish it was one place to the right on my paycheck.

[Bret]

Easiest way to do it is to fire 5 or so factory rounds and measure them with a Hornady headspace comparator.
For gas guns, bump it back between .003 and .005.

+1
This has done the trick for me. I don't use mixed brass though, so I don't know if that would make a difference.

[purple bikerr]

I have a Hornady Headspace comparator, and all of my brass is PPU once fired. I measured approx. 100 pieces and they all sized to the same dimensions. But all the cases are right at 1.464". I do have to set my die back another .006.

[Bret]

The first thing that you need to understand about the Hornady comparator is that it does not provide an absolute headspace value. It simply provides differences between headspace measurements. Fortunately, you don't need an absolute headspace value, just the differences. If you were going to fire your reloaded ammo in multiple rifles or even rifles that you don't have fired cases from, it would be a good idea to resize it back to below the factory maximum size. However, you're not in that situation. You have one rifle that you need to size the cases for. You simply need to size them 0.004" to 0.005" below the fired case values that you're reading on your comparator. Any more than that and you're just overworking the cases for (in theory) less accuracy potential.

[markm]

If you were going to fire your reloaded ammo in multiple rifles or even rifles that you don't have fired cases from, it would be a good idea to resize it back to below the factory maximum size.

Exactly. I've always just set my die so that it averages in between the min/max on a head space gauge. I don't know why guys like to torture themselves with extra gadgets.

Even when setting up bolt gun dies, I simply use the gun as a gauge.