Constantly Changing Headspace Dimension

[Phil3]

I am still struggling to find the proper shoulder setback on my AR-15. This should be easy. Measure fired cases with a solid load (not too light), and set the shoulder back .003 - .005" from that dimension. The problem is that the gun is very inconsistent on what the fired dimension is. It can vary by .004". The only consistent thing is one solid load with Vhitavouri N133 did result in 15 cases, all identical in size. I also measured headspace with a case and spent primer partially inserted. I closed and locked the bolt, withdrew the case, and measured headspace with primer acting as base of case. It is the same dimension as the Vhitavorui loads. So far so good, until I measured factory fired ammo cases. Those cases are about .003" longer than the Vhitavouri fired cases. ??? I am guessing the factory ammo is even hotter than the Vhitavouri loads and that my chamber is as long as those cases measure. But even those cases vary a fair amount from one to the next.

Bottom line, I am not confident where shoulder setback should be. How can I know for SURE?

Phil

 

[jonbearman]

Phil,what kind of cases are you using? How many times did you fire to start the measuring process? I full length every time and take the longest in the group and bump that one back like .004 thou.Then I know that they will all fit.Did you start with virgin brasss or once f

 

[bigedp51]

The majority of AR15s are over gassed, this means the bolt can start to unlock and move to the rear while there is still pressure in the barrel. This means the fired case can end up "longer" than the chamber because the residual pressure pushes the shoulder forward. Reloaders who shoot the M1A also have this same problem with cases longer than the chamber. The M1 Garand doesn't have this problem because the gas port is located nearer to the muzzle and the bullet leaves the barrel before the bolt starts to move.

You stated the factory loaded ammunition must be hotter, I think it wasn't "hotter" but loaded with a "slower" burning powder that raised your port pressure and allowed the cases to become longer than the chamber when fired.

Question, do any of these fired cases have case head brass swipes?

http://www.shootingillustrated.com/index.php/8720/case-head-swipe/

 

[bigedp51]

You also have your reloads and the factory ammunition you fired.

Use the primer trick on the longer once fired factory cartridges, how much head clearance do you have or is the primer flush with the base of the case.

Check all the different length cases you have using the primer trick, what is the "average" overall length with the primer seated with the bolt.

As long as your are using the same load in the same type cases you should have a constant length. If you have mixed once fired brass you will also have varying lengths.

Are you using new virgin brass or once fired LC cases with spring back issues?

 

[mac86951]

+1 with BigEd, overgassed extraction could be a culprit!

 

[stubbicatt]

I didn't read all the replies, but measure this headspace dimension with a deprimed case. The primer may have backed out some and skew your readings.

Good luck.

 

[effendude]

Put a CSS carrier weight system in the bolt carrier to delay the unlocking of the bolt. This will help with the overgassing/premature unlocking problem. I always size my AR brass to minimum specs. The AR platform has a floating firing pin just like the Garands and M1A rifles so slamfires, while very rare, are possible. Because fired gasses come directly into the action, AR rifle chambers are nearly always dirty and this will also affect measurements, clearance and headspace. These rifles are unlike a bolt rifle where the chamber stays clean. I strongly encourage you to size to SAAMI minimum specs for safety sake.

Scott

 

[XTR]

One other thing to do is to re-seat your primers or decap w/o sizing to measure headspace. If there is a bit of primer slip or crater protrusion it will make your measurements come in all over the place.

 

[Phil3]

To jonbearman: The brass is PMC and Blackhills. The PMC was new factory ammo and fired once out of the AR. The Blackhills was factory reload and new. All mixed up now, so can’t tell which is which, so not sure how many times these were fired before I got the ammo.

To bigedp51: Your “overgassing” statement is a potential explanation of why I can use your spent primer method of measuring headspace only to find out that ejected cases are sometimes longer. The AR has the proper size gas port hole in the barrel, and is a rifle length gas system, with a 22” long barrel. The slower burning powder makes sense too. And YES, some of the factory ammo (will need to see which ones) DO have head swipes.

I will find a long case, use your primer measuring method and see what that does. I rather expect the primer will be flush, but we will see. Good idea. I will also need to see what brands of brass are doing what, but I have resized some already, so not sure how much data I can collect. Brass brands in use are PMC (new ammo, fired 1x from AR), Blackhills new and reloads from Blackhills (fired 1x from AR), WCC (new ammo, fired 1x from AR), and Winchester (my reloads, fired 1x from AR).

To stubbicatt: I measure all the cases without any primer.

To effendude: I have a Wilson case gauge, which helps to see the min and max SAAMI spec. The cases I have resized all fit within that range. So I should be safe.

To XTR: I used a Sinclair depriming die, so no resizing was done with removal of the primers. I did it this way so I could measure fired cases, without having sizing done. Later, when I get everything right, I will use my resizing die with decapper in place. Saves a step.

Thank you.

Phil

 

[bigedp51]

If your gas block on the barrel has a set screw instead of being pinned. Loosen it and pull it out enough to block the gas port, If pinned punch the pins out and pull out . This effectively makes it a single shot. You'll have to manually eject the rounds after fireing, but you will have a more credible headspace measurement. Randy

The more credible method is using the primer method and getting your chambers actual headspace rather than wondering if the bolt twitched or moved at 55,000 psi or how much spring back your brass has.

Someone else already suggested he should use "SAAMI minimum specs for safety sake" but the AR15 uses military headspace settings which are a wee bit longer. :

 

[effendude]

Phil,
I will say it more simply this time. I honestly believe you are wasting much time and effort trying to measure case headspace length in a gas impingment gun. Depending on the load; powder, bullet weight, pressure at the time the bullet passes the port, all are variables that affect what you are trying to measure. Stop treating your AR like a bolt gun, it doesn't operate that way and it won't care how the brass is sized as long as it is consistent and fits the chamber properly with room to spare. AR chambers dirty up fast and you can't do anything about it. If the brass isn't sized to minimum specs, sooner or later you WILL have trouble.

Scott

 

[bigedp51]

Colt 5.56 Field gauge, 1.4736

Below, a Colt Field gauge in a "adjusted" Hornady cartridge case headspace gauge reading actual headspace.

Below a fired Federal M193 case and average length, they vary .004 to .005 depending on the ammunition manufacture. I also load for three diffrent AR15 rifles so the reloaded ammunition must work in all three rifles.

Below the same case after .003 shoulder bump.

Below a unfired Federal M193 cartridge.

They are not benchrest rifles and the have the bad habit of throwing perfectly good brass away and make you go looking for it.

Have you given any thought to something that looks the same and lets you keep your brass?

 

[Phil3]

Thanks to all for the useful tips and approaches. Really quite helpful! I certainly have some homework to do.

To rewinder: My gun has a clamp on gas block (Evolution Gun Works). Doing what you say would be easy. Thanks.

To bigedp51: My AR15 does not use a 5.56 spec barrel/chamber. It is a true 223 Remington. I wanted it that way from Krieger, and I rechecked with them again with the serial number. They confirmed it is not 5.56, but 223 minimum SAAMI. But, for those with 5.56, you are right.

To effendude: I understand your point and I am not trying to shoot for a snug case fit in the AR. I absolutely do not want a case stuck in the chamber! I also understand when you say “…as long as it is consistent and fits the chamber properly with room to spare…”. I don’t want the fit to be so loose (excessive headspace) that I risk blowing the head off the case. I don’t want too much “room to spare”. Perhaps the safest thing to do is just to make sure the cases fit within the min and max on my Wilson case gauge. The chamber on the rifle is a SAAMI min spec 223 (not 5.56), as is the case gauge, so proper fit with the gauge should leave me safe. I would still like to know what the dimension is within the gun and intend to find out with bigedp51’s primer method, by using the approach of rwh. I just want to see how that compares with the Wilson gauge and it is a number I can use to reference any shoulder setback to.

To rwh: I like this. “…The last time I rebarreled I took the ejector out of the bolt and tried to close the bolt on different size cases with my fingers until I had the exact size of the chamber and then set the die .003 back from that…”. I will try it.

To bigedp51: Yes, I have some rifles that keep the brass. Three bolt actions. But, am constructing a PVC frame and net to catch the AR brass. Not about to leave that on the ground! Thanks for the photos. Very nice and clear. My measuring set up looks identical, except I have the green tool on the caliper from Sinclair. My typical measurement on the 223 is around 1.4285".

Phil

 

[gdbleb]

This mirrors my experience when I started HP. Bought a case of Black Hills reloads (blue box) w/ SMK 69's. It shot very well and I progressed but once I got set-up reloading and measuring, had the same observations as you. Try measuring un-fired stuff for headspace. You may benefit by upgrading your match brass to a single manufacturer. Much more uniformity. Buy new WCC - LC - Lapua

 

[Raythemanroe]

Unless the brass is the same lot and fired the same amount wouldn't spring back vary, also wouldn't the amount of expansion vary? I like the idea of converting to single shot to rule out gun problems and then use brass that is all the same firing and lot.. my 2 cents


Ray

 

[243winxb]

Common problem with AR15 reloaders. Hope this helps.

 

[Phil3]

Because I THOUGHT I knew what the proper shoulder setback should be, I resized most all of my old cases, so I can't execute my plan for determining chamber size. My plan was to use a series of "long" cases, and set each one back to a specific dimension and see what chambers. Example:

1.4335" Will not chamber
1.4325" Will not chamber
1.4315" Will not chamber
1.4305" Will not chamber
1.4295" Chambers
1.4285" Chambers
1.4275" Chambers
1.4265" Chambers

Then I remembered I had unfired Hornady ammo, most of which won't chamber in my stock Howa 223. I sampled 10 rounds and found one measuring 1.4335". This round is so long it clearly sticks above the top limit of the Wilson case gauge. I don't think the bullet in the case should affect this measurement (I hope) when in the Wilson gauge. I measured some Blackhills factory ammo using calipers and the Wilson gauge. It was considerably "shorter" than the Hornady and was within spec on the Wilson gauge. And it chambers in the AR. I have my doubts about the Hornady. We will see.

Being too neat and tidy, I got rid of all my spent primers so can't use bigedp51's primer measurement method. I did this last time with a live primer, which made me nervous, but maybe I will just remove the firing pin from the bolt carrier. Still, live anything in the gun while working I the garage unnerves me a bit.

One question.

Can I safely resize the Hornady ammo (with decapping pin removed) while the primer, powder, and bullet are still in it. This sounds like a bad idea, and will not do it unless I am assured it is OK. If not OK, I will have to yank the bullet out of the Hornady case (not easy as crimped), empty it of powder, but the primer is still in it. I can keep slowly resizing the case down until it chambers, noting the dimension this happens.

Phil

 

[918v]

You can safely resize Hornady ammo in a a Redding body die as it will not size the neck. A standard die will size the neck and squish your bullet down by .010" or so.

You can do the live primer method, just be gentle and use the forward assist to close the bolt. If you don't have one, take the upper off and use your finger to push on the bolt carrier until the bolt closes. The primer will not fire unless it is hit hard in the center.