Question about pressure.

[Mr_Flintstone]

Is there any way to figure (or estimate) the pressure on a load without fancy electronics? I need to check a load to make sure it's safe in a 2" 38 Special. I got the load data from a Load Development publication from loaddata.com for 200 gr LRN. If anyone knows how, or has software to check, here are the specs:

Cartridge=.38 Special
Case Vol=23.7 gr
Case length=1.150
Bullet weight=190 gr LFP
Bullet length=.750"
Seating depth=.450 (base to middle of cannelure)
COL=1.45"
Load=3.0 gr Titegroup
Crimp=medium roll crimp.

I test fired one from a .38 Spl +P pistol, and it felt like a +P round, although it's supposed to be safe for standard .38 Special. Any help is greatly appreciated.

 

[DWalt]

Yes and no. There is an internal ballistics program called "Quickload" that can be used to estimate peak chamber pressure (and also MV) for any load you input to it, but it's a bit pricy. There are also some strain gauge-based pressure measurement electronics that are touted to work fairly well, but those are not cheap either. And probably best confined to use on rifles, as it depends on "gluing" strain gauges onto the barrel. And I guarantee you aren't likely to have the funding to afford the piezo gauge pressure measurement equipment that the ammunition makers have and use.

I ran your data on Quickload, but I had to use a 180 grain Sierra jacketed bullet (it does not have a 190 grain .357 bullet in its database). It gave a peak chamber pressure of about 13000 psi, which is within SAAMI limits, MV shows to be 717 ft/sec from a 6" barrel. Obviously, I cannot vouch for the accuracy of Quickload. I have found that in many cases, Quickload is probably on the optimistic side of reality.

 

[DwayneV]

There is no way to measure pressure without strain gauges or receiver ported for crushing copper cylinders.

 

[Mr_Flintstone]

There doesn't seem to be any signs of over pressure; but I doubt I'd see any on 38 Special unless I got to .357 magnum levels. You guys probably have a lot more experience than me. Does this seem to like it would likely be +P level or lower? The article I read said it would push a 200 grain bullet 627 fps from a 2" barrel, and 710 fps from a 3" barrel; but the seat depth was .413. The author said it was tested in a 2-inch Colt Detective Special and a 3-inch S&W model 36.

 

[DWalt]

Did you bother to read my Quickload response?

 

[Mr_Flintstone]

Did you bother to read my Quickload response?

Sorry, I see it now. Ive been having problems with my iPad not loading all the replies. I just saw the notice at the top of the page explaining why.

Anyway, thanks for the quick reply; sorry I didn't see it sooner. With the pressure that low on 180 gr jacketed, then 190 gr lead should be good too.

 

[rockquarry]

Bullseye and Titegroup are quite close in burning rates. A Lyman cast bullet book from the mid-'70s shows a maximum charge of 3.3 grs. Bullseye with the Lyman 195 gr. LRN, and a CUP of 15,400, muzzle velocity = 770, but that's from about an 8" pressure barrel. That's probably at least as accurate as QuickLoad data would be. Now, if Lyman were to run their pressure test today, perhaps with different, maybe more accurate equipment, powder charge weight might change and so may pressure.

I've used Titegroup and Bullseye with cast bullets and would GUESS your load is likely a very safe one if used in a gun in good, sound condition.

However, it's important to remember this is the Internet, sometimes the very worst source for load data. I would dig until I found reliable load data from a reputable source, preferably on paper, despite the fact that a lot of people nowadays don't like to use paper data.

 

[DWalt]

"and a CUP of 15,400, muzzle velocity = 770, but that's from about an 8" pressure barrel. That's probably at least as accurate as QuickLoad data would be."

Except CUP is not comparable to newer piezo gauge measurements for determining peak chamber pressure. I doubt any ammunition manufacturer in the civilized world has used the CUP crusher measurement method for at least the last 30 years. When I worked for Hercules, we were using piezo pressure gauges as far back as 1969.

 

[rockquarry]

That's straight out of the book and I make no claim to be a ballistics expert.

 

[smithrjd]

Quick load gets close, but is not always on point. A Chronograph is the best way with a pistol, other than not trying to get a max load. If it is fast then you are getting way up there. A rifle one can see a extractor mark, hard bolt lift etc. Better to stay within published loads for a given platform and barrel length.

 

[DWalt]

That's straight out of the book and I make no claim to be a ballistics expert.

CUP is only a crude analog approximation of average chamber pressure. Sort of like trying to use an hourglass to accurately measure time. But back in the pre-electronic days it was the best method available. Depending upon a number of factors, CUP results typically understate true peak chamber pressure by at least 15%, meaning that a CUP of 15,000 represents a true peak chamber pressure of at least 17,500 psi or possibly more. To complicate matters further, there are at least three different chamber pressure measurement methods used with Piezo gauges, and they all produce slightly different results, mainly because measurements are taken at different locations. I won't get into that, but any of them are far superior to CUP. Quickload calculates peak chamber pressures in PSI, not CUP.

While there is no good way to use it for measuring chamber pressure, the fact is that MV itself has a fairly linear relationship with peak chamber pressure for the same bullet. So if you make up a handload which produces an MV in your gun close to that of a factory load having the same bullet type and weight, you can be reasonably certain that the handload is safe, regardless of what powder you use.