How to read Primer’s

[dgeesaman]

This is exactly the method I use. At the time, buying a blade mic seemed expensive, but compared to some of the tools I see people popping for today, it's a trivial amount of money compared to the info it gives you.

Why a blade mic? The case body is generally round x tapered so no matter what geometry your mic is, it contacts at two points.

David

 

[jelenko]

I get that the various pressure signs can have causes other than just pressure.
But, to confirm, if they start to show as the powder charge is increased, wouldn't they be for sure signs of pressure? E.g., if there were no pressure signs at a low charge and they started appearing at some point as the powder charge was increased, you'd have high confidence that the signs were, in fact, indicating too high pressure, yes?
[Recognizing that typically soft brass - e.g., Remington - would show the sign too early]

 

[mikecr]

Pressure signs and pressure problems are different things.
A pressure sign is something looked for that indicates xxKpsi of pressure.
A pressure problem is a build (gun or ammo) issue showing up with increasing pressure.
Both or either can present at safe pressures.

You can get a pressure sign from a strain gauge, calibrated to the barrel, and validated with a chronograph.
You can validate from fired brass, with all known and understood about that brass and chamber.

Problems are just that. Weak chain links you find as you work up a challenge to them.
If you removed all problems, then it is plausible to continue upward in pressure. I do -for testing.
But of course you don't want to settle so high that a simple change could put you at risk of big failure.

It seems that with a good gun build, and hard brass, I can run up to SAAMI max pressures, safely, and get a good number of, or potentially infinite, reloads out of brass.
The hard part today is finding hard brass..
Does anyone offer a hard brass alloy today?

 

[Orr89rocz]

Are there any labs that do testing on loads you can send in? I understand variances in chambers and barrels will show different pressures but atleast it would be a good starting point

 

[big5ifty]

Why a blade mic? The case body is generally round x tapered so no matter what geometry your mic is, it contacts at two points.

The type depends on what brass you measuring.

Some brass has a slight taper. A normal anvil works to measure close to the extractor groove.

If your brass is a straight taper, a normal anvil may contact a bit too far away from the extractor groove.

For a belt, you want to measure the bottom of the belt next to the groove, which a normal anvil won't be able to.

 

[dgeesaman]

The type depends on what brass you measuring.

Some brass has a slight taper. A normal anvil works to measure close to the extractor groove.

If your brass is a straight taper, a normal anvil may contact a bit too far away from the extractor groove.

For a belt, you want to measure the bottom of the belt next to the groove, which a normal anvil won't be able to.

I will argue that the majority of us load tapered cases (at least in such a way where we approach pressure limits) and have sufficiently accurate micrometers for the task. The “requirement” of blade type mics caused me to write this off.

 

[Doom]

Primers showing pressure signs, like sometimes cratering and flattening, are signs that the primer is seeing excessive pressure. That may indicate the load is over SAAMI pressure. Or it could mean that the wrong primer is being used. Also creating isn't necessarily an indication since clearance between the bolt and firing pin can show cratering.

 

[Phys86]

@414gates and @dgeesaman , what do you mean by tapered cases and load tapered cases?

I've got a cartridge where my virgin brass is a straight taper from edge of extractor groove to shoulder. After fire forming, the web area just ahead of the groove edge is 0.002 larger and then it straight tapers from there to shoulder. Example, after fire forming, the edge of extractor groove on Peterson 284win brass is 0.499 and at roughly 0.250" from the base of the head it's 0.501 then straight taper to shoulder.

Is this what you're referring to?

 

[jelenko]

Some brass has a slight taper. A normal anvil works to measure close to the extractor groove.

Phew. That saved me $300.

 

[mikecr]

@414gates and @dgeesaman
I've got a cartridge where my virgin brass is a straight taper from edge of extractor groove to shoulder. After fire forming, the web area just ahead of the groove edge is 0.002 larger and then it straight tapers from there to shoulder. Example, after fire forming, the edge of extractor groove on Peterson 284win brass is 0.499 and at roughly 0.250" from the base of the head it's 0.501 then straight taper to shoulder.

I don't mean to interrupt, but you're describing what I referred to as the web-line, for MyMax testing.
Basic calipers work just fine for that.

To measure the case head for change, and hold a meaning, you would need a blade mic and jig to set a datum,, and you would have to know the brass alloy and adapt a calculated CUP process in measure.
That's trying to directly estimate actual pressure.
What good that does us, I don't know. I personally don't care what the pressure is, and only want to know what's safe and problem free in MY gun.

 

[big5ifty]

@414gates and @dgeesaman , what do you mean by tapered cases and load tapered cases?

I've got a cartridge where my virgin brass is a straight taper from edge of extractor groove to shoulder. After fire forming, the web area just ahead of the groove edge is 0.002 larger and then it straight tapers from there to shoulder. Example, after fire forming, the edge of extractor groove on Peterson 284win brass is 0.499 and at roughly 0.250" from the base of the head it's 0.501 then straight taper to shoulder.

Is this what you're referring to?

For example : RUM brass is slightly rebated, so a flat anvil works.

The anvil type you need depends on your brass.

 

[mikecr]

So if you get a mic reading on fired brass of 0.55000", what pressure is that?
How would you calculate it?

For illustration, this is the web-Line:

 

[Bronsin]

Why a blade mic? The case body is generally round x tapered so no matter what geometry your mic is, it contacts at two points.

David

With a blade mic, you can measure the extractor groove and get a measurement that no other mic can reach, but I don't recommend it. The brass is much thinner there than the web, and will show more expansion for the same charge. Instead, I just move up to the web area and measure there. Sometimes that web area can be really narrow on small cases, and a blade mic set right next to the groove will easily measure it. The rim can be larger than the web on some cases, so a regular mic won't work there.

Measuring expansion doesn't tell you how much pressure you have. It just tells you when the brass you are using has been pushed past it's limit. Make a ladder in .1gr increments and measure the results. That's how you determine what the limits are for your brass in your rifle.

As an example, I recently tested LT-32 in my 6PPC. A powder I had never used in anything, with a new to me cartridge. I started at 28.0grs and loaded all the way up to 29.5grs. I figured I would get a heavy bolt lift or a click at the top of the bolt lift to signal when loads were getting too high. Neither of those happened. The first clue I had that I was over pressure was when the last case left the primer in the bolt raceway.

Starting at 28.3 and all the way up to 28.9, the expansion was only .0002, from 29.0 to 29.3, it only gained .0002 more, then 29.4 added another .0002. So from 28.3 to 29.3, the cases had only expanded .0006. Next up was 29.5, and that .1gr alone expanded the case .0007, knocking the primer clean out. I settled on 29.1grs as 29.3 can loosen the pockets even though it still shoots great. No reason to beat up the brass for that tiny gain in FPS.

Now, I have an expansion measurement that I can use for any other powder in this gun. So far N133 hasn't gotten close, and I haven't tried H322 yet, probably won't either. Some brass won't take much pressure either. I used Keith Glasscock's method of fireforming on these cases and couldn't be happier with it. My pockets are all still tight, and even after 6 firings the pocket uniformer doesn't do anything other than remove carbon. Blade mics aren't cheap, but they are the only way to know how much your brass will take. They're a damn sight cheaper than an induction annealer, labradar, or primer seater at 600 bucks.

 

[mikecr]

That makes sense Bronsin.
With MyMax as described I see growth that plateaus, and then a step change upward (way higher growth rate). We're looking for the same thing on different scales.

 

[big5ifty]

So if you get a mic reading on fired brass of 0.55000", what pressure is that?
How would you calculate it?

I measure brass case expansion between virgin and fired brass around the primer pocket, not pressure.

Expansion is proportional to pressure.

You measure virgin brass first, then the fired load.

There are various numbers out there that you can use as a guideline if provided by a major manufacturer. For example, they may say 6/10,000 in a .223 Rem is indicative of max pressure.

The larger the cartridge, the higher that number can be.