How to measure chamber pressure?

[Walt Krafft]

I got mine for playing with the Sig hybrid case. The case is no longer the failure point. All the things we have been taught to measure pressure do not exist with the steel headed case.

 

[fscrook]

I have a ptII.

If the machine said it was a good load, then downrange results were also good. If the machine said it wasn't a good load, the load may still perform well downrange : Sometimes you'll test different primers, or annealing , seating depths, tension, and find no discernible difference in chronograph or downrange results. Maybe both ES/SD and group size were excellent despite changing load characteristics. So how do you pick ? As a result , the ptII makes sense. But it also makes sense to just accept whatever and move on.

i do not believe using a ptII will give you a true measurement of psi : If your testing some exotic clambering you'll have no baseline for comparison. And I had problems with 223. Not a single 223 ammunition type i tested came anywhere close to the published psi. There were almost always 10,000-8000 under. I tried different ARs, numerous types of ammo, strain gage placement, and tweaking input data. I spoke to jim on several occasions about this, he was full of information and a great guy to talk to, but i never walked away with a definitive answer for why the numbers didnt add up. I dont own a 223 bolt, so i never tried one of those. I imagine some people probably have had success with 223 and getting something close to SAAMI rating, but i never did. Dispite not having the same issue with other cartridges that gave reasonably close enough comparison results, i threw the baby out with the bathwater in trusting the ptII for a true psi measurement.

But I don't think reading the case heads for signs of pressure is really the best way to go : If the brass is soft , or there are big gaps in the firing pin or extractor/ejector, you'll see flow marks that don't truly indicate excessive pressure. If you bush the pin you won't see flow marks at all. So, damned if you do and damned if you dont.

 

[DumpHollow]

So, I have to ask, what’s a “ptll”?

 

[gunsandgunsmithing]

So, I have to ask, what’s a “ptll”?

Pressure Trace II

 

[209jones]

There is a pretty good thread about using it on Accurate Reloading from years back, think it was when they were testing the stuff like 450B&M etc, and Cutting Edge and North Fork bullets (Michael?), they tested quite a bit of different stuff and bullets and a few different guns and talk about the foibles of the pressure trace system they used. It may still be a kept up thread, haven't looked for it in a long time, think it was in big bore section. There is also a thread in there about rifling transfer ghosting to exterior of barrel and solid copper bullets in a double rifle, fella named Sam, who I think worked with Michael from B&M, turned some barrels down to ridiculous wall thicknesses and shot them to try to prove or disprove it. Most of that stuff spanned about 10-15 yrs ago.

 

[Webster]

What Walt said. It'll get you in the ballpark if it's a standard chamber that you can calibrate to factory ammo, but it's not intended to give absolute pressure values.

Someone on this website has a good article on how he measures pressure with stress gauges. See if you can search and find the post. The strain gauge method requires a very expensive high speed oscilliscope. I'll see if I can find the post.

 

[Webster]

Hi All,

I understand there are multiple ways to check for signs of pressure after firing a round, but I was wondering if there is some kind of a tool which can help measure actual pressure inside the chamber when firing?

Any feedback, inputs will be helpful

thanks
harjeet

Post by CATSHOOTER 23 Aug 2013 reply #6

The above fragmanets are the sum of it.

Even the industry (Olin-Winchester, et al) cannot measure pressure accurately.

We can come close, but the differences in steel, the the differences in gauges, plus the effect of hysteresis (strain tapes over steel are bad, but copper plugs were worse) means that we cannot get accurate measurements... we can only come close - maybe +/- 10%.

Some years ago, SAAMI made up a batch of test loads - all the same lot of brass, same lot of primers, lot of powder, etc... and sent some to every manufacturer and test lab in the country, for a pressure test of the cartridges. The returned answers ran from ~42,000 Kpsia, to ~61,000 Kpsia (or thereabouts)

But is it really necessary for any reason, other to satisfy the curiosity of the anal compulsive, and the Mensa intellectual, that we have pressure measurements, other than relative/comparative?

What difference does it make if our loads are actually 48,000 Kpsia, or 53,000 Kpsia, or 63,000 Kpsia? None to us. Many benchrest and match (and varmint) shooters are running loads well up into the mid 60's, and the only effect is... well, not much at all!

The only effect of pressure (assuming decent design mechanical), is the level of pressure that our system begins to fail, that that "point of failure" is the case.

But the case is not uniform, so the failure point varies between lots, and more so, between makers.

So it makes absolute sense to use the signs of the failure point, to be our guidance in determining pressure limits, and not an abstract number, who's lineage and parentage, is more than somewhat suspect.

ADDED LATER:
Some old military rifles had cast metal receivers. They are much more likley to fail than forged steel. A friend had a Mosant rifle receiver crack while we were at the range. He was lucky no gas escaped. Just a large crack no pieces airborne.

 

[jelenko]

Interesting.
In all this time, the OP hasn't said peep since his original post.

 

[Webster]

Interesting.
In all this time, the OP hasn't said peep since his original post.

I used to like reading his post. I think it's been at least 5 years since he was on the website.

 

[XTR]

An example that might be applicable for a loader today. Shooting F-TR we’ve all learned in the last 9 yrs that Varget/VV150 has proven loads at about 2650 fps with 200 gr Bergers seated long in custom chambers. We pretty much all have a load there. The pressure is reasonable in small primer brass. We also know that somewhere just above 2700 fps (where the next node sits) brass life begins to suffer

Enter the 208 Hybrids. How hard can you push them? Toss in a new powder option (say Re15.5) now what is reasonable?

Load up your known 200-20x load in a barrel with a pressure trace and shoot a string of 10.

Now use the same barrel with a series loaded with Re15.5 and see where the pressure crosses. You can reliably find your equivalent point in around 10 shots. Now you know where your pressure is compared to a known, use it and go do load development.

use the same process for a new bullet like the 208, or if you’re playing with 215s in a chamber with a 225 freebore.

I get useful data with a minimum of components and trips to the range. A couple of the folks with whom I’ve shared my data have reported quickly getting good results.

As for the 223. Russ Theurer told me yrs ago he did a lotof testing with the 223 and the case was just too short to get the sensor in a reliable location. If I were trying I’d probably look out an inch or two past the chamber where The thickness is consistent. Again, not a chamber pressure but looking for a reliable reference. I haven’t tried it, but that’s where I’d start.

 

[ebb]

The OP hasn't been seen in 11 months, I suspect he was looking for permission from a machine to shoot a load he already knew was too hot.
With all the physical evidence, book data, and quick load all the other shooters on the net talking about what their max is it seems to me it would be obvious. But many have a deep seeded need to go faster than would seem prudent. I am not sure if I understand this need but I think most know that their goal is too hot and unsafe yet they want to go there anyway.