Tuning headspace in the AR-15 platform

[Nitrider]

This question is a little convoluted, but is a window into my reloading mania. In advance, thanks for taking the time to read it. I appreciate the help.

I recently set back the headspace of some reloads causing the occasional double feed. This led on a quest to better understand headspace and it's effect on accuracy and function of the rifle.

My journey began by measuring headspace using a Hornady headspace guage. It works well and led me to believe that headspace should be around 1.451"-1.455" according to factory loaded .223/5.56 ammo. I then began measuring the headspace of cases that had been fired through my rifle and found that it was 1.459 +/- .001"

So, question #1: If my fired cartridge's headspace measures 1.459", what should I adjust my resizing die [headspace-wise] to optimize function and accuracy?

Moving on...

In order to tune my cartridges headspace, I purchased a feeler gauge set to help adjust the resizing die. I started experimenting with different headspace measurements to see if they chambered properly. To my surprise, when chambering a dummy round made of a piece of resized brass [at 1.458"] and a 55gr bullet, the headspace measurement shrank after being chambered. Specifically, it reduced itself from 1.458" to 1.454" after releasing the bolt hold-open and letting spring power chamber the round. In contrast, when riding the charging handle forward and slowly loading a round into the chamber, the headspace measurement does not change at all.

Question #2: Why is my headspace measurement shrinking after chambering a round?!

Question #3: What do you believe this rifle's actual headspace is? Or, how would you go about determining my rifle's exact headspace?

For what it's worth, I've tried chambering longer headspaced brass and gotten cases stuck. I've also tried chambering much shorter headspaced cases until it caused failure to extract issues. I've at least determined a sweet spot to keep the rifle reliable.

Another thought on headspace in the AR platform is that cartridge overall length is limited to magazine dimensions and as such, loading the bullet until it is a few thousandths off the rifling is not really an option. Since loading the bullet just shy of the rifling is not an option, the next best place to look for accuracy is properly headspacing cartridges.

Question #4: Will adjusting headspace result in better accuracy?

Again, sorry for such a convoluted post, but the questions feel too interconnected to separate.

[bigedp51]

Fire a assortment of factory ammunition and measure their average length with your Hornady gauge. I showed you this in your first posting, the basic thumb rule is to bump the case shoulder back .003 to .006 from its fired length for a semi-auto rifle. When I bump my fired Lake City cases back .003 using a small base die they are .002 longer than a unfired Federal M193 cartridge case.

Your cases are shorter when chambered because the weight and speed of the bolt slams the cartridge into the chamber and the soft shoulder of the case is pushed backwards.

As long as your cartridges fall in between .003 to .006 shorter than the chamber don't worry about headspace and accuracy.

The late Jim Hull of the Sierra Bullets testing laboratory was also a world class competitive shooter and did not believe in neck sizing only. And he would jokingly say "The cartridge should fit the chamber like a rat turd in a violin case" meaning he full length resized all his cases with adequate shoulder setback for easy chambering.

You have a AR15 and not a bench rest rifle, so just keep it simple and don't over think anything and you will be OK. rifle

I have a AR15 A2 HBAR and a standard AR15 carbine plus a Savage .223 bolt action and I save the tighter headspace cases with .001 to .002 shoulder bump and the better brass for the bolt action. You have a AR15 rifle that chews up your rims and throws perfectly good brass away and makes you go look for it.

So just resize your cases with the correct shoulder bump, shoot the rifle and don't over think and complicate matters. As time goes on you will learn what your rifle likes and doesn't like and you will be in fat city.

[Nitrider]

bigedp51- "Rat turd in a violin case", I'm going to remember that one. Ha ha!

Thank you for the well thought through reply. I'll take a deep breath, put my OCD on the back burner, and go back to reloading per usual.

While I'm not trying to eek out every last bit of accuracy from this rifle, I do want to make sure that I'm doing things properly.

I will follow this proceedure when reloading.

Polish brass in media tumbler

Lube/deprime/resize to .003-.006" smaller than fired brass headspace dimensions in a single stage press

Polish in tumber again to remove lube

Trim brass to 1.753"/chamfer and deburr using Trim-it II

Swage primer pocket [only if necessary]

Prime/drop powder/seat bullet in Dillon 550 [Note: Bullet seated to max COAL that the magazine will allow, usually 2.250"]

Does that sound about right? Am I missing anything along the way?

[adamge]

I've only reloaded about 1200 rounds .223 total so I'm rather new to this. My process is similar but I ultrasonic clean instead of tumble, and I do the second clean after lube/resize/deprime-trim-chamfer-pocket. To me this makes more sense because the trim, chamfer, and pocket steps all generate little particles of junk that I don't want in the case.

[Steve RA]

I load to 2.260 as they will fit in my magazines. This puts the cannelure on the Hornady 55gr SPs quite a ways out of the case and always elicits questions if anyone sees them. I also size .003 less than a fired case.

[Nitrider]

I've only reloaded about 1200 rounds .223 total so I'm rather new to this. My process is similar but I ultrasonic clean instead of tumble, and I do the second clean after lube/resize/deprime-trim-chamfer-pocket. To me this makes more sense because the trim, chamfer, and pocket steps all generate little particles of junk that I don't want in the case.

adamge- Good idea on cleaning the second time after trimming. I wonder if case lube may build up in the Trim-it as I use it and mess with trim length.

Thus far, I haven't noticed any residual brass shavings in my brass as I reload, but you never know...

I load to 2.260 as they will fit in my magazines. This puts the cannelure on the Hornady 55gr SPs quite a ways out of the case and always elicits questions if anyone sees them. I also size .003 less than a fired case.

Steve RA- I assume that loading out to 2.260 doesn't cause feeding issues in your AR and that you've had good accuracy results reloading this way, right?

Also, does loading out longer help with accuracy or does it increase case capacity? Both?

[lwink]

Steve RA- I assume that loading out to 2.260 doesn't cause feeding issues in your AR and that you've had good accuracy results reloading this way, right?

Also, does loading out longer help with accuracy or does it increase case capacity? Both?

Loading longer does increase case capacity and theoretically then you can put in more powder to burn, but it also takes more powder to make the same pressure. In a short barrel I doubt that this is really a point worth considering for too much time as you are blowing a lot of extra powder already. This would be more substantial if you had maxed out barrel length vs burn rate of your cartridge.

More importantly it does affect accuracy. Some bullets like to be closer to the lands (rifling) while others prefer a small "jump" and others still are pretty tolerant of larger jumps. Can't say that longer is more accurate, or shorter is more accurate, its all about your particular bullet choice and chamber cut. Another point that should be stated is pressure increases when the bullet hits rifling as this causes resistance, so loading too long and jamming bullets in the lands must be done knowingly and with caution as not to exceed safe pressures.

Finally, since this is an autoloader you must assess magazine fit and bullet feeding. If you load a 40gr vmax to the same length as a 62 or 75 grain hpbt you're asking for trouble. Either you haven't properly seated the small bullet and it may move when in the magazine or hitting the feed ramp, or you may have greatly overseated the longer heavy bullet thereby reducing the internal case volume and possibly setting yourself up for dangerous pressures. Remember, all these "overall lengths" of loaded cartridges are not all that important, but the base to ogive is actually what you have to be concerned about.

If you want to measure your chamber properly (I think these other guys gave you great info on case sizing) you can buy a tool to do that and another to then measure bullets from base to ogive and you will always know how much jump or jam your bullets have. I doubt many jam in autoloaders, seems like a bad idea. I personally measure with a cleaning rod and caliper, which is not 100% accurate but I've verified within 2-3 thousandths of an inch. Insert a bullet (bullet only, not whole cartridge, vmax, Amax, or some polymer tip is best) into chamber and apply just enough pressure with a short pistol cleaning rod or unsharpened pencil so you know it has just touched the rifling. Don't jam it in. From muzzle end insert cleaning rod with blunt jag and make contact with the nose of the bullet. Mark cleaning rod at muzzle with fine pen or sharp edge of tape. remove rod carefully and bullet. Close chamber and bolt with firing pin retracted, measure from muzzle to contact with bolt face with the same rod and jag and mark again. Measure from mark to mark with calipers (these need to be super fine marks and consistent across the muzzle). This is your OAL to touching lands with THAT BULLET. I load that bullet then (either real or dummy) into sized brass and until I get the same OAL from base to bullet tip that I measured on cleaning rod. Use your ogive measuring tool to get the ogive length and now this number is the one you save and can use for all further loads with any bullets. You know this length is on the rifling, so if you want to experiment with jump try 5,10,15,30 thousandths off or whatever you'd like and see how accuracy is changed.

Hope that made sense, otherwise pm me and I can try explaining more. This info is also taken from bolt guns, but the ballistics will apply, just not too familiar with feed issues you may incur.

[adamge]

thx wink, good write up.

[Steve RA]

I have no feeding issues. On the Hornady 55gr SPs loading to 2.260 - besides fitting the mags I use - puts the base of the bullet at the juncture of neck and shoulder in the case. Seems to be pretty accurate but is improving as it's a new gun and doesn't have many rounds thru it yet.

[dauntedfuture]

Headspace on a rifle is a fixed distance and its from your bolt face to a measurement about 1/2 in the shoulder of the case. When you are talking about sizing cases, you are not adjusting headspace, your just sizing your brass more or less. Specific to your issue; most double feeds are caused by bad magazines so i would start there. If you are getting a double feed then that means that two loaded rounds are attempting to chamber at the same time. If you have a fired case and a live round then thats a failure to extract (if the fired case is stuck in the chamber) or a failure to eject (fired case and live round fighting to get into the chamber).

Using the Hornade guage gives you a measureing point as a reference; your rifles "headspace" is NOT 1.459, thats just a reference. To answer your question you want to size cases that have been fired in your rifle to -.002-.003. I emphasise that you have to use cases fired in your rifle. An alternative is size a case and try to chamber and eject it using a "new" fired from your rifle case each time and stop screwing the die down when things work. From there size 5 and try them all. Then try some random cases and see if the all work.

Your "headspace" on the loaded round shrinks because there is enough force to size the case when you slam the bolt closed. In a bolt action rifle it would be hard to close and you can gaul the lugs.

Properly adjusting/ setting up your dies will primarily influence function of the rifle and or case life. Excessive sizing can reduce case life. In general, as long as all cases are sized the same you will not see an effect on accuracy. Cases that are not sized enough can also stick in the chamber, resulting in extractor marks that will mimic signs of high pressure as well.

Without getting into the weeds; set your dies up properly, FL size your brass and go shoot.