The truth about "pressure signs"

So I'm already in hot water over on the milsurp thread, so I might as well keep digging!

Over the years I've come across various rehashings of the old methods of pressure reading (sticky bolt lift, primer cratering/flattening, shiny spots on case heads etc.) and some of those examples have been right here on this fine website. I am concerned that many folks seem to take this stuff as gospel, when in fact most of it has been soundly dispelled as myth and tea leaves. To wit:

Primer reading -- seems to be the most commonly used method, probably because it takes the least effort. It may be the most useless, too. The idea is that a cratered or flattened primer can be a sign of excess pressure: cratering because the primer cup flows into the firing pin hole, and flattening because pressure forces the primer to back into the breech face. The problems are several. First, not all primers are made with the same toughness of material, so a "safe" load -- according to primer condition -- turns into a "dangerous" load when a different primer is used. Did pressures change? Nope. So what did you learn from the primer? Not a thing. Secondly, cratered primers can show up with very mild loads, in a gun with lots of clearance around the firing pin, or even in a gun with a heavy firing pin fall. It's quite common for your old Mauser to put a real splash into a primer, simply because those old military rifles were designed for reliability above all else. This is neither good nor bad, it just is. And those "cratered" primers from Old Betsy don't mean a thing.

Now, pierced or "blown" primers are indeed telling you something. Assuming the rifle is in halfway decent shape, a pierced primer should be taken as an absolute indication of overpressure. The question, though, is "how much", and the only answer (again assuming rifle in good shape) is way too much. Something is seriously wrong here, either an accidental overload, incorrect powder, or similar misadventure. A pierced primer tells the handloader to stop right there, back up and figure out what's going on.

Then there is hard bolt lift -- supposedly indicating either that pressure is so high that forcing the brass case to "stick" to the chamber walls, and/or that the case head has extruded into the extractor cut and the extruded bit then has to be sheared by the bolt in order to extract (hence the shiny spot on the case head.) The trouble here is that, much like primers, brass cases vary in toughness. A soft case (Norma is a common offender) may well flow at safe pressure levels, while a very tough case (Lake City, Lapua) may not show any sign of pressure until SAAMI specs have been massively exceeded. The case itself is responsible for very little of the strength of the rifle/case combo, so soft cases are just as safe and useful as hard ones.

Moreover, the condition of your chamber can make a big difference as well. A rough, pitted, or rusty chamber is going to give you sticky extraction with almost any load. A precise, polished match chamber is going to tell you the opposite lie: that pressures are just fine, even if they are sky-high. (This, BTW, is why you'll find benchresters complaining about loading manuals written by lawyers -- a custom benchrest rifle isn't going to give any of the typical pressure signs until 85,000+ PSI.)

And as for the shiny spot on the case head? It's the same deal: soft brass will flow more than hard brass, and a gun with a large extractor cutout is going to show more extrusion than one with a small, tightly fitted one. Going by the shiny spot, an 80,000 PSI load is "safe" in the match rifle, while the 45,000 PSI load is "overpressure" in the old milsurp. Both "truths" are false.

Which brings us to measuring case heads. This method was actually used by the reloading manual publishers for many years. It depends upon an experienced user with a blade micrometer, taking precise and repeatable measurements at the correct area of the case, and using the same lot of cases. Few reloaders meet all those criteria, and many who think they do probably don't. Beyond that, there are still problems, like work hardening. Moreover, more recent testing has shown that even loads put together by experienced professionals using this technique have exceeded 70,000 or even 80,000 PSI, which exceeds SAAMI limits for any cartridge I am aware of. It's a slightly better method than the others listed, but it's still no substitute for actual pressure testing.

In summation, reading pressure signs is synonymous with pressure guessing, with your rifle -- not to mention health -- on the line. The idea that the hobbyist handloader can learn more about pressure by looking at his cases than the professional ballistician can learn by using several hundred thousand dollars of purpose built pressure testing equipment is dangerous and ludicrous.

 

Now, having written all of that, I'll mention the two "methods" that do work.

The first is simply obeying the manual. If you carefully work up a load to the manual maximum, without any pressure signs, in a modern rifle, you are almost certainly safe. I know how boring that is to a certain type of handloading adventurer, but there you go. IMO, if a person wants more velocity, he should get a different rifle. The craze for turning your .30-'06 into a .300 Magnum utterly defeats me, but whatever.

The other "method" involves velocity. Using your chronograph, you slowly work your way up to the maximum published velocity for your barrel length. Again assuming a rifle in good condition, and no signs of pressure, getting your 180/.30-'06 bullet up to 2700 FPS practically ensures a safe load. If the chrono says that bullet is going 2900 FPS, you can bet the farm that your pressures are exceeding SAAMI specs.

Which brings up one last point: SAAMI vs. "safe". The benchresters, among others, have proven that the two are not perfectly synchronous. In my time in the benchrest game -- now more than a decade ago -- it was common for shooters to grossly exceed SAAMI specs for their loads, and many of these folks could reuse the same set of cases for the whole year -- dozens of reloadings. And I never saw a blown rifle. This really just points to the facts outlined in the first post: in a carefully crafted match gun, you can get away with a lot. The same is also true, to a lesser extent, in any modern sporter from a major company. Most of these guns have a huge margin of safety, and can stand up to the adventurous handloader with no real trouble. The problem, of course, is that the pressure guessers have no real idea how much pressure they are actually developing, and no real idea if this cartridge is the one that's finally going to cause the gun to let go.

I personally don't see the sense in that kind of guessing game, considering the consequences.

HTH!

 

The point is that we don't have "several hundred thousand dollars of purpose built pressure testing equipment." We have to go by what we see. That means watching for a wide array of *POTENTIAL* warning signs, checking FPS on the chrony, and cross-checking our loads and load data with multiple sources. Common sense and experience also play a large role.

SAAMI standards, particularly for European military rounds, are frankly ridiculous. They have for example neutered the 8x57JS into something akin to a .30-30 for absolutely no good reason.

 

I've always understood that the sudden onset of the symptoms you listed could be a sign of over pressure. If you are getting them from the git-go with normal or soft loads is one thing, but if at some point you start getting hard extraction, cratered primers (obviously using the same primers as before), etc., you might want to back off a tad. They're not something I'm going to ignore.

 

And if you don't normally see cratered primers in the firearm you are shooting and all of a sudden you see cratered primers because you upped the powder charge in an attempt to squeek out just a bit more fps or you are working up a new load on a known rifle. I would call cratered primers an indication of over pressure in that case.

But mostly I agree with you.

I do have a revolver that starts complaining at just under maximum powder charge...

 

How about loose primer pockets after a single loading?

I have loaded some max 22-250 loads that only after a single loading a new primer would just fall right out.

I think loose primer pockets, while not being able to see right after you fire a round, are a pretty good indicator of high pressures.

 

And now to further complicate things, we have new powders (RE-17) that distribute the pressure over a longer period. This yields higher velocities than we have ever been able to achieve with most powders. Seems like an over-pressure load with one of these new powders might have the potential to really damage an action.

 

Quick, contact Hornady and tell them their 180gr .30-06 Light Magnum load at 2900fps exceeds SAAMI specs. I'll give you one thing, .38 Special, you are relentless.

Don

 

Smokeless propellants, gunpowder being a subset, have an exponential burn rate. Or maybe the slope of the pressure curve changes exponentially, I forget which is the correct description.

But what it really comes down to is that pressures don’t increase in increments like 1, 2, 3, 4, 5, 6 but increase in increments like 1, 2, 4, 16, 256, 65,536. All the devices available to us are crude measuring tools to guess the rapid rise in pressures. Of course we are going to be off when we try to measure something down to the tenth of an inch using the Pharaoh’s cubit.

But that is all we have.

Read them tea leaves and predict the future.

 

myth and tea leaves

http://www.thehighroad.us/showthread.php?t=375160 Not myth or tea leaves. Pressure Signs
The bolt will be hard to open when you are over pressure. The primer will not have a rounded edge. The web area of the brass has expanded. There is an ejector mark on the brass head. Case head separation. Primer flowing into & around firing pin. Look at the photos of some signs here. http://www.photobucket.com/joe1944usa Click for larger photos.



This guy had primer pressure signs but keep shooting till he got this.

 

As far as primers having different thicknesses, any idiot who swaps components near or at max charges without working up their loads from starting charges is just that: An idiot who deserves what they get. That doesn't negate the usefulness of people watching for CHANGES in the look of fired primers as they work up their loads.

 

So let me try and understand your post. All pressure "measurment" techniques that don't involve using a transducer are bogus. However, by using them and following the manual or matching test barrel velocity then a load will be safe. Safe being that it doesn't blow up your gun, rather than staying within the confines of the SAAMI specification.

Is that really what you are trying to say?

 

Bingo. Nobody has ever said that these pressure signs are absolute measurements. I figured everybody knew they are relative measurements only. Which is why I was really confused while reading the OP.

 

+1. For example, I cast bullets. If my bullets drop from the mould with wrinkles, the lead and/or my mould is not hot enough. If the bullets are frosted when they drop, the lead and/or my mould is too hot. I don't need to know what the exact temperature is.

Don

 

Why not use case head expansion as the indirect measure of pressure? It seems relatively reliable, by all accounts...

 

I cite the example of 7x57 vs 270 win.

A lot of folks make the mistake of believing that some form of pressure indication will show the instant SAAMI max pressure spec is reached. The problem in this is neither your rifle nor your cases know a thing about this arbitrary number.

If a person has two similar rifles one in 270 and the other in 7x57 and you don't see overpressure signs with the 270 shooting max loads why in the wourld would you expect the 7x57 to show you anything shooting pretty much the same piece of brass. In other words if the 270 doesn't show you anything at 65,000psi why would you expect the Mauser with it's 51K rating to tell you anything till it's overloaded by at LEAST 14Kpsi

This example is given to illustrate how just through comparison to high pressure rifle cartridges that pressure signs will not show till you've already drastically gone over the line.

38's advice above is sound, the only thing you can do is stay within published and cross referenced data and use your chronograph.

Well look at it this way if you're getting a 180grainer to 2900fps matching or exceeding light magnum ammo with a handload then yes you can in fact bet the farm you're overpressure. Supposedly light magnum ammo does it's thing by using large quantities of slower propellants loaded by some form of magic that allows for hyper compressed charges

 

Some form of magic? Sorry, but of all the things I've learned in 30+ years of reloading, I must have missed the course in magic. However, you are right about large quantities of slow propellants. And you know what? I guarantee that the Hornady Light Magnum load is below the .30-06's 60k SAAMI pressure spec, just as my 2900fps 190SMK load is.

Don

 

I shot benchrest for many years myself and I do have to admit that it changed my views on pressure and maximum loadings. However, there are a lot of factors to consider when making determinations. If you are using a powder that is a little on the fast side for the case capacity and bore diameter you are shooting you can still ruin your day by being less than diligent. On the other hand if you are using a powder that fits your bore diameter and case capacity, there will be a lot of latitude that you can take advantage of. The key is knowing the difference. Reloading manuals are written to provide safe loading information for the average reloader and if followed they will do just that.

That doesn't mean that if you add .1 grain more of powder than the manual recommends that you are suicidal.

For those of us who like to "wildcat" we don't have the benefit of loading manuals to follow for other than the most general of information.

YMMV

 

And how have you prooven this? USSR

 

Maybe, maybe not.

Pressure exists as a curve. If the curve is relatively sharp, then you get less overall velocity. If the curve is relatively broad and flat, then you get more velocity. In many instances, we can gain velocity by increasing the dwell of the effective pressure curve without necessarily increasing the peak pressure.

 

On the other hand, if the rifle can safely handle 65K in .270, an identical rifle in .30-06 or 7X57 should be safe at the same pressure level. MAP for the .30-06 and 7X57 are set low because there are many old rifles out there that are not safe at higher pressures.

The danger in loading a 7X57 hot in a modern Model 700 Rem is not that the rifle can't handle higher pressures, it's that when you depart from published data, you're in uncharted territory.

 

I'm going to stay out of this discussion about rifle pressure, simply because I am not much of a rifle reloader yet. Having said that--

There is that case head expansion / diameter measurement subject. I know that was a popular technique in the Sixties and Seventies for handgun loads--Ken Waters used it.

However, somewhere in the past few years, I believe I have read a study of this--e.g., done with pressure barrels, a run of same-brand brass, etc.--and there simply was no good correlation between diameter expansion and excess pressure. I'll see if I can chase down a link on this issue....

Jim H.

 

Hornady loads their Light Magnum series with powders you cannot get, and techniques you cannot use. But you knew that. If that's the best you can do, I'll consider my opinions unchallenged.

Your thread detailing the pressure guessing methods you use in working up benchrest loads was irresponsible, as is your repetition of the idea that you alone have figured out a way to drive bullets hundreds of feet per second faster than any published load while still maintaining SAAMI specs. Your "guarantee" is useless, because it is based upon the exact guessing methods shown time and again to be without merit. I think it's high time people like you stopped encouraging folks to push the limits of safety based upon little more than tea leaves.

If you want a .300 Magnum, just go buy one. They're making more every day.

 

No. I already said what I was trying to say. You cannot grossly exceed published limits while "knowing" you are staying within SAAMI pressure limits based upon signs like brass and primer condition.

 

It was actually used to develop loads by many of the reloading manual folks -- which is one of the reasons why published loads have been cut back quite a bit over the last few decades. Actual pressure testing equipment showed that even loads developed by professional ballisticians using the case head measurement technique could be grossly overpressure. Any technique based upon changes to the brass case are invalidated by the fact that brass cases differ one to another and change even during the process of firing.