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Is there any way to use base or expansion ring measurements on a formed rifle case to decide where to stop (from a pressure standpoint) if you do NOT have a factory cartridge to measure?

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DB Bill is right on!! There is no reliable way to do this, whether you have a factory cartridge or not! NO CASE expands the same amount as any other case, even if pressures are exactly the same! Often a round that has a HIGHER MEASURED CUP pressure will expand the so-called "pressure ring" LESS than a round with a LOWER MEASURED CUP pressure expands a different case! And, since work-hardening changes a case after each successive shot, even the SAME case can't be used for this!!

People who determine what is an acceptable pressure level using this method are deluding themselves. A lot of "experts" who should know better are still doing this! (Ken Waters is one of these "experts"!!)

[ 09-30-2003, 22:37: Message edited by: eldeguello ]

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For decades, I defended the notion of miking cases to get an idea of the internal pressures being developed. I'm the editor who originally published both Bob Hagel's and Ken Waters' articles on how they do it. I also published Waters' "Pet Loads" article as a supplement for his Pet Loads book.
At the time, Waters and I both considered Hagel's method extremely risky. I still do (because it IS!). I supported Waters' more moderate approach. I've since learned how foolish and unreliable any variation of this basic technique is.

� Many cases don't expand enough, even at 80,000 lb/sq in., to warn of risky or excessive pressures.

� Catastrophic failures of overloaded rifles may occur with either the first over-hot round, or they may occur only after years of repeated use of over-hot loads. In the latter type of failure, the rifle has appeared "safe" with these loads, clear up until the time one round "caused" the failure "for no apparent reason."

� Cases work-harden in use. Repeated use makes them become brittle in the crucial portion exposed in the breech � typically 0.200 inch of the head of the case. Cases already too hard to show "excessive" expansion here (some, even at 80,000 lb/sq in.) are especially likely to become brittle in repeated firings and reloadings, and spew wild gas and bits of brass into a shooter's face. I have on hand now a rifle utterly demolished when half the head of the case blew back through the action. The rest of the case is still in the apparently unharmed barrel, but the receiver is in many pieces, and my friend still has one piece of brass in his face (a larger chunk was surgically removed).

� The maximum safe limit for many rifles and cartridges is well below the level of peak pressures that many cases can handle without any discernible or measurable indication of excess.

� The less experienced you are in the use of this method, the greater is the certainty that miking your cases will inevitably lead you to accept dangerously high pressures as "safe."

Careful lab tests of many typical "pet" loads, developed by attention to traditionally accepted "signs" of pressure, have shown their peak pressures to be 70,000 to 75,000 lb/sq in. The highest SAAMI "safe" pressure I know of, for any cartridge or rifle, is 65,000 lb/sq in. Most are lower. Many are much lower.

Some carefully lab-tested loads, developed by miking case rims, webs, and expansion rings, have developed 80,000 lb/sq in. without measurable expansion.

Stay well below the maximum charges listed in the manuals, and you'll be worlds safer without significant sacrifice in down-range performance. No micrometer is a reliable pressure gauge.

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I just did a bunch of the same research, and came to the same conclusions, by shooting loads worked up with "home" pressure testing methods (from bolt lift to measuring case heads) in a professional pressure lab. Could find no consistent correlation between case expansion (or even bolt lift!) and pressure. An article will appear in the next HANDLOADER on the experiments--which will also corroborate Blaine's contention that a chronograph is the best indicator of pressure.

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And being immensely privileged to be privy to a lot of factual findings -- both classic and recent -- about interior ballistics, factual material that hasn't found its way into the gun magazines and isn't obvious enough that every opiner in print or cyberspace sees its validity at first glance, I feel duty-bound to share the impact of these facts even though such sharing inspires onslaughts of doubts and dissenting opinions. It's worth noting, IMO, that none of these doubts or dissenting opinions comes from a dependable, well equipped experimenter who has tested and examined the validity of case expansion with good pressure-measuring equipment.

When several careful and dependable experimenters, each well equipped and confirming the others' findings, report that miking case expansion to "read" pressure has in their experience produced two significant bodies of serious error, I take all that as observed fact, not conjecture or opinion.

Those two bodies of repeatedly observed error are --
(a) case-expansion readings that indicate dangerously high pressures, when the pressure guns indicate pressures well below maximums
and
(b) case-expansion readings that DON'T indicate excessive pressures, when the pressure guns indicate pressures at AND ABOVE proof-load levels -- DANGEROUS loads that case-expansion readings do NOT detect, for example 70,000, 75,000, even 90,000 lb/sq in. in cartridges SAAMI-rated at safe only below 60,000 lb/sq in.

So my opinion of case expansion's worth doesn't matter. The facts do matter. And I don't have to have discovered those facts myself. Also, my acceptance of these facts isn't determined or affected by the dissenting opinions of otherwise well informed and intelligent people who happen NOT to have learned these same facts.

So my opinion is that the statement "Case expansion is a dangerously misleading and undependable method of trying to 'read' peak chamber pressures" is a well and repeatedly proven fact, based on well and repeatedly proven facts (not opinions based on old, opinion-based writings). I also opine with some confidence that this opinion is sturdily supported by enough repeatedly observed facts to qualify it as a final opinion.

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