Measuring Pressure From Chronograph

[pdoggeth]

Hi folks.

So far from what I've read, there's too many variables and complications to extrapolate pressure from just the velocity and other measurements. But that's ok, I don't need the exact pressure, I just would like a ball park figure.

So, anyone have a formula to get "in-the-ballpark" pressure from velocity and bullet & powder measurements (hopefully simple -- been years since I've done physics) ?

If no formula exists, then what use is a chronograph for a reloader then? Just to make sure your loads are "consistent" based on the velocity?

[JMP]

No, a chronograph will not provide this. A chronograph measures velocity only. Please note that you can get the same velocity with two very different pressures. The pressure is not a nice linear system as it builds in your chamber. Rather, it's a hyperjerk system that makes it nearly impossible to model with anything such as you mentioned. I'd love to hear if anyone knows different.

For now, it isn't perfect, but the best way is to use QuickLoad. This uses numerical methods based on a lot of testing. To my knowledge, it will be the closest you can get.

[pdoggeth]

Yeah I was afraid of that. There is some mystery powder that I was thinking of trying out (SB 620), but since it has almost no data and reloading information, I'm a little hesitant. The main concern I have was over pressure, and I didn't want to rely on just the primer and case bulging. Was hoping a chronograph could help me out. However, if there's no way to extrapolate pressure from a chronny...

Looks like I better play it safe then. I'll pass on the powder for now =).

[ExtremeX]

then what use is a chronograph for a reloader then? Just to make sure your loads are "consistent" based on the velocity?

A chrono is a pretty useful tool for a reloader... to be honest I went without one for a very long time but after getting one it opens doors to data/information I didn't have in the past.

It only measures the velocity of the projectile, but from that you can compile other data like standard deviation, extreme spread, average velocity.

From that you can analyze ammo that you load to see if its a viable candidate for long range shooting and give you a head start when developing ballistic data / drop tables. For example you may opt for a load with a lower velocity because of a tighter SD/ED than a faster load with a higher SD/ES to improve on vertical stringing at distance.

If you are making defense or hunting ammo, you can verify you are getting the velocity you need for good terminal ballistics.

[pacrat]

Looks like I better play it safe then. I'll pass on the powder for now =).

Good decision, load safe, shoot safe, have fun.

Blow up gun, not safe, not fun.

JM2c

[ar15barrels]

Quickload works IF (there are a LOT of IF's here...)

IF you have the powder, cartridge and load modeled correctly in Quickload AND the quickload results are matching MULTIPLE different loads of that powder when measured on the chronograph.

When you reach a point where quickload is accurately predicting what your chronograph is giving, then you can run the pressures up to your desired max pressure in quickload and see what velocity it says you would be getting.
After you have that, you can work up the load to that velocity and see if the charge is matching quickload.
IF you get multiple simulations matching multiple actual chrono sessions, the chance is pretty good that your actual pressures are very close to your Quickload predicted (modeled) pressures.

[phish]

There is a commercial system sold here: https://www.shootingsoftware.com/pressure.htm.

It'll run you about $600, but it is the only off the shelf tool that I know of that will measure chamber pressure.

If you're a DIY type and good with electronics, there are the cheap micro controller kits that can be used along with the associated hardware to go with it. Vaughn made such a system that he wrote about in chapter 1 of his book.

[LynnJr]

If you want to spend the money you can buy a used Oehler 43 with the strain Gauge that attaches to your action.
You will learn more about reloading with a 43 than you ever thought possible but they run $1000 - $4000 used.

It might be easier to just find out what SB 620 is/was used for and go from there.

[Eljay]

Looks like I better play it safe then. I'll pass on the powder for now =).

Yeah, I wouldn't go there. A chrono is mostly useful for sanity checking your load against known data and making sure you got the result you expected, not for going out into unexplored horizons.

[rsrocket1]

Google is your friend

It looks like it is close to W231 which is a very popular powder. If you are new to reloading, $100/5# is tempting, but you should pass. If offered for considerably less, I'd personally buy it and work up loads for it. You could write the manufacturer/distributor for any sample load data.

But if you are uncomfortable with it, just pass. Think about it, you could get "gouged" by paying $30/pound for run of the mill $20 powder, but that would load near 1200 pistol rounds. Not worth the risk.

[mark501w]

I used it to determine if high pressure indications produced high velocity, it didn't.

[pdoggeth]

Coming back to this whole situation, I realize I have one pound of HP-38 unopened. That should be the same as Win 231, which is what SB620 "supposedly" is based on. Let me bring up the following hypothetical:

I load up some test rounds (let's just say 9mm 115gr xtreme RN copper plated) with HP-38 and find an ideal charge based on known data that is safe (Lyman's). I then go and measure the velocity of these rounds. Let's just assume that the chronny readings make sense and match up to what is given from the known load data.

Now let's say I get the mystery powder SB620, work up safely to the same charge and the same projectiles (as well as same OAL, primers, etc), and then measure those rounds through a chronny. If the velocity turns out to be about the same as my known load setup, would it be safe to assume that the two powders are pretty much identical?

[CSACANNONEER]

Now let's say I get the mystery powder SB620, work up safely to the same charge and the same projectiles (as well as same OAL, primers, etc), and then measure those rounds through a chronny. If the velocity turns out to be about the same as my known load setup, would it be safe to assume that the two powders are pretty much identical?

No, not unless you do the same test through a large variety of barrels with different lengths.

[pdoggeth]

No, not unless you do the same test through a large variety of barrels with different lengths.

Drats =/ I'm going to assume 3, 3.5, and 4" barrels isn't enough variation.

[ExtremeX]

Now let's say I get the mystery powder SB620, work up safely to the same charge and the same projectiles (as well as same OAL, primers, etc), and then measure those rounds through a chronny. If the velocity turns out to be about the same as my known load setup, would it be safe to assume that the two powders are pretty much identical?