Cartridge headspace in an AR

[Ape_Factory]

Ok...I'm trying to wrap my head around the headspace concept. I'm not so much aiming for ultimate precision but for practical accuracy and loading AR's exclusively, no bolt guns. I hear most people load .002-.003 below a fired case. This seems to be a workable tolerance that allows problem-free feeding of cartridges.

From reading various threads it seems it takes at least two, maybe three firings before the cartridge headspace will settle and be consistent across multiple cases.

I'm loading PMC and Lake City brass for .223 and .308. I've taken measurements from once fired, twice fired and processed, ready to drop powder and a bullet brass. I processed most of the PMC brass and I purchased "processed" LC brass that I annealed, unified, etc...

Going to keep my questions to the .223 for the time being but I have some interesting observations on .308 as I have some factory cartridges from various manufacturers.

Anyway...with the PMC .223, I didn't take any measurements from the factory-loaded ammo prior to firing unfortunately. I'm measuring 10 random cases and taking the average.
Fired once, processed/resized, ready to load-1.44885"
Fired once, factory ammo unprocessed-1.45665
Reloaded, fired again, unprocessed-1.456

Full disclosure, I didn't even consider case headspace during the first round of resizing. So...I'm looking at a .00715 setback from once fired with the PMC I've processed. Seems like too much headspace and my primers, while not pushed out, do flatten fairly readily and I'm staying within published charges, referring to Lyman, Hornady, Sierra and Nosler, nothing over their maximum loads. For round two of reloading, I'm starting from scratch as I've dealt with neck tension and a few other issues that I've sorted.

Strangely, on the Lake City, which was fired once prior to me receiving it, didn't grow when reloaded and fired out of my rifle.
Fired once, fully processed, ready to reload-1.454
Fired twice, unprocessed, primer removed-1.454

Taking the LC out another decimal position, it actually came in slightly shorter on average so I may go back and measure again. I don't see how that's actually possible? My digital calipers aren't super expensive. Maybe time to invest in a better caliper or change the battery.

For measuring my rifle's headspace, I have the Hornady OAL gauge, modified cases and I do have the bullet comparator set as well as the headspace gauges.

I cleaned the barrel and chamber prior to taking measurements but I can't seem to get a consistent reading using the OAL gauge and I'm not even sure how useful that measurement is given I'm measuring at the ogive and that'll vary with the bullet being used. I seem to get more consistent readings with the Sierra 69TMK vs. the Hornady 55gr VMAX. Ultimately, the measurement will vary on where the bullet is positioned when I insert the modified case on the end of the OAL gauge. If I seat the bullet short, down in the case, insert the case and gently push the bullet forward with the OAL's rod, I'll get a different reading from seating the bullet long and moving the OAL gauge rod to meet up with the bullet. It's a Wylde chamber but my understanding is that shouldn't matter for headspace measurements. I will get a very slight marring on the bullet if I push a bit harder with the OAL gauge. It almost seems like it seats twice. I get the initial contact but if I exert a bit more force, it'll seat further in without getting the bullet stuck in the chamber.

So...starting from scratch for round two, my plan is to take five rounds of already reloaded ammo cartridges, write the headspace on the cartridge, fire them and measure after. Given that they're too short already, wondering if that'll throw off my subsequent measurements vs. a case that was sized .003 off of fired?

For the LC brass, which was resized by someone else, I'll find five cases which have the same headspace measurement and do the same. It's .002 off the fired PMC brass but not sure they're comparable. Thoughts?

 

[Gustav7]

Wooooof man.... I think you're over thinking this A LOT. Its always good to be cautious, but your return on investment here is lacking.

Others may have different experiences with different calibers, but I've loaded thousands of rounds for my 223 AR's and I've never had brass that didn't completely fireform the first firing. That being said, all my loads are middle of the Lyman book or higher.

In the end, it doesn't matter that theres different brass or different measurements loaded vs. unloaded. It only matters that your Sizing die is setup to bump shoulders at least 2thou off of a fired case from YOUR rifle's chamber. For autos, I do closer to 3 thou to give wiggle room for springback or stubborn cases.

Take 5 rounds, shoot them, clean them off, decap them, measure their headspace. Set up your sizing die to bump 3 thou back from that number. Size all cases that way. If a case you size is already smaller, then it doesn't matter as it will fit in your chamber just fine.

This is how I've always loaded bulk ammo. LC, WCC, FC, Fiocchi, S&B, Wolf, Rem, Win, etc.... all headstamps. They get sized 3 thou back according to my Smallest AR chamber (out of 3 AR's right now), and I don't worry about it.


If you really wanna see if your fireforming is off, then load 5 rounds and shoot them, clean them, and decap them. Measure their headspace and write it down. Size them minimally and load them, shoot them, clean them, decap them and measure them again. If your 1x fired measurement is +/-.0005 of the 2x fired, then your cases are fireforming properly the first time.

Your calipers are accurate to +/-0.001 (1 thou)....but they show resolution to a half thou. DON'T worry about differences of +/- 0.0005 (half thou)

 

[Ape_Factory]

Thanks! Makes sense. My issue lies in the fact I didn't take headspace into account and adjusted the die so that my bump was .00715 (approx) for my first round of case prep and subsequent reloads which I'm assuming is too much and can cause safety issues.

I get pressure signs fairly early in the ladder, especially with the Lyman loads. Just seems those should be fine but the indicators seem to say otherwise and they start with a much higher charge than say a recipe from Hornady. I've experienced a few double taps for yet to be determined reasons (could be mechanical, could be my loads or primers) and at least one bonafide slam fire. Running CCI 400 primers based on my friend never having any issues with them on his AR's, 10's of thousands of rounds so trying to eliminate headspace as a possible contributor.

Last night was the first night I took the time and measured cartridge shoulder bump and head space after ordering the Hornady headspace gauge set.

I was surprised the Lake City brass didn't change after one firing. I don't know if that's because I'm am or almost at zero off my actual head space with it or that's just the way the brass is.

Development of .308 rounds has been the complete opposite, no issues so far and it was easy to measure the headspace on that rifle. I feel like I have a handle on that round far better than the .223.

I still have each and every casing I've reloaded and fired from the .223 AR, whether it's PMC or LC. The headspace numbers weren't 100% consistent either, hence taking an average from 10 cases. I've probably shot around 200 rounds of reloaded PMC and about the same amount of Lake City testing out various loads and bullet weights. I've not had any actual issues per se except for the occasional double tap and the one slam fire. No case head separation, no mild or catastrophic failures and understandably, I'd like to keep it that way.

I have a good-sized pile of PMC fully-processed brass which is already resized (with too much shoulder bump at .00715) and primed so I can't adjust headspace at this point. The Lake City looks to be about right where I'd need it so I'm moving on to those cases. Rather bitter as I was being "guided" through the process initially and that person left out the whole headspace/die setup thing. So...taking things into my own hands now and doing a deep dive. The PMC stuff seems fine with say the first three charges in a ladder but beyond that, I start to get flattened primers and worse. I'm going through a bunch of the fired PMC and discarding it today as well.

 

[axarob44]

How are you getting headspace readings to 5 decimal places?

 

[Ape_Factory]

Those are just averages. I get three decimal places out to the tenth, fourth is 5 or 0. At this point I'm rounding up.

Just measured a lot of prepped LC brass with no primers and had a range from 1.462 to 1.456. Most fell within the 1.457-1.458 range which may work. Just don't know which number to gauge off of...logically it seems I'd do it off the largest number for a fired case, not the smallest?

 

[Ape_Factory]

Here's an idea as to how all over the map the "processed" Lake City brass was. Starts out at 1.460 on the left and works its way down to 1.456, the two on the far right were 1.461 and 1.462. Live and learn. That was just the first handful I grabbed. I went and sorted the rest of the unprimed brass.



I did more measuring of fired LC and PMC cases.
LC 1.4576 avg
PMC 1.458 avg
PMC Factory loaded 1.459

The LC brass is probably thicker and doesn't expand as readily as the PMC.

I'm going to assume my chamber is 1.460. Since I want the gun to cycle and fire when it's dirty, I'm going to size the remaining LC unprimed brass at 1.455 and use CCI41 primers and go from there.

I'll have to sort the processed and primed LC and PMC cases to see if any can be salvaged.

 

[axarob44]

My vote goes to the Burris Zee rings. Half minute all day long at a mile if I do my part.

Ok...I'm trying to wrap my head around the headspace concept. I'm not so much aiming for ultimate precision but for practical accuracy and loading AR's exclusively, no bolt guns. I hear most people load .002-.003 below a fired case. This seems to be a workable tolerance that allows problem-free feeding of cartridges.

From reading various threads it seems it takes at least two, maybe three firings before the cartridge headspace will settle and be consistent across multiple cases.

I'm loading PMC and Lake City brass for .223 and .308. I've taken measurements from once fired, twice fired and processed, ready to drop powder and a bullet brass. I processed most of the PMC brass and I purchased "processed" LC brass that I annealed, unified, etc...

Going to keep my questions to the .223 for the time being but I have some interesting observations on .308 as I have some factory cartridges from various manufacturers.

Anyway...with the PMC .223, I didn't take any measurements from the factory-loaded ammo prior to firing unfortunately. I'm measuring 10 random cases and taking the average.
Fired once, processed/resized, ready to load-1.44885"
Fired once, factory ammo unprocessed-1.45665
Reloaded, fired again, unprocessed-1.456

Full disclosure, I didn't even consider case headspace during the first round of resizing. So...I'm looking at a .00715 setback from once fired with the PMC I've processed. Seems like too much headspace and my primers, while not pushed out, do flatten fairly readily and I'm staying within published charges, referring to Lyman, Hornady, Sierra and Nosler, nothing over their maximum loads. For round two of reloading, I'm starting from scratch as I've dealt with neck tension and a few other issues that I've sorted.

Strangely, on the Lake City, which was fired once prior to me receiving it, didn't grow when reloaded and fired out of my rifle.
Fired once, fully processed, ready to reload-1.454
Fired twice, unprocessed, primer removed-1.454

Taking the LC out another decimal position, it actually came in slightly shorter on average so I may go back and measure again. I don't see how that's actually possible? My digital calipers aren't super expensive. Maybe time to invest in a better caliper or change the battery.

For measuring my rifle's headspace, I have the Hornady OAL gauge, modified cases and I do have the bullet comparator set as well as the headspace gauges.

I cleaned the barrel and chamber prior to taking measurements but I can't seem to get a consistent reading using the OAL gauge and I'm not even sure how useful that measurement is given I'm measuring at the ogive and that'll vary with the bullet being used. I seem to get more consistent readings with the Sierra 69TMK vs. the Hornady 55gr VMAX. Ultimately, the measurement will vary on where the bullet is positioned when I insert the modified case on the end of the OAL gauge. If I seat the bullet short, down in the case, insert the case and gently push the bullet forward with the OAL's rod, I'll get a different reading from seating the bullet long and moving the OAL gauge rod to meet up with the bullet. It's a Wylde chamber but my understanding is that shouldn't matter for headspace measurements. I will get a very slight marring on the bullet if I push a bit harder with the OAL gauge. It almost seems like it seats twice. I get the initial contact but if I exert a bit more force, it'll seat further in without getting the bullet stuck in the chamber.

So...starting from scratch for round two, my plan is to take five rounds of already reloaded ammo cartridges, write the headspace on the cartridge, fire them and measure after. Given that they're too short already, wondering if that'll throw off my subsequent measurements vs. a case that was sized .003 off of fired?

For the LC brass, which was resized by someone else, I'll find five cases which have the same headspace measurement and do the same. It's .002 off the fired PMC brass but not sure they're comparable. Thoughts?

How are you getting these numbers? I mean what tool do you use that reads that fine?

 

[Ape_Factory]

Normal digital calipers and the Hornady anvil/headspace gauge set with metal body. My calipers go out four decimal places.

 

[axarob44]

Ok, but where are you getting the 5th number and any number in the 4th position other than 0 or 5?

 

[Ape_Factory]

Just ignore, I was adding up 10 different measurements and taking the average. I may have typed in an errant digit.

 

[axarob44]

Ok sorry, was just wondering where you got that fifth number. I was confused. I guess if you were that OCD to take it out that far, but not really needed, just make a little easier on yourself.

 

[whatsupdoc]

1) After a cartridge is fired ONCE it is headspaced to that particular chamber.
2) Always decap before taking any measurements as the primer may protrude and skew your readings.
3) Keep a couple of unmolested fired cases from your rifle as a reference for that chamber. If there is a issue later you will have a record of the new chamber.
4) The fired case datum to base length should be the same regardless of case brand for a particular chamber.

Keep it simple.

1) Measure the datum of your fired cases.
2) Bump your shoulder back .003
3) Check and trim the case length if necessary.
4) Check that the processed case fits the chamber.

 

[Ape_Factory]

That sounds so simple but...what happens when the fired case measurements vary? Do you go off the low or the high or an average?

I would assume the amount of powder can have an effect as well. Interestingly, the factory loaded PMC Bronze cartridges were the most consistent although I'm testing a bunch of different bullet types and charges to determine what my rifle prefers.

I currently don't have a trimmer but have a Giraud ordered. Should have it in December.

 

[supercorndogs]

Maximum. The biggest case is obviously going to be closest to chamber dimensions.

 

[Gustav7]

That sounds so simple but...what happens when the fired case measurements vary? Do you go off the low or the high or an average?

I would assume the amount of powder can have an effect as well. Interestingly, the factory loaded PMC Bronze cartridges were the most consistent although I'm testing a bunch of different bullet types and charges to determine what my rifle prefers.

I currently don't have a trimmer but have a Giraud ordered. Should have it in December.

Understanding why you even resize cases in the first place may help. The ONLY reason you resize cases is so that they'll fit back in your gun when you go to shoot them. The neck is resized so you can fit another bullet into the case and it will hold, thats the 2-4 thou of neck tension. The reason you bump shoulders and size the body of the case is so that they'll fit back in your chamber...especially with autos.

So if we understand that logic, then you'll understand what @supercorndogs is saying. The ONLY thing that matters is that each case is sized at minimum 2-3 thou shorter/below your chamber dimensions. If your chamber is 1.460"... then the only thing that matters is that every case measures 1.44"-1.43" or lower. We don't care if its shorter, because that won't give you problems...but when you resize cases and they are measuring 1.46" or higher, then thats a problem...and you would need screw your sizing die down more.

Does that all make sense?

 

[supercorndogs]

There is a big section in the front of reloading manuals on theory. As gustav said, once we understand why we do things, sometimes they make more sense.

 

[Ape_Factory]

I definitely understand why we resize and I took quite a bit of time researching die types before settling on my current setup. That concept was never an issue and I had a clear understanding of that at the very start of my reloading journey.

My issues were three fold. First, that shoulder bump was a "thing" and just adjusting the die down to the base of the press and opening up a an 8th or so is not the way to do it; determining a shoulder bump value based on fired case measurements that were all over the place and lastly, the dangers of too much (or too little) cartridge headspace. Too little is pretty obvious but too much, less so.

Does having too much cartridge headspace introduce other issues? No one has answered that yet.

Since I've resized a bunch of PMC cases already (most coming in about 1.450-1.453), and fired many of them, I want to make sure that excessive headspace will not or is not contributing to other issues, false reading of the tea leaves so to speak and other endless rabbit holes.

For instance, I've read too much headspace can cause flattened primers. As the powder ignites, the excessive cartridge headspace allows the primer to be pushed out slightly before being slammed against the bolt face which flattens the primer.

Does too much headspace contribute to slam fires or "double taps"? Could it lead to false high pressure signs in other ways?

As I mentioned earlier, I resized some of the LC brass to 1.455" and loaded those up. I'll try to get to the range today to test and will report back.

As for all the PMC that's now, IMO, too short, what to do with it? Light plinking loads to fire form it back up? Still have the CCI 400 primer issues with those so will have to use them when the range is mostly empty.

 

[Gustav7]

Does having too much cartridge headspace introduce other issues? No one has answered that yet.

Does too much headspace contribute to slam fires or "double taps"? Could it lead to false high pressure signs in other ways?

As for all the PMC that's now, IMO, too short, what to do with it? Light plinking loads to fire form it back up? Still have the CCI 400 primer issues with those so will have to use them when the range is mostly empty.

Too much headspace over works your brass and can lead to case head separation. IMO, I don't think 7 thou of headspace is too much provided you don't do it excessively while running ramped up loads. I reloaded a few thousand 223 multiple times back when I first started reloading using your method. I don't remember what they actually sized too but it was more than 3 thou. I think I was sizing around 4-6 thou and I had multiple reloads on that brass....but it was also middle of the book loads. Just enough to reliably cycle my AR's...

I would fire what you have sized, and just correct your methods to 3 thou bump from here on out.

If you're primers are seated below flush, you should be fine... HOWEVER, those CCI 400's can contribute to that. So if you're primers are seated just barely flush or flush, a little excessive headspace, AND a thin cupped primer like a 400, that could be the culmination of events that gives you a slam fire.

If you're making plinking loads, I would run middle of the book anyways, so yes you could do that with the PMC's if you want.

Generally speaking, MORE headspace causes LESS pressure. The more the brass has to move, the more energy is taken up in moving that brass, which creates less peak pressure on the chamber.

Hope that helps

 

[Ape_Factory]

It does Gustav! Thank you again.

I tend to stick to middle of the road or even the lower end of things if I can get an accurate cartridge out of it. I am most definitely not pushing things.

On the primers, I ditched my Lyman hand primer (works well with my AR10 rounds though) and went with one where I can adjust seating depth so I check all of them before loading now. That didn't really solve the double tap issue. Still have them occasionally and only on the .223. Maybe once in 40 rounds but sometimes more. Don't know if it's the mechanics of the gun, my reloading technique (or lack thereof) or a combination of all of the above. Still working that one out definitively. I do have CCI 41's which is what I'll test out today with the correctly re-resized brass, LOL.

 

[Gustav7]

It does Gustav! Thank you again.

I tend to stick to middle of the road or even the lower end of things if I can get an accurate cartridge out of it. I am most definitely not pushing things.

On the primers, I ditched my Lyman hand primer (works well with my AR10 rounds though) and went with one where I can adjust seating depth so I check all of them before loading now. That didn't really solve the double tap issue. Still have them occasionally and only on the .223. Maybe once in 40 rounds but sometimes more. Don't know if it's the mechanics of the gun, my reloading technique (or lack thereof) or a combination of all of the above. Still working that one out definitively. I do have CCI 41's which is what I'll test out today with the correctly re-resized brass, LOL.

Just make sure the primers are below flush. I usually seat them around 3-5 thou below flush. You can measure with your calipers with the bottom piece (if you were not aware).

The double taps sound more like a trigger manipulation issue? Do you have a super light trigger on your AR?

 

[Ape_Factory]

Four pound drop in. Sent trigger in to have it checked but it still does it. Lightweight enhanced bolt carrier group and have messed with buffer weights to no avail. None of the off the shelf factory loads had an issue. I went back and looked at all the primers. When it does the double tap, the primer strike area becomes an outie. I’ll post photo later.

 

[Ape_Factory]

Back from the range, haven't taken any measurements yet but my groupings were not what I hoped they would be overall. I had been using my friend's Chargemaster 1500 but I just purchased my own. Might have to spend more time dialing mine in. I'm already leaving it on permanently and doing a few of the "usual" mods to make them more accurate.

The good news, no slam fires on the CCI 400 rounds. I went through 30 and had the entire range to myself so was able to shoot more than once every five seconds. I hand selected cartridges that were 1.452 to 1.453, enough to fill one 30 round magazine, for the test. Grouped decently too so I must have done something right there.

The only difference is I ran the BCG almost completely dry and I cleaned it thoroughly, including the firing pin channel. It's a fairly new BCG and I generally clean it every few hundred rounds. I did notice some lube had migrated to the firing pin, probably from the cam pin, last cleaning. This time I used M7 Pro instead of CLP. Wondering if the lube on the firing pin (yes, I know not to lube the firing pin!) might have been contributing.

CCI41 primed cartridges were fine as well (expected). I took velocity readings for everything but haven't digested those yet. My SD and ES was higher than it has been in the past though which was unexpected. I had a few loads which were in the low 10's from past attempts.

I brought the 308 out at the end and shot some factory PMC, American Eagle and five rounds Federal Gold Match 169gr. out of the POF Revolution DI and I think the SD was 7 and it was a really tight group. Yep, I have some work to do. Interestingly, the velocity for the Fed Gold was in the 2400 range out of a 16" barrel.

 

[Ape_Factory]

Had a chance to pull my brass out and examine. No overpressure signs but again, I'm running light to medium loads. I traced my possible less than stellar groupings down to (possibly) my failure to use the neck sizing mandrel on this batch and seating the bullets at mag length instead of Hornady's recommendation (50 & 55gr VMAX). The bullets seated fine so it didn't dawn on me that I'd forgotten to set the neck tension.

So...all my fired headspace lengths came in at 1.458 for this batch after firing. I went super anal and took 250 rounds of the primed LC brass and unified the shoulder bump at 1.455 and this time, I used the Redding mandrel to set the neck tension. I don't use the resizing bullet in the Hornady resizing die and just remove it, relying on the Redding mandrel.

I then took the time to sort the brass by OAL since I do use a crimping die for a very light crimp. They're all over the map, ranging from 1.742 to 1.752, all well below 1.460. Not ideal if you crimp hence the time taken to sort by length. Hopefully I can get these all the same over time or once the Giraud is in my possession, I'll pick a length which will get as many of them at the same measurement as possible. Most of them group in the 1.747-1.750 range. Typical bell curve which is interesting.

I'm assuming switching between CCI41 and CCI400's will affect my results so I'll have to do a little comparison between the two primers, same load, same bullet. Can I expect a change in pressure switching between primers?

 

[TheOfficeT-Rex]

I'm assuming switching between CCI41 and CCI400's will affect my results so I'll have to do a little comparison between the two primers, same load, same bullet. Can I expect a change in pressure switching between primers?

Yes, because of the initial ignition provided by each primer. IIRC, 41's are magnum level primers, and will provide a longer/hotter initial flame than the 400s. Shouldn't be too big an issue unless you are near the top of allowable pressure, but you will likely see it on the chronograph.

 

[spife7980]

Your current pressure may be too much for the thinner 400 primer to reliably contain. Just watch for punctures.

 

[Ape_Factory]

Have not had any punctures so far at all. Last two trips to the range, no double taps. I'll have to remember to use minimal lube on the RCA BCG and keep the firing pin clean.

I think I have the .223 stuff figured out. Once the Giraud trimmer gets here I should be able to whittle down my SD and get a bit more consistency overall.

Moving on to .308. I have factory ammo. I have a few Federal Gold 168gr. SMK's left (they had an SD of 9.6 for five rounds) and measuring those, along with PMC Bronze, I'm getting a shoulder measurement of 1.621 on both. Same with some Federal XM80 although with those, there's some variance.

I then went to my freshly shot cases, some of the Federal Gold, American Eagle and PMC. I'm getting upwards of 1.633" which is a HUGE jump over the factory ammo new. Can't imagine my rifle's headspace (AR10) is that large so I started doing some reading. Seems while violently ejecting the spent cases, they can lengthen.

Is this something that's specific to an AR10 and the 308 cartridge or does it transfer over to an AR15 and the .223/5.56 round? If so, could explain why my case readings were all over the map. I have zero factory ammo to compare it to so if anyone has some nice stuff lying around, match grade, I'd love to know the headspace/shoulder measurements on those cases.

I'm going to base my resized brass off of the 1.621 length as the Federal Match was dead on and the PMC shoots pretty well for what it is and the cost. This is five rounds shot in fairly quick succession with my mediocre skills and the Revolution DI mounted on a $30 bipod @ 100 yards. Pretty sure the one of the left (first shot) would have been more inline with the rest if there were a more talented person behind the gun.


On the chronograph, the Fed Gold shot 2493, 2481, 2494, 2489, and 2471. Average was 2493 with an ES of 23 and SD of 9.6. I know some of you guys crush those numbers but I'm going for practical accuracy out of an AR10 that'll function no matter what. Can't really tell if the Fed Gold is crimped; I'm assuming it is but it doesn't look like it. The PMC is obviously crimped. But I figure this is a good target to aim for in terms of my own hand loads. I think I'll be pretty happy if I can get an SD consistently at 20 or below while loading a large number of rounds. That's my hope anyway.