I see a lot of people asking about how to determine headspace, where to size their brass, "What case gauge should I get?", "Do I cam over with the dies smashing into the shell holder?"....

Using the Hornady Headspace Comparator Set, you can really save yourself a lot of trouble and quickly get your sizing die dialed in nicely for reliable feeding in your rifle, especially with a gas gun.  I'll use 6.5 Grendel as an example.

I think this method really cuts to the core of what we're trying to do with sizing, namely where we're placing the shoulder so we get consistent headspace that will actually fit in the chamber and allow the bolt to lock over the reloaded cartridge. For the 6mm PPC family and its .350" datum line on the center of the shoulder per SAAMI, we use the B350 bushing provided by Hornady in the kit.

I thought some pics of how the process works would be worth a thousand words:

Bushing installed in my calipers, indicated to 0.000" off the 2" mark, which is the length the tool will fill between the blades after being installed.

L-R: Unfired AA 123gr Scenar load, 1x fired Lapua brass, reloaded cartridge with enough shoulder setback to chamber and lock-up


Unfired Lapua brassed cartridge headspace measurement: 1.205"





Fired Lapua brass, unsized headspace measurement: 1.218"



Sized to fit and lock-up in my chambers with a bolt that has the ejector removed: 1.210" (I can get away with 1.212" and still lock up, so this is a little excessive sized back .008")


So unfired/virgin:  1.205"
Fired: 1.218"
Sized: 1.212" is where I have to go to get reliable lock-up

I use a stripped bolt with confirmed identical face depth and lug lengths to check my sizing process, not a case gauge.  I turn the die down on full extension ram with an unsized case on it until contact with the shoulder.  Drop the ram, and turn down in increments, measuring the shoulder location as I go.

For me, even .005" of setback will not chamber reliably with the stripped bolt, so I have to go to .006", and it was surprisingly consistent across several different barrels.

i agree with rob.  i've used those hornady guages and that method as long as i can remember.  works great.  but i only bump shoulders back from fired about .002 and they chamber fine.

any chance you need to trim necks?  something doesn't seem right.  maybe color your cases with a dry erase marker and chamber them and see where they're rubbing?

Excellent work illustrating how this works - It's also uncanny because I was doing the exact same thing two hours ago.  I am just wondering if that fired brass came out of the larue rig?  

I bought a larue 6.5 kit, and my once fired Hornady cases measured 1.220 on the .350 bushing. I bumped them back to 1.215 and the bolt closed on top just fine, but how do I know if that's the correct amount or if it's excessive?  Any tips for those who lack a duplicate bolt other than pulling off the extractor on the one I have?

Sudboy

The sized cases drop into the chamber and fall out just fine even when sized minimally for headspace, and none of them grow anywhere near the chamber length to the necks.

As I have incrementally sized the cases down when setting my sizing die up, .005" setback on the shoulder will not reliably allow a stripped bolt to rotate into lock across a large lot of brass, but onesies and twosies just barely will lock up.

I think I'm getting these results because I actually check with a stripped bolt, whereas most people are using BCG inertia to get lock.  Doesn't leave any room for dirty. dusty environment, which is what I shoot in a lot.

I just checked with a another piece of fired brass.  No tumble, just universal de-primed.

Spray-lubed, let it dry.  

Fired headspace:  1.220"

Sized it down to 1.216"

I could barely get the stripped bolt to lock around it, but it felt like spring tension on the brass.

The sized case falls out of the chamber.

I also checked with the loads I have that are sized with .003" shoulder setback, which didn't run in my 16", won't allow bolt lock in an 18", or my 22", or my bolt gun.

Their shoulder location from the base is 1.217", from a 1.220" fired size.

Base diameter above the extractor groove is .442" on the case I just sized down .004".

I'm looking at the SAAMI diagrams right now, and maximum cartridge at that datum point is .4410", minimum chamber at that point is .4426".

The cartridges that won't lock up will also not drop free from my 22" Lilja Grendel barrel, and needed cleaning rod removal.

I'm using the Hornady New Dimension dies.  

I've been thinking about getting Redding bushing dies lately though.

I just took black permanent marker to one of the reloaded cartridges that won't chamber in any of the barrels (.003" setback shoulder).

You had me second-guessing myself about possibly needing a small-based die.

I was also interested to see where it would mark the neck and shoulder.

I firmly forced it into the chamber.

Attempted to lock up with stripped bolt, which it won't of course.

Drops free from chamber.

No marks on the brass on the neck, shoulder, or base.

Head-to-neck dimension is 1.512", well within the 1.530" chamber length to neck from bolt face.

Talked with a good buddy of mine over lunch about this.

He's an engineer and machinist who put himself through 2 different gunsmithing schools, is competing in PRS with his own cartridge and placing among the top 100 Nationally.

He wants me to take some 600grit over granite and run some case heads over it to see if I'm getting extractor rim deformation that is contributing to case length, and causing the .005-.006" setback requirement.

That could explain the springy feel I get when forcing that .0055" shoulder setback case into the chamber with a stripped bolt.

That does seem like an awful lot. In my .223 Wylde AR, I get reliable function by setting the shoulders back only .003" per the Hornady headspace comparitor. (Using a Dillon FL sizing die)

I use the Hornady comparators without issue. I always test measured empty casings in a rifle before loading the batch. I think people mix up the 1/8 to 1/4 turn more vs. 1 turn back on sizing/seating die settings resulting in smashed casings. It was one of my first DOHH moments in reloading.

Have to be mindful of shell plates too. Years ago, I mixed the shellplate that I used for priming with the one for my press. I still got a good cam when sizing but the rounds required some force to chamber. I wasn't getting the 2-3 thou bump that I normally got. The rounds shot like crap as well.

From that experience, I learned to keep track of shellplates for sizing (marked w/ sharpie) and to get the Hornady comparator kit to confirm how much the shoulder moved.


Interested in the OP's findings.

The OP's instructions are well written. Much better than most product manuals.

thanks for posting this.  That's the part that left me scratching my head and questioning if I was using the tool right - because I too have read that one should target a 0.002-0.005 setback for ultra supreme performance.  So I did that using the tool - and then had my AR lock up on me because my bolt wouldn't close.  So finally just stopped using the damned things and went back to kissing the shell-holder to the bottom of the die and calling it good.

It's good to see your explanation that you have to do minimum of at least 0.006" setback, before it'll even work.

I had a sharp edge on my extractor that would leave a raised mark on my brass causing the same issue.  Once I cleaned it up I can reliably size at .004" in my 708 ar.

You may have same problem or the ejector is leaving a raised portion also, I knocked the edges down on mine.

I shot a bunch of my hand loads through my Howa Mini bolt gun Grendel last weekend.

These have the shoulders pushed back .006" from fire-formed in my AR chambers.

Some of them had a very difficult time locking up in the action, with pronounced bolt turn resistance.

It will be interesting to see how the fire-formed brass from the bolt gun compares.

ETA:  Just measured them.  Doesn't appear to be any shoulder movement in the bolt gun, which is no surprise given that they were maxed out in the chamber already.

This has been my experience with my .223 AR's as well and I agree that the headspace comparator is invaluable.  

If I test sized cases by removing the extractor and then press on the rear of the BCG with my fingers to test bolt lock up I end up needing to go .004-.005 under fired case dimension to slip the bolt into battery without a fight.

For example my Daniel Defense fired case headspace dimension is 1.463 using the comparator.  The rule of thumb would have me at 1.460 for a safe minimally sized case.  However I noticed that those 1.460 cases were difficult to extract if I didn't fire them.  That got me thinking.

With a clean barrel/chamber I removed the extractor & ejector and reassembled the BCG then started testing various pieces of brass that were sized in .0005 increments as measured (very painstakingly) with the comparator on a Mitutoyo caliper.  After measuring a few hundred cases I had a collection that I marked with a sharpie to not forget their dimension and keep for headspace testing.

I pushed the BCG forward into the upper with a single finger feeling for binding. Here were my results.

1.460 I could force it into battery by using both thumbs to shove the BCG forward but extraction of the BCG was an issue indicating the bolt lugs were jammed against the extension lugs.  Anything bigger than this would not go into battery without me tapping the back of the BCG with a mallet.

1.459 I could gently "snap" it into battery but could feel the bolt lugs grating across the extension lugs slightly.

1.458 I could slip the BCG into battery with no extra effort and it felt exactly the same as it did when I tried it without a piece of brass.

All of these cases fell out of the chamber once I got the bolt out indicating that headspace was in fact the limiting factor and not something like a too fat base getting wedged in the chamber.

For giggles I tried this same test with a case fired in this chamber but unsized.  It was impossible to lock the bolt into battery and needed a cleaning rod to pop the case free as it was wedged to the chamber walls.

Think about what that means.  If I continued to use brass that was .003 under fired dimension I'd be forcing my bolt lugs into a very tight almost interference fit with the receiver lugs which only functions because the inertia of the BCG slamming forward squashes the case enough to let the bolt close with a lot of friction against the bolt lugs.

None of that testing would have been possible without the comparator.  If anyone reloads and cares about the precision of their ammo, life of their brass and life of their bolt they should get one.

Very interesting.  I was expecting more responses condemning my results, but it went quiet.

I suspect that's because people shooting on clean ranges or cleaner conditions have been getting away with .003" setback from carrier inertia.

I shoot in a lot of different temperatures as well, so I can see shifts from well below freezing to 100°F over the year, with a lot of precipitation, usually in pretty cold conditions.

If I were to rely on .003" setback, I'd have a lot of Failure to Chamber malfs, and have had quite a few when setting back .003".

The reality is that most people don't have the tools to disassemble their bolts, so it rarely happens.

I have 2 different AR15 bolt disassembly tools from Brownell's and even a spare bolt to use for this purpose that I have confirmed has the same lug lengths and face depth as my other bolts for 6.5 Grendel.

Totally agree.  I edited my post to add that I removed the extractor and ejector (forgot the ejector).

It's a time consuming pain when this whole hobby of reloading already eats a lot of time to do it really well.

My brother in law has some other friends who reload and he's described some of what I do and is met with a mix of "never heard of that" or "sounds like a waste of time".  So be it.

Most folks just get a small base die if they're having chambering problems assuming that their problem is the case is too fat at the bottom when it really is just getting sized not quite enough.

After seeing the results of my study which I came to independently of yours (great minds and all that) I'll never go without testing headspace the "right" way again.

One thing I'll do the next time I generate some empty brass from my rifle (teaching an Appleseed in a few weeks so I'll get some before and during that) is to deprime before measuring fired brass just in case the expended primer is causing a bad reading by standing proud a thousandth.  That'll answer the one little doubt that says I may not have gotten the best measurement of fired case dimension that I could have.  When I first got my original fired case dimension I did it with a single brand of factory ammo (I think it was Federal XM 193) and left the fired primer in.

I also got a Hornady caliper anvil to go along with the comparator.  Hopefully that does as advertised and gives me a more stable base than just the caliper blade to make good readings easier.

Attached File

Same here.

I've gone out as cold as 12 degrees F and as high as 105 F.  If my son is shooting an Appleseed that means the AR is getting over 600 rounds and laying on a mat when not shooting with a chamber flag in it (mandatory when you have 30 shooters on the line).  I see a lot of rifles choke and the owner is shocked because it's been 100% reliable before (of course it was always more accurate before too!).

If my kid's not shooting I'm often loaning it to someone whose rifle keeps shitting the bed.

If you can't argue it's best to be quiet.  Now some rifles will be different such as my  .223wylde. It doesn't require the set back that my grendel or 6.8 does. And as someone pointed out shell holders can ruin your results.

When in doubt use the bent paper clip to test for the separation ridge. You just use a sharp pick and scratch the inner head of the case to feel for the separation head. Been doing it for 30 years and have only had one case separate.

I have a built AR-15 and a Remington R25 (308w) both with custom barrels (Xcaliber).
The AR15 is chambered for 223 rem. I have great luck with using Redding standard f/l bushing die. Only bumping the shoulders back .002" from 1.463"..
The R25 is a different story. Standard f/l bushing die had a .006" setback to function reliably. Groups were terribly inconsistent.
 I have recently dusted off my 308w small base die and began chasing the headspace with far more consistent results... Good Luck

The pic of the comparator at .008" setback is not what I set back to, and I pointed out that it is excessive.

However, I do have to set back more than .003" to get lock-up.

I need at least .006" of setback.

I also check my cases with the paper clip arm, and have even sectioned cases in the past out of batches to determine if there is thinning of the web.