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View Full Version : Varying headspace in loading for a NATO AR-15 chamber (FIGURED IT OUT THANKS CG)


trigger hippie
07-24-2012, 11:19 AM
Greetings. I have a slightly technical question, so I thought I'd explain what I'm using to see if that makes a difference.

So I have recently started reloading and acquiring reloading infrastructure. I shoot an 18" AR with a 1:7 twist Noveske SS barrel and I've been disappointed with the accuracy of M193 and M855; I don't recall off-hand but about 3-4 MOA groups at best sound right.

Since Mk262 Mod 1 is still about a dollar a bang, I've been developing 69 grain and 77 grain Sierra Match King loads so that I can afford to practice. I conducted multiple ladder tests with both bullets (as well as with 55 grain Speer Match bullets I don't plan to use extensively).

I trim every round (even new rounds) every time I fire and reload brass, turn necks to about 0.110" to 0.130" with 0.001" run-out depending on the brass, debur flash holes. I use exclusively CCI Small Rifle #300 primers; there's a box of CCI BR-4 primers on my shelf but I don't want to have to re-develop a load for them.

At any rate, I'm finding that the "sweet spot" for all my loads is consistently right at about 1/3rds up the way from minimum load. Consequently I'm loading 24.0 grains of Varget for my 69 grain SMKs (well under the 26.0 gr max in the Lyman manual), and 23.5 grains of RL-15 for my 77-grain SMKs. Frequently, the groups in my ladder test were sub-1/2 MOA; I settled on the 23.5 grain loading for 77 grain SMKs when I saw consistent 0.25" - 0.50" 5-shot groups within the 22.5 - 23.9 load range. 23.5 grains is on the hot end of this optimum band but still well short of the published max, so that on a hot day with a hot rifle it hopefully shouldn't become an unsafe charge level.

The hillbilly ballistician in me suspects that the reason I'm getting these mid-range loadings is because the benchmark ranges I'm working from in the Lyman load book assume a 24" barrel test platform; the hottest loads they recommend simply aren't combusting fully in an 18" barrel and are still expanding while the bullet was still exiting the barrel, causing a kind of tail wind that made the hottest loads consistently drop about 0.1 - 0.2 mils. I'm not sure how I can verify that this mechanism is at work at my level of experimentation but there does seem to be circumstantial evidence in my loadings.

So I started out with a cheap Lee Breech Lock kit and was quickly dismayed by how inconsistent (+/- 0.010") the seating die was and how slow the scale was. Since then I upgraded to an RCBS ChargeMaster 1500 combo scale/dispenser and Redding Competition seating die, with a VLD seating stem I use for my 77-grain loads. I look for at most +/- 0.003" variance in OAL - 2.260", with 2.257" and 2.263" being borderline reject rounds. Consequently my failure rate is about 10 - 20% - much better than the 40 - 60% rejection rates I saw with the Lee die.

Now that I've found the optimum charge, I want to begin working with headspace dimensions. I have an RCBS case micrometer and gauge with which I've obtained chamber dimensions (by using the case mic on several fired cases). Here my problems begin.

The kit comes with a fired-case gauge, and a sort of collapsible dummy round that, when chambered, gives chambering dimensions. Using the fired-case gauge, I find my headspacing value is 1.4729" (based on a fired-case case reading of 0.0093" added to minimum .223 chamber value of 1.4636). This is on the high end of the SAAMI range for .223 Rem but I assume it's like that because it's a 5.56 NATO chambering. OK, I figure, what do I do with that value? It occurs to me, for instance, that I could use the headspace mic on a batch basis and sort my brass by headspacing... a giant PITA.

I believe the Lee re-sizing die I am using is a neck-resizing die - that is, it contains a cylindrical needle-like assembly which plunges down into the case and back up, widening the neck for bullet-seating. The plunger is secured to the die by being mounted in hollow screw cut in a cross-section which tightens as it is screwed in. Am I even able to alter headspace in such a setting?

So, OK... maybe I can't alter headspacing... how about bullet seating depth? And here is another mystery.

The problem is, I can't chamber the damn dummy round in my AR. I've tried simply putting it into the chamber (haha, predictable jam), I tried taking out the BCG and gently inserting it until it's flush with the star chamber (simply results in pushing it down to its minimum size), I tried taking out the bolt carrier group and inserting it underneath the extractor hook (won't stay because of ejector spring tension). How can I single-load this dummy round in my AR? I can't seem to find anything that explains this stupidly simple process.

I want to make sure that I precisely understand the language in how I do this: Am I varying bullet seating depth by altering cartridge overall length?

When people talk about "jamming or jumping" to the lands of the barrel (or barrel extension in my case), is this being achieved by altering the micrometer settings on the bullet seating die?

If this is the case, why even bother obtaining a chamber dimension value at all? Without using my weird dummy-round thing, I could run off a test sequence of 77-grain SMKs looking for optimal seating depth, looking like:

LOADING: .556 NATO MK262-clone seat depth test
CASES: 50 new Hornady brass necks trimmed/turned/deburred
PRIMER: CCI 300 small rifle
POWDER: Reloader-15
CHARGE LEVEL: 23.5 grains
TEST VARIABLE: Cartridge OAL

CHARGE LEVELS:
Nine at each OAL:
2.255 OAL
2.256
2.257
2.258
2.259
2.260
2.261
2.262
2.263
2.264
2.265

So I simply fire three three-shot groups at each OAL and select the best three for extended testing in 10-round groups, then select the best of those for my final OAL. Does this logic check out?

joelogic
07-24-2012, 11:38 AM
TL;DR
I seat to max mag length. I get 1MOA with milsurp powder, Hornady 55gr bullets, and wolf primers. Good enough for me.

If you want to load just off the lands it may to too long for the mag.

gau17
07-24-2012, 12:23 PM
You're loading for an AR and you're turning necks? Why?
You should be sizing the brass to SAAMI spec for better reliability on semi-auto.

Something is wrong with your rifle if it's shooting 3-4 MOA. That is a lot.

CS Sports
07-24-2012, 12:43 PM
Do yourself a favor and pick up the book "Handloading for Competition" by Glen Zediker. Read it cover to cover before you do any more loading, as you have several misconceptions that are hindering your advancement.

As for your belief that "the hottest loads they recommend simply aren't combusting fully in an 18" barrel and are still expanding while the bullet was still exiting the barrel", this just isn't the case. All of the powder burns in a very short time, FAR before the bullet ever gets anywhere near the end of the barrel. However, the gas is still expanding and will continue to do so FAR past the point that the bullet exits the barrel.

Barrels vibrate in predictable and consistent patterns. The goal of altering charge weight is to find a load where the bullet consistently exits the barrel when the barrel is at rest or very close to it (The top or bottom of the vibration cycle, if you will).

You should NOT be trying to vary headspace on your loaded rounds, as your rifle's headspace won't vary once set. Adjust your sizing die to bump the shoulder to whatever point you want and leave it be. Some people like a snug fit in the chamber, some like a few thou clearance. Depends on the application.

"When people talk about "jamming or jumping" to the lands of the barrel (or barrel extension in my case), is this being achieved by altering the micrometer settings on the bullet seating die?"

Yes, but the bullet is "jumping" to the lands of your barrel and not your barrel extension. The barrel extension is immaterial int his discussion as it is a fixed dimension that you can't alter.

"If this is the case, why even bother obtaining a chamber dimension value at all? Without using my weird dummy-round thing, I could run off a test sequence of 77-grain SMKs looking for optimal seating depth, looking like:"

Headspace and bullet seating depth are two very different things and should not be confused.

Your mistake is in measuring the OAL to the tip of the meplat. Unless you have uniformed the meplats (and even then), you will get a different OAL because the seating die indexes off a point on the ogive the bullet. OAL can and will very wildly with different shape bullets, so that two cartridges with identical OALs can have huge differences in the "bullet jump" to the lands.

CS Sports
07-24-2012, 12:47 PM
Forgot to mention, don't trim unfired brass. Even if you get it all uniform, after the first firing it will be all over the place again. Wait to trim your brass until after it has been fired.

gau17
07-24-2012, 1:10 PM
OAL for magazine is 2.260. are you going to be doing single feed?

+ 1 CS Sports.

trigger hippie
07-24-2012, 1:37 PM
CS Sports, thank you, that is exactly the answer I am looking for. I'll go ahead and pick that book up.

My original post does slightly confuse OAL and headspace, something I tried to pick up in an edit. Since .223 headspaces on the shoulder, what I'm having difficulty grasping is how it's even possible to alter cartridge dimensions to exploit my knowledge of my rifle's chamber dimensions; it would seem that altering the location of the cartridge shoulder is beyond the ability of my Lee die set and tools.

The RCBS mic and gauge documentation suggest that I use my headspace reading of 0.0093" to alter my full-length resizing die settings. I believe I have a resizing die that only works the neck of the case.

What I'm asking is, is the only usage I have for this headspace number to discriminate between cases which might misfeed in my rifle (cases over 0.0093 + 0.04 = 0.0493" runout plus RCBS-recommended additional 0.01 - 0.02" in length)? From what I've researched it seems like you start to run into diminishing returns and potential additional fault points when you start messing with critical cartridge dimensions.

With regard to OAL your answer with regard to measurement explains a ton about why I've been getting (what seems to me to be) runout in overall length.

What I'm trying - unsuccessfully it would seem? - is to derive the most accurate seating depth for my rifle's chamber. While I've been doing my pressure testing based on the basic printed reloading information I have at hand I've been keeping those dimensions as uniform as possible - thus trimming everything to 1.750" exact, even new brass (on the assumption that I'll be using, later on, fired brass that I'll need to trim anyway). I only see a runout of about 0.001 on new Hornady cases anyway so I might as well stop trimming those when new.

Now, mind you, I'm going to look this up in an authoritative reference before I deploy it, so you aren't risking blowing me up by giving me inaccurate info, but I guess what my question really is asking is whether OAL is the control variable for experimenting with bullet engagement to barrel lands. Are variations in OAL really substantively effective in accuracy and worth controlling?

joelogic
07-24-2012, 1:48 PM
Are variations in OAL really substantively effective in accuracy and worth controlling?

Yes, but it may be longer than mag length. People usually slot the neck of the brass, place a bullet in the neck as far out as possible, chamber the round, and back off a few thousands.

However when chasing MOA in a gas gun just remember the law of diminishing returns.

trigger hippie
07-24-2012, 1:54 PM
Are variations in OAL really substantively effective in accuracy and worth controlling?

Yes, but it may be longer than mag length. People usually slot the neck of the brass, place a bullet in the neck as far out as possible, chamber the round, and back off a few thousands.

However when chasing MOA in a gas gun just remember the law of diminishing returns.

Ahhhh, there we go.

How do you single chamber a round - work it under the extractor, or simply throw it in the chamber with bolt locked back and release the bolt?

joelogic
07-24-2012, 2:18 PM
The latter.

gau17
07-24-2012, 2:30 PM
Now, mind you, I'm going to look this up in an authoritative reference before I deploy it, so you aren't risking blowing me up by giving me inaccurate info, but I guess what my question really is asking is whether OAL is the control variable for experimenting with bullet engagement to barrel lands. Are variations in OAL really substantively effective in accuracy and worth controlling?

If your goal is to chase the lands, you cannot rely on OAL. You must use a comparator to take the measurement. As mentioned previously, bullet tips can vary. The tool takes the measurement from the bullet's Ogive to the base of the cartridge.

CS Sports
07-24-2012, 2:54 PM
Some neck dies will allow you to bump shoulders back, but will not "squeeze" them to a smaller diameter. I will assume that you are talking about bumping the shoulders back.

Just set the die up to bump it back "just enough" then don't worry about it. All you are really trying to do here is set the die up so that it resizes the case enough to chamber reliably while not too much as to induce case head separation.

Quit worrying so much about trim length. Yes, they should all be close but we are not talking about a 6ppc bench gun, we are talking about an AR. As long as the length is not too long, it's fine.

Make sure you are chamfering/deburring after trimming. burrs or sharp edges on a case neck can damage the base of the bullet leading to erratic accuracy.

"Are variations in OAL really substantively effective in accuracy and worth controlling?"

Yes, absolutely. Not only is altering seating depth important, it is critical to really dial in a load.

You are somewhat constrained with the AR (or any magazine fed) rifle, so we need to work within the limitations of the platform. Your first objective is to find the max overall length that will function through the rifle/magazine. Since that is the max functional OAL, it makes no sense to try loads with a longer OAL. Next, see if that OAL is touching the lands. To do this you just need a fired case and a sharpie (google it, there are videos that will explain the process better than I can type it out).

If you are touching the lands, back off the bullet until you are just touching or slightly shy. (With a bolt gun you can get away with jamming the bullets into the lands, but that is a dicey proposition with many ARs).

Once you find the max OAL that will function with your rifle, that is also barely touching the lands, start load testing working backwards. I typically start by finding my max safe charge weight at the max OAL, then work backwards with OAL until I find my accuracy node. Obviously I am using OAL and seating depth interchangeably here, they are not the same, but for our purposes let's just run with it.

You mention "runout" several times, but I'm not sure you are using that term in the way typically associated with handloading. "Runout" in handloading typically refers to the measurement of the difference in concentricity between the case body and the bullet. Find a flat, true surface and roll a loaded round on it. Any "wiggle" you see at the bullet tip is runout. Eliminating runout is an entirely separate and equally in-depth discussion. Runout in handloading is not typically variance of case length, headspace dimension or OAL/seating depth.

LAstly, once you find your ideal seating depth, make a dummy round (I even use bearing retaining compound inside the case neck). Use that as a reference to quickly set up/check your seating die. If you shoot different bullets, do this with each bullet type to allow quick set changes when switching between bullet types.

trigger hippie
07-24-2012, 5:09 PM
You are somewhat constrained with the AR (or any magazine fed) rifle, so we need to work within the limitations of the platform. Your first objective is to find the max overall length that will function through the rifle/magazine. Since that is the max functional OAL, it makes no sense to try loads with a longer OAL. Next, see if that OAL is touching the lands. To do this you just need a fired case and a sharpie (google it, there are videos that will explain the process better than I can type it out).

If you are touching the lands, back off the bullet until you are just touching or slightly shy. (With a bolt gun you can get away with jamming the bullets into the lands, but that is a dicey proposition with many ARs).

Once you find the max OAL that will function with your rifle, that is also barely touching the lands, start load testing working backwards. I typically start by finding my max safe charge weight at the max OAL, then work backwards with OAL until I find my accuracy node. Obviously I am using OAL and seating depth interchangeably here, they are not the same, but for our purposes let's just run with it.

You mention "runout" several times, but I'm not sure you are using that term in the way typically associated with handloading. "Runout" in handloading typically refers to the measurement of the difference in concentricity between the case body and the bullet. Find a flat, true surface and roll a loaded round on it. Any "wiggle" you see at the bullet tip is runout. Eliminating runout is an entirely separate and equally in-depth discussion. Runout in handloading is not typically variance of case length, headspace dimension or OAL/seating depth.

LAstly, once you find your ideal seating depth, make a dummy round (I even use bearing retaining compound inside the case neck). Use that as a reference to quickly set up/check your seating die. If you shoot different bullets, do this with each bullet type to allow quick set changes when switching between bullet types.

I'm going to read this twice again then do this when I get home. Will report. Thank you for your patience in curing my dangerous ignorance :)

mroels
07-24-2012, 10:17 PM
I've only been reloading about 6 months, got about 1k rounds under my belt, so take my opinion with a grain of salt. But I too had a little bit of difficulty understanding the RCBS mic and where to seat the shoulder. The annoying thing is that the RCBS manual doesn't really explain what the values on the mic mean. Or specifically what it means to your reloads. But for the AR platform just set shoulder back to ANSI minimum or so that a resized case reads 0 on the mic.

What I noticed was that:
Unfired factory ammo - PMC/ BHA/ AE - consistently measured at -0.03 or -0.003" below ANSI minimum, presumably to allow for consistent loading in a wide variety of weapons systems.

My once fired brass measured at +0.02-+0.03 with a couple of odd ones at -0.01

The way it was explained to me this meant that my chamber is probably +0.003" to +0.005" above ANSI minimum, but that after expansion the brass shrinks back a little again.

I set the die to bump the shoulder right back to ANSI minimum. Although I was told I'd be ok setting back to +0.02 if I really wanted to size closer to my chamber's specs.

I have noticed that multiple fired cases don't expand as much so the shoulder doesn't need to be set back as much.


btw make sure to use the mic before applying resizing lube, while you don't lube the shoulders i've noticed that it does affect the measurements.



I haven't played to much with OAL, i've noticed that most of my favorite factory match grade ammo brands are all at 2.20". Also from what i've read and noticed trying to use RCBS mic to measure ogive is difficult. The little bullet thing they give you doesn't like it when the bolt is closed too hard or too soft, i'm getting very inconsistent readings.

trigger hippie
07-24-2012, 10:44 PM
I haven't played to much with OAL, i've noticed that most of my favorite factory match grade ammo brands are all at 2.20". Also from what i've read and noticed trying to use RCBS mic to measure ogive is difficult. The little bullet thing they give you doesn't like it when the bolt is closed too hard or too soft, i'm getting very inconsistent readings.

THANK YOU! I thought I was somehow doing it wrong. This thing was driving me nuts! There's a method someone mentions above for obtaining the same measurement which uses pigment and an inert bullet - I'm actually trying to find a video or instructions where this is done.

So are you saying that "riding" the bolt closed to reduce the seating force works? Hmm...

mroels
07-24-2012, 11:08 PM
I've tried riding the bolt and still getting varying measurements. Tried forward assist and getting differing measuments. Sometimes the extractor doesn't catch so I'm not sure if the bullet thing is seated against the bolt face. Overall I think this feature just wasn't designed for auto loaders.

trigger hippie
08-25-2012, 7:15 PM
Thread necromancy ftw. Finally figured this out.

My max cartridge length is 2.2767. I obtain this by repeatedly using the RCBS precision mic (which is PLAINLY made for bolt guns) for seven measurements, discarding the low and high values. The RCBS mic gives me 2.1545. Next, since I know that the mic is riding on top of the bolt and not engaged by the extractor, I measure the depth of the bolt face and I obtain 0.1222. Conclusion, the largest round I can chamber is 2.2767; this is about 0.017 over the size of a magazine. These will have to be single-fed rounds.

Next steps are to manufacture rounds for a ladder test, from 2.260 (minimum recommended length for SMK 77gn) to 2.277, in 0.01 increments.

I'm thinking I'll end up using this single-feed formula on new brass that I'll save for long-distance shooting. if I know how they deviate from my standard "tactical" brass-throwaway load then I can use them on long-range shots in mixed stages - though single-loading during a run and gun stage seems like it would be a severe challenge, so I can't really see using it in a 3-gun shoot; this seems like it'll be more of an F-class or NRA high power load.

joelogic
08-25-2012, 7:40 PM
What is the point of all of this? People load sub MOA ammo that is mag length all the time.
This is with 77gr SMK, pulled surplus powder, wolf primers, range pick up brass, and a nut behind the trigger at 100yds.
http://i332.photobucket.com/albums/m353/joelogic/979c0f9f.jpg