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.

[Coal Dragger]

Easiest way to produce in-spec 5.56NATO ammo is to invest $25.00 or so in a good .223/5.56mm case gauge. Set your resizing die to bump the shoulder back until the case rim is just even with the low step on the case gauge. I've found that brass with the shoulder to rim length any longer, even .001"-.002", is not going to function as well as brass that is bumped back to the minimum length for head space. Most factory ammo that I have case gauged also aligns with the low step on the case gauge.

[Krazykarl]

I use rcbs precision micrometer to set my resizing dies for correct shoulder set back. I can record the measured average number for five cases in order to document barrel wear.

[Eric D.]

I prefer to size to a case gauge as well. In my experience there's too much variation in how much setback you get based on brass lot, age, number of firings, headstamp, etc. to go for a specific value with any statistical confidence. This is really important for loading for a semi-auto because there aren't many things that will choke your gun faster and more frequently than a case that hasn't been sized back far enough.

[markm]

I prefer to size to a case gauge as well.

Absolutely. Anything else is nuts... at least for setting a die up for a Semi auto. I try to take a five case sample since not all pieces of brass will have the same "spring back".

[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.

[T2C]

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.

If possible, check the comparator with a chamber headspace gauge. After comparing the Stoney Point comparator with a 30-06 insert to chamber headspace gauges I discovered several thousandths of an inch difference. I logged the differences and take them into account when using the comparator.

Here are the readings I observed:

30-06 Headspace Gauge/ Stoney Point Measurement
Go --------2.0487”/ 2.041”
No Go – 2.0547” / 2.046”
Field ----2.0577”/ 2.049”

As others have recommended, it's a good idea to check resized brass with a cartridge headspace gauge. Until you have a cartridge headspace gauge, you can compare the headspace reading of resized brass with the headspace reading of brass on an unfired factory cartridge from a major ammunition company.

[eightmillimeter]

Plinking ammo? ... just set the die so the press handle cams over, don't even measure the headspace 'cause it just don't matter.

Trying to hold a 6 inch x ring in service rifle comps... then bump it back .003-.004.

[Eric D.]

That's bad advice. Depending on the dies and shell holder cam-over occurs at different places. If I set my Dillon sizing die up to cam over on the Rockchucker the brass would be sized back way too far to be even safe to shoot.


Plinking ammo? ... just set the die so the press handle cams over, don't even measure the headspace 'cause it just don't matter.


That Hornady comparator is a terrible tool to validate the measurements of headspace gauges. The inserts aren't bored accurately enough to take a reading from the established datum line on the case shoulder. They also tend to have radii that allow the insert to seat further down on the case shoulder than where the datum line is referenced resulting in shorter measurements as you observed. Someone above mentioned that tool isn't meant for absolute measurements but rather, relative ones where you use it to measure a fire-formed case and then a sized case and the difference between the two is what counts. Differences between true headspace values and what the tool reads don't really matter.


If possible, check the comparator with a chamber headspace gauge. After comparing the Stoney Point comparator with a 30-06 insert to chamber headspace gauges I discovered several thousandths of an inch difference. I logged the differences and take them into account when using the comparator.

Here are the readings I observed:

30-06 Headspace Gauge/ Stoney Point Measurement
Go --------2.0487”/ 2.041”
No Go – 2.0547” / 2.046”
Field ----2.0577”/ 2.049”

As others have recommended, it's a good idea to check resized brass with a cartridge headspace gauge. Until you have a cartridge headspace gauge, you can compare the headspace reading of resized brass with the headspace reading of brass on an unfired factory cartridge from a major ammunition company.

[markm]

Agreed on can over being a bad idea. If it's too much sizing, you're going to get a lot of case separation.

[Krazykarl]

Especially with tapered cases.

Missed markms quote....

[LMT/556]

Agreed on can over being a bad idea. If it's too much sizing, you're going to get a lot of case separation.
Watch for shiny rings on your brass, if observed use a feeler to locate a ridge.
I was used to sizing -.003 for a PRI which measured 1.458 fired. My new Noveske 20" measured 1.461 fired, the PRI die setup caused stretching of my Hornady brass, observed as a shiny ring about a third from the base. If you can feel them with a feeler the brass is trashed.

[Lefty223]

I can record the measured average number for five cases in order to document barrel wear.
You're talking about cartridge overall length to the ogive ... they're talking headspace to the shoulder.

Good point ... but wrong topic ;)

[Bret]

Watch for shiny rings on your brass, if observed use a feeler to locate a ridge.
Poor man's feeler gauge: Take a paper clip and straighten it out. Sharpen the end to make a wedge. Bend the very end so that the wedge is perpendicular to the bend. Insert it in the case and you can feel rings where the brass has thinned.

[Lefty223]

Insert it in the case and you can feel rings where the brass has thinned.
... usually just above the rim, maybe 1/4" or so ... , but that end of the case, closer to the rim, away from the shoulder.

[markm]

Easiest way to produce in-spec 5.56NATO ammo is to invest $25.00 or so in a good .223/5.56mm case gauge. Set your resizing die to bump the shoulder back until the case rim is just even with the low step on the case gauge. I've found that brass with the shoulder to rim length any longer, even .001"-.002", is not going to function as well as brass that is bumped back to the minimum length for head space. Most factory ammo that I have case gauged also aligns with the low step on the case gauge.

This. No need for other tools, unless you are into gear or obsessed with quantifying every thing.

[LMT/556]

My rounds are towards the upper step of my dillon case gauge but are loaded for a single rifle. Having the Hornady comparator and a separate case gauge just allows double checking against different means, both should agree or make sense when comparing measurements.

[eightmillimeter]

Press cam-over has nothing to do with how much a case gets sized in one press to the next. The only thing it guarantees is full contact between the bottom of the die and the top of the shell holder. It's the only place in the press stroke that is 100% consistent.

When you start measuring your headspace on sized cases eventually you will get to a point where you have that die adjusted just perfectly and every case you size is coming in -.004 but depending on the brass and a few other things you'll get stragglers at -.002 and mysteries at -.005. It's not always spring back. Sometimes the brass is just worked harder and since you have the die screwed back to get that headspace perfect the slop in the ram will give you the variable readings. This is why Redding makes competition shell holders of varying thickness so you can set your headspace AND still cam the press over. Using that setup and fresh or annealed brass you can almost guarantee that every case will come out of the die the same size without springback.

I've been in the game a long damn time and over the years It becomes obvious that reloading for a gas gun is not rocket science and there is no reason to make it so. Any good SAAMI spec dies will work just fine, follow the instructions that come with the set, inspect your cases with a case gauge... You may get one more firing out of a case (maybe) by sizing it to -.003 as opposed to just FL sizing but what did you gain? By the time a 5.56 case has been reloaded 5-6 times you're already way past the designed service life. Just pick a number of times to reload that you are comfortable with and toss em, sleep easy.

[Bret]

This. No need for other tools, unless you are into gear or obsessed with quantifying every thing.
If you want to fine tune your headspace adjustment for loads in each rifle, then you'll need more than just a yes or no gauge. For example, I have an AR180 that has headspace that measures approximately 0.010" greater than most 5.56x45 AR15's. I shoot new factory loaded ammo in it and then resize the cases so that they're only 0.002" to 0.003" less than the AR180's chamber. I can then repeat the process over and over with the brass lasting through numerous loadings. These same cartridges won't fit in most 5.56x45 chambers. If I did resize the brass back so that it would fit in a case gauge each time, then it would stretch much more, wouldn't last as long, and would likely result in less accuracy.

[LMT/556]

What I'm doing also. One size does not fit all and maintain long case life, as demonstrated by rings on my Hornady brass.

[markm]

Unless you're annealing every loading, you'll never get fine tuning on the bump. As pointed out, uneven work hardening leaves you with finding the best average bump. I set my die by checking five pieces in the gauge.

For specific/bolt gun chambers, I size to the chamber fit. Knowing the nominal bump has no value for me.

[bigedp51]

Chambers and dies vary in size and the advantage of the Hornady cartridge case headspace gauge is being able to measure fired cases. Then adjust the die for proper shoulder bump and not push the case shoulder back too far.

Below I'm measuring a fired case from my AR15, I then adjust the die for .003 to .004 shoulder setback using Redding competition shell holders. Each of these competition shell holders allows the die to contact the shell holder reaching press cam over. This takes any slop out of the press and gives more constant shoulder location.

http://i.imgur.com/H0SXHH8.jpg

Below in the past I would use feeler gauges adjusting the die down until the correct shoulder bump was achieved. The problem with this method is the air gap between the die and shell holder that allows any flex in the press to effect the amount of shoulder bump.

http://i.imgur.com/7FfXhJ7.jpg

There is nothing wrong with markm's method of just using a simple drop in case gauge. "BUT" with more toys (gauges) I find I have to bump the shoulder back less and get less brass spring back after sizing. I use the +.004 competition shell holder and I'm loading for three different AR15 rifles.

Head clearance is the air space between the rear of the case and the bolt face. And excessive head clearance will allow the case to stretch and cause case head separations. Meaning being able to measure your fired cases with the Hornady gauge allows you have minimum shoulder bump and less head clearance.

http://i.imgur.com/HK76WCp.jpg

And guess which case gauge is better at checking case diameter. I use the red JP Enterprise for my final "plop test" of my loaded rounds, this gauge is made with a finish chamber reamer and is a smaller diameter than the Wilson or Dillon gauge.

http://i.imgur.com/KSB3ZvP.jpg

markm, "He who dies with the most toys wins." :D