Reading primers - pressure question


December 30, 2011, 11:57 PM
From today's range session I'm left with a question about signs of overpressure.

After inspecting my ejected cases, I noticed the one round I fired loaded at max had a widened & flattened primer. I'm a little confused about it though.

The load is 55gr Hornady FMJ, H335, CCI #41 primer, & LC brass. Fired from a 20" AR-15 with 5.56mm chamber.

The chrony results were off & everything was reading higher than last trip including factory rounds from the same lot. It's probably because the range is uneven on hilly ground so I'm shooting from a nice level platform but the backstop is a little lower so the rounds are passing through the chrony at a downward angle. I probably had the angle set a little off from the angle of the shot. That's all I can think of because my loads at the same charges and the XM193 were both reading right around 135 fps higher. Anyway, the average FPS for these batches were:

24.8 gr - 3054 fps
24.9 gr - 3102 fps
25.1 gr - 3212 fps
25.3 gr - 3299 fps
XM193 - 3405 fps

I'd say 100 - 135 fps lower is reality.

Anyway...... The round I fired at max load (25.3 gr per my data) has a primer that looks (to me) like its showing signs of overpressure. The XM193 is still much hotter. The couple I pulled & weighed had charges of 26.9 gr & I thought H335 & what's in LC ammo were very similar. Even so, XM193 should build a lot more pressure than a published max .223 load. So why would I get a flattened primer on my max load and no issues with XM193?

Or am I reading this wrong & that's a normal primer? (it looks different enough to me to at least ask). I don't really intend to make large batches at max load - I just thought I'd see how much difference there was. Should I try a few more or leave it alone & stay under 25.3 gr?

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December 31, 2011, 07:00 AM
Sorry CMV, I don't see a flat primer. Primers can be hard to read as different primers have harder or softer cups. Your Mil spec primers are pretty tough compaired to a Federal primer. Not that I recommend a Federal primer in a AR.

The cut crimps you have do leave a rather large gap between the edge of the primer and the base of the case. A flat primer will fill in a normal swaged pocket gap and appear to be flat all cross the base of the cartridge. There will be no rounded edges on the primer.

Checking a Speer 13 you still have a ways to go to reach max.

I hardly ever make it to a max load. I find I want to stop increasing powder some where in the middle of the road.

if you think you have gone as far as you want to, then there is no reason to keep adding powder.

December 31, 2011, 07:05 AM
You'd have to be significantly downhill for it to make a difference on you velocity readings and even then it wouldn't matter until the bullet was at terminal velocity. Compared to drag, gravity is insignificant. Different lighting on the chronograph, different atmospherics or even a calibration problem would be the most likely explanation of your velocity differences.

That primer doesn't look out of the ordinary. Setting the shoulder of the case back a little more than the others could easily be to blame for that bit of flattening. It could also be that primer was a little soft or a myriad of other things. Primers are a very poor indication of pressure.

December 31, 2011, 07:10 AM
This might qualify as a flattened primer. I got a few of these from a batch of handloaded .327 fed mag. Loads were well within published data, but I had some loose primer pockets, possibly due to my lubing my priming tool too much. I don't know. I reloaded the cases and will test carefully. May need to go to a SRP instead of SPP in that caliber. That's what Fed uses in factory ammo. Sorry for the hijack.

December 31, 2011, 08:27 AM
Primers are unreliable in estimating pressure.

The only positive signs of overpressure are pierced primers, leaking primers, blown primers. If you have those indications you are way over pressure.

In a manually operated rifle sticking cases are a positive indication of overpressure conditions.

If your primer pockets expand so much that primers are a loose fit, then your pressures are too high.

If you want to be safe, look at your data books and stay within the velocities there.

If your cases look like this, pressures are way too high!

December 31, 2011, 09:37 AM
My load is hotter than your load!You win. :)

I do not see anything in the OP's primers to worry about either. Differences in primer cup hardness could show that much difference is how flat they get, and the OP's are not "flat", still showing a nice round edge.

If you start getting sticky extraction, excessive velocity, etc, then you have a problem. Up till then you could be OK or a little over max pressures, but you would never know it by looking at the primers.

This might qualify as a flattened primer.
Oh yea, close to a real problem there.

December 31, 2011, 10:07 AM
This is what I was thinking could be giving me inconsistent chrony results:

Angle is exaggerated, but if I'm at an angle to the target and the chrony isn't sitting at the same angle as the flight path, that would make the flight path from sensor to sensor a little longer giving a slower reading. I didn't think gravity was causing it :)

But I was there a couple hrs later in the day yesterday and the sun was at a different angle so that might be it too. I guess it will be a good practice to always start with a few rounds of factory ammo to make sure they're measuring close to normal before believing what the chrony tells me.

So that primer shape isn't anything to worry about? It looked different than any other I've fired so far so was cause for concern.

My brass was 1x fired LC I bought locally. Some of it is, but a lot was already reloaded once. There were also about 300 pieces of crimped FC headstamp mixed in. The crimps are all different. Some are in place, some look to have been cut out with a chamfer tool, and some have been swaged or it was new brass with no crimp (can't tell for sure but some look like there never was a crimp). If that's going to give me primers of all different shapes after firing, I'll stop worrying about it so much and watch for other things.

Or do I need to watch for overpressure at all? If I'm staying .3 or more under max load and using 5.56 chambers can I now consider them safe from a pressure standpoint since 150 or so have been fired? I have been stopping after each string, gathering up all the brass, & inspecting it.

Friendly, Don't Fire!
December 31, 2011, 10:17 AM
The angle would have to be a lot more substantial at bullet-velocity speeds!
As shown, I bet the speed would not even vary by one foot per second!

Now, if one were to measure the speed of a snail walking between point A to point B, they may have some kind of variance due to the angle, i.e., like four inches per minute difference between the straight path and the angled path out of a total of perhaps fourteen inches in 22 minutes or more!

Third EDIT:
By the looks of your primers, you are in safe territory as far as I am concerned and the way I visually judge my primers and case bottoms for my loads.

When I see the imprint of tool marks embedded into the flattened primer and even tool marks impressed into the case bottom, stop there and do not shoot any more of those loads as you are more than likely working in the DANGER ZONE!

December 31, 2011, 02:58 PM
CMV, The closeup of the 25.3 show some signs of pressure (radius missing) but not over pressure like the other have said. When you get flattening where it fills in the gap you've crossed the line. With that said primers are not the most reliable way to read over pressure signs. Walkalone mentioned the ones that are better indication on a bolt rifle. I think your safe but continue to look for signs. If the ambient temp goes up you may have a different results.

December 31, 2011, 03:05 PM
cmv, what brand of chronograph are you using?


Lost Sheep
December 31, 2011, 04:15 PM
Friendly, don't fire,

The difference in speed would be proportional to the cosine of the amount of angle off from true alignment. And always a LESSER velocity, never more.

A lot more than single digit fps, though, at rifle velocities. The angle can be a lot more than most people would estimate.

If the chronograph is a Chrony, though, you can get substantially higher velocity readings if the instrument is not fully open. A partially closed Chrony has the screens' sighting a lot closer to each other, and the effect is more pronounced the higher up the bullet's flight path is, the more the effect.

Also, temperature of the ammunition can easily make 100 fps or more difference in velocity. Some powders are more temperature sensitive than others.

Lost Sheep

Friendly, Don't Fire!
December 31, 2011, 04:33 PM
I did not indicate which way the speed would fluctuate, I did not say the speed would be faster or slower, only that it would not vary as much as one might think.:D

December 31, 2011, 06:13 PM
I'm using a Chrony F1 mounted on a nice camera tripod.

If I was sitting on the shooting bench, had the rifle shouldered and pointed downrange with the barrel level, I'd be holding about 12-15' over the target at 100 yds. 15' over 100 yds isn't a huge decline but it's enough that if the chrony is level, the flight path is crossing thru it at an angle. Not much of one, but it's there. whether that's enough to throw the readings off or not I don't know.

It's been unusually warm here for Dec. 55 - 60. Anyway, saw a consistent fps spike in my loads and the factory loads. +/- 10 shouldn't do that with XM193 or H335 right? Anyway, i'm thinking it was something to do with the chrony this time & not the 1st time although it appeared to be set up the exact same way. XM193 was showing around 3400 fps & it shouldn't go that fast. The "it wasn't unfolded all the way" theory is as good as any because that would explain it, but since I had it sitting on the bench while installing the legs & diffusers it would have been hard not to have it fully extended. But it is possible since its new & kind of stiff.

I'm also watching for cycling/ejecting problems, bulges, or splits in the brass along with inspecting primers. Since I'm new to this figured better to be safe & ask because this one primer looks different than all the others so far.

Friendly, Don't Fire!
December 31, 2011, 06:18 PM
I purchased a long, low red metal toolbox that I put my Chrony f1 in without ever having to fold it up for storage. I prefer to leave it open, the rods and shades fit in there (shades assembled) as well.

January 1, 2012, 11:49 AM
Not knowing what primer LC uses, you cannot read the CCI#41 to compare against the LC.

Pierced primers occurr at pressures way in excess of what you want to run at. Waiting for a pierced primer, then backing off a hair is not a good reloading practice.

I suggest you get a factory load using components you can obtain, i.e. FC, and rework your load.

January 1, 2012, 01:04 PM
The thing I see immediately is excess material cut away from the primer pocket. That's the problem with using a chamfer tool, excess material can be removed too easily. The primers themselves look OK to me from what I can tell from the photos. (No cratering of the firing pin indention.)

January 2, 2012, 10:41 AM
The XM ammo is loaded to 5.56 as far as I know and your book is probably for .223. Hence the higher velocity of the factory loads when you are at book max.
The military used 26gr or more of WC844 powder. I want to say 26.5 but I'm at work and can't consult my data. WC844 is allegedly equivalent to H335. So in a 5.56-chambered AR with rifle-length gas system, and from looking at your pics, you should be good at your max load. YMMV

Look for signs such as the imprint of the bolt face/ejector/extractor on the head of the cartridge. When you begin to see the light imprint of the hole in the bolt face for the ejector stamped into your brass, it's about time to stop raising the bar. Deep impressions are excessive pressure and you need to back off your loads. I've found these signs to be more reliable than primers especially in semi-autos.

I have a lot of ammo I loaded using WC844 that was developed in my 20" Colt that the carbine-length ARs don't like at all due to the shorter amount of time the bolt stays locked on the 16" guns. They were trying to open too soon while the chamber was still pressured, ripping the rim off the shell with the extractor. I fixed the extraction with heavier buffers, but my carbines still show pressure signs with that ammo while the 20" is perfect. All my ARs are 5.56 chambered--or at least that's what they are marked.
All this to say that each gun is different and it will tell you when it's time to stop raising pressure, you just have to be alert and able to recognize the signs. You have done well by stopping when unsure and asking others.

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