Low Pressure vs. High Pressure Cartridges

[keys85]

I know the 38 Special is low pressure, and the .357 sig is high pressure. Some loading manuals/cartridge descriptions point this out and make mention to or cautions (when necessary) about such. But...I've never read any literature on what exactly this means in terms of design and performance, other than that you're more likely to wear out your gun and/or have a more likelihood of a pressure related problem with a high pressure round. What are the pros/cons of each, and why is it that some low pressure rounds like the .45acp can achieve nearly identical ballistics to the high pressure .40 SW? Is case volume in anyway related to pressure and performance?

[WESHOOT2]

Case volume can relate to pressure.
It can relate to performance.

I am not clear how the 40 S&W and 45 ACP "achieve nearly identical ballistics".

What, more specifically, are you trying to learn?
(Do you have any reloading manuals?)

[jen_lynn24]

Hi Everyone. I Have Rifles And Extras To Sell, How Do I Post TheM? Jen

[Sevens]

Jen, you are at the right site, but you are in the wrong area of the site. Use drop-down menus or click from the home page to find the buy/sell areas of the site.

Keys85-- high vs. low pressure rounds. The reason we have both is mostly due to the long history of the caliber. With the old, low pressure ones (.38 Special and .45 auto are perfect examples), they were designed in a day where smokeless powder wasn't as efficient as it is these days, and in an age when metallurgy wasn't as advanced as it is now and firearms couldn't take the same level of abuse they take safely today.

The 9mm breaks those rules a bit... nearly as old as the .38 Special and running about twice the pressure.

Pros to high pressure rounds: You get more power in a smaller size which typically translates to higher capacity and smaller overall handgun size. You get high velocity which gives you more bullet energy than low velocity. You end up with a flatter shooting round which effectively gives you a longer viable range compared to a bullet moving more slowly. And no, you don't get a shorter working life from the firearm since the firearm is modern with current metal technology.

Cons to high pressure rounds: You can't safely shoot them in older firearms, or you can shorten the life of that firearm. Typically, there simply aren't older guns built for the newer, high pressure rounds. Also, you need to be much more aware at the reloading bench because you have much less room for error before things get very unsafe. This is with regard both to powder levels and to bullet seating depth/COAL which does much more to affect internal working pressures.

The .38 Special will never be a high pressure round because of the existence of many old firearms. SAAMI doesn't want some new shooter to buy a new box of ultra-hot .38 Specials and blow up his Grandpa's old S&W top break.

Look at the .357 and compare it to a .38 Special. It's a bit longer. That added length is not what makes it a hotter round. That added length keeps people from stuffing them in to older guns chambered for .38 Spl. The .357 is hotter because it's a higher pressure round. The slight added capacity doesn't hurt it, but it's not the reason it's hot. It's hot because it works at a much higher pressure. It's longer to keep people from blowing up old firearms at twice or more the working pressure.

As for the assertion that some low pressure rounds can achieve identical ballistics to high pressure rounds? I think you are using the wrong words here to say what you mean. There's pretty much no way that can happen. You can see similar energy figures, because energy is simply a mathematical product of bullet speed and bullet mass. But to be ballistically identical, you'd have to take a projectile of a very similar weight and diameter and send it at a very similar velocity and I think you'll have a tough time doing that at radically different working pressures.

[Shoney]

Your question How does pressure relate to volume in ammunition?

pV=nRT

where

p is the absolute pressure [Pa],
V is the volume [m3] of the vessel containing moles of gas, here it is the volume you allow in the shell case
n is the amount of substance of gas [mol], here it is in solid form as powder
R is the gas constant [8.314 472 m3·Pa·K−1·mol−1],
T is the temperature in kelvin [K].
BORED YET?

Think of it as:
Pressure in the case X Volume of case = 2 Irrelevant numbers RT (for our purposes) X the amount of gas n as being in solid form as powder until the "big bang";
1. the gas constant R remains the same;
2. the temperature T remains constant;
For practical purposes the equation should read PV=n

Now lets do some math. Remember, the two sides of the equation must balance. PV=n

Presume you are shooting your pet load
A. Increased powder = Increased Pressure Increased powder loads produce more gas n, and volume V inside the case remains the same. That means the Pressure must increase to match the increase in gas.
B. Decreased Volume = Increased Pressure Back to the pet load, Now decrease the volume V (by seating the bullet deeper) and keep the gas n the same, in order for the equation to balance, the pressure P must go up.

Bottom line on loading all cartridges at or near max pressure.
1. You must be careful not to exceed max pressures, because this tends to rupture cases, damaging weapons and body parts. Even if the cases do not rupture, over pressure loads are abusive to equipment and fatigue metal into failure. Make sure you’re following recipes closely.
2. It has been shown that when max loads are exceeded, exceptionally high pressure spikes can occur. It has been suggested by some that high pressure rounds are more susceptible to these pressure spies.
3. Semi-auto bullets can easily set back into cases if care is not taken during loading. Crimp/neck tension is important to prevent set backs, because as the equations show, decreased case volumes increase pressure and it can be to a catastrophic level. Wheel guns have problems with bullets moving forward.
Good Shooting!

[keys85]

So, then would it be logical to say when working up a load that the minimum COL of a [pistol] round that reliably feeds in your gun would be most efficient?

vs. a round seated longer, with a larger volume, thus, the need for more powder to achieve the same ballistics?

Kind of like engines - only thinking of horsepower like energy and disregarding the concept of torque. Ford produced a 400 Cu Inch engine in '77 that made a whopping 210 or so horsepower. Got about 14 miles per gallon tops. Then in 1990, cadillac's last year for their 273 cubic inch engine, with 180 horsepower and much higher efficiency. The compression ratios were different, meaning cylinder pressures were different. The cadillac had high compression with smaller cylinder volume, Ford was low compression with a lot of volume. But in the end, the Ford made nearly as much power as the Cadillac but used a heck more fuel to do it.

What I'm getting at is say you wanted a .45 ACP to fly at 830 ft/sec. You can load up some rounds at 1.270" and others at 1.190". Both feed reliable in YOUR gun. Wouldnt it be logical to say that the 1.270" loading would require MORE powder?

[WESHOOT2]

....it may not withstand the scrutiny of actual testing.

How many (others) have had a projectile go slower after adding more powder?
Or loading shorter?


"Test then know; unless you test you guess"

[Sevens]

I agree-- it might be logical to say that, but it may not be accurate.

Accurate pressure testing requires a lot of high dollar precision equipment. There's older, less accurate methods which involve measuring the expansion of the brass, but to really know what is going on, you'd need tools and equipment beyond the scope of our benches.

From our bench, the best we can typically do is to use MANY sources for our data, start low and increase gradually. A chrono can help, but it won't give all the answers.