Semi-auto vs revolver vs derringer--all else equal

aaaaa

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I have been wondering what type of gun would have the most velocity out the barrel if all else is equal. We can start with guns that are chambered for 9mm to make the cartridge equal, but the problem comes in that revolver barrel length does not include the chamber, but semi-auto and derringer barrel length does. How to equalize that?

Then we want to look at the types of semi-auto. I am not that knowledgeable but seems you have blow back and delayed blow back, which could produce variation in velocity for the same cartridge. Maybe there are other semi-auto configurations too.

My feeling is the semi-auto loses some velocity from the pressure kicking the slide back and the revolver loses some velocity from the cylinder gap, so it seems the derringer is going to have the most velocity of the three.

So maybe I solved the problem and there is no more to discuss, if so, short thread. Oh, but what about the difference between revolver and semi-auto, which has more velocity? I suspect we need to have a standard cylinder gap to consider else it could vary. And then each semi-auto had different spring tension, slide weight. Maybe this is impossible to narrow down.

Ok this had been bouncing around in my head for a while. Now I have spat it out. We'll see if any interesting discussion ensues.
 
This article looks at the ‘free bore’ effect in revolvers.

‘It was apparent that these bullets achieved significant velocity before losing gas at the barrel/cylinder gap and hitting the rifling. The same bullets fired in a semiauto will encounter the resistance of the rifling before they are fully out of the cartridge case and have to make up velocity farther down the bore. It looked like we had an answer. Long throats plus short cartridges mean some revolvers achieve parity with semiauto velocities through a “freebore effect.”’

 
9mm Bond Arms 3'' derringer - 2 rounds
9mm Ruger LCRx 3'' revolver - 5 rounds
9mm Sig 365 3.1'' barrel - 11 rounds - wait a second, revolver barrel doe not include chamber, semi does.
A 9mm loaded round is at least 1'' - so a 4'' semi is a more fair comparison versus the 3'' barrel revolver
9mm Glock 19 4'' barrel - 16 rounds

For sure, the Glock 19 is going to have better velocity after the 2nd round is fired from the derringer and after the 5th from the revolver. ;) 😁
 
As in many things, there are a lot of variables; barrel length and dimensions, chamber dimensions, even the infield fly rule.
I have clocked the same load in revolvers and autos; sometimes the revolver brings bigger numbers.
A wholly sealed, fixed barrel should have an advantage, but derringers are an awful choice as a defensive arm, in the Year of our Lord 2024.
Moon
 
I have been wondering what type of gun would have the most velocity out the barrel if all else is equal.
If all else is equal then I suppose the gun with the highest average pressure throughout the bullet's travel while it's in the barrel will have the highest velocity.

We can start with guns that are chambered for 9mm to make the cartridge equal, but the problem comes in that revolver barrel length does not include the chamber, but semi-auto and derringer barrel length does. How to equalize that?
I guess a fair comparison would be to measure from the base of the seated cartridge to the muzzle exit, that seems fair to me... without getting too weird about it.

...
Then we want to look at the types of semi-auto. I am not that knowledgeable but seems you have blow back and delayed blow back, which could produce variation in velocity for the same cartridge. Maybe there are other semi-auto configurations too.
I don't think we would see any meaningful variations in velocity brought on by gun type. There's plenty of other things that can throw a fly in the velocity ointment.

...
My feeling is the semi-auto loses some velocity from the pressure kicking the slide back and the revolver loses some velocity from the cylinder gap, so it seems the derringer is going to have the most velocity of the three.
Could you explain this idea in more detail?

...
So maybe I solved the problem and there is no more to discuss, if so, short thread. Oh, but what about the difference between revolver and semi-auto, which has more velocity? I suspect we need to have a standard cylinder gap to consider else it could vary. And then each semi-auto had different spring tension, slide weight. Maybe this is impossible to narrow down.
You already narrowed it down above when you stipulated, "If all else is equal".
Once you start introducing variables and changes, it's not equal anymore. Then the answer becomes, "It depends".

Anyway, just my random thoughts on the matter. 🤔
 
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To quote the great Yogi Berra
"In theory, theory and practice are the same. In practice they ain't"
Theorizing about velocity vs barrel length is great right up until you break out the chronograph and a shorter barrel clocks faster than a longer one.
 
In semiautomatic actions, the chamber doesn't open until the bullet has exited the barrel. Doesn't matter if its blowback or locked breech. Even a "blowback" is delayed by the mass of the slide/breech and the hammer/striker/recuperator (recoil) springs until the bullet is gone and taken most of the pressure with it.
If the breech opened when the bullet was still in the gun, you'd have a catastrophic blowout. Some locked-breech semiautos have less perceived recoil because the waste energy used to operate the action is spread out over time before it is transmitted to your hand, but the total force is the same.

A semiautomatic should have no significant loss of velocity compared to a static breech gun like a derringer, for a given barrel length.
 
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In semiautomatic actions, the chamber doesn't open until the bullet has exited the barrel. Doesn't matter if its blowback or locked breech. Even a "blowback" is delayed by the mass of the slide/breech and the hammer/striker/recuperator (recoil) springs until the bullet is gone and taken most of the pressure with it.
If the breech opened when the bullet was still in the gun, you'd have a catastrophic blowout. Some semiautos have less perceived recoil because the waste energy used to operate the action is spread out over time before it is transmitted to your hand, but the total force is the same.

A semiautomatic should have no significant loss of velocity compared to a static breech gun like a derringer, for a given barrel length.
Wow! I want to see that catastrophic blowout on You Tube, :D with appropriate safety precautions of course.

So what prevents the slide moving back sooner than the bullet leaving the barrel? Is it at that point that the pressure of hot gasses inside the barrel suddenly is released out the muzzle, thereby forcing the slide back? When the bullet is still in the barrel, only the mass of the bullet accelerating works against the slide.

This also suggests to me that the cylinder gap in a revolver somewhat mitigates recoil by letting off some of the hot gasses that otherwise would be contributing to recoil.
 
"My feeling is the semi-auto loses some velocity from the pressure kicking the slide back and the revolver loses some velocity from the cylinder gap, so it seems the derringer is going to have the most velocity of the three."
Could you explain this idea in more detail?

Welp, after NIGHTLORD40K's post it looks like the semi-auto does not lose velocity from the slide kicking back since that happens after the bullet has left the barrel.
 
This article looks at the ‘free bore’ effect in revolvers.

‘It was apparent that these bullets achieved significant velocity before losing gas at the barrel/cylinder gap and hitting the rifling. The same bullets fired in a semiauto will encounter the resistance of the rifling before they are fully out of the cartridge case and have to make up velocity farther down the bore. It looked like we had an answer. Long throats plus short cartridges mean some revolvers achieve parity with semiauto velocities through a “freebore effect.”’

Fascinating article. On the topic of freebore, I assume it has to be relatively close tolerance, not like the .410 shot shell chamber in a Bond Arms derringer shooting a .45LC where the presumed freebore might be a bit to loose to produce the freebore efffect and instead destabilizes the bullet so that the little bit of rifling at the end of the barrel does not produce the nice effect but the bullet tends to tumble, or so I have read somewhere is the problem of .45 LC out a Bond .45/.410 barrel.
 
Wow! I want to see that catastrophic blowout on You Tube, :D with appropriate safety precautions of course.

So what prevents the slide moving back sooner than the bullet leaving the barrel? Is it at that point that the pressure of hot gasses inside the barrel suddenly is released out the muzzle, thereby forcing the slide back? When the bullet is still in the barrel, only the mass of the bullet accelerating works against the slide.

This also suggests to me that the cylinder gap in a revolver somewhat mitigates recoil by letting off some of the hot gasses that otherwise would be contributing to recoil.
Newtons First Law, usually expressed as an object at rest stays at rest unless acted upon governs the breech (and all other solid matter). The mass of the breech block gives it a degree of static inertia that must be overcome before it will move, and the springs provide mechanical resistance as they store energy during compression.

In a blowback, the pistols' designers calculate how much mass and spring tension is needed to keep the breech closed until pressure drops to a safe level and cycling can begin. This is one reason why cartridges meant for small blowback pistols must be kept within a narrow range of bullet weights and charges or you throw off the math and blowbacks are seldom chambered for powerful cartridges because the reciprocating mass becomes too heavy.
The designers must also account for mechanical delay from friction of the case swell in the chamber before pressure drops. Plated, coated, lubricated, or cases made from different materials can change the equation somewhat.

Thus, despite their simplicity and few moving parts, designing a blowback that is reliable with a range of ammunition is actually quite difficult and requires precise calculations. Its also why they tend to be more ammo sensitive and have more perceived recoil than locked breech designs.
 
9mm Sig 365 3.1'' barrel - 11 rounds - wait a second, revolver barrel doe not include chamber, semi does.
A 9mm loaded round is at least 1'' - so a 4'' semi is a more fair comparison versus the 3'' barrel revolver
This. Revolver barrels are measured from forcing cone to muzzle. Auto loaders (and derringers) are measured from the rear of the chamber to the muzzle. So revolvers have a full chamber of advantage in effective barrel length over the others from the start.
 
So what prevents the slide moving back sooner than the bullet leaving the barrel?
The slide does move before the bullet leaves the barrel, it's just a short distance because of the mass difference between the slide and bullet and isn't even close to unlocking a locked breach or moving a straight blowback enough to pull the thick web of the case out of the support of the chamber.
As I said theory is one thing but you're talking about the attempt to accurately measure in .01 fps when the equipment isn't capable of that accuracy and the round to round consistency is considered great when it's less than 10.
 
The slide does move before the bullet leaves the barrel, it's just a short distance because of the mass difference between the slide and bullet and isn't even close to unlocking a locked breach or moving a straight blowback enough to pull the thick web of the case out of the support of the chamber.

Yep. The slide DOES move as soon as the bullet leaves the casing and begins traveling down the barrel. By the time it leaves the barrel of a typical handgun, the slide will only have moved back by a couple millimeters.

 
There is a theoretically tiny loss of velocity imparted on the projectile as the mass of the reciprocating slide alone has a lower ratio to the bullet mass than say, the entire mass of a revolver shooting the same cartridge. This effect is almost certainly lesser than the barrel/cylinder gap losses of a revolver. Not enough for any practical concern.

You would get a similar theoretical difference in imparted muzzle velocity between a 5" barrel J-frame and say, a 5" N-frame revolver, both shooting identical .357 mag cartridges. The larger mass of the N-frame would theoretically bump up the muzzle velocity by maybe 1 fps or less, all else being absolutely identical.
 
a5,

I think you are looking at this from a completely wrong perspective. What do you want to do with this gun. Derringers are poor choices for almost anything. They usually have a poor grip shape, have only two shots which may shoot to different point of aim, are very slow to reload (no speedloaders and very few are double action, so shooting them can be complicated.

Forget about velocity. If ;you actually measure it, the difference between a compact pistol with a similar length barrel will be small, if any. Most small 9m.m. pistols are recoil operated, so gas loss is probably nill.

Also, remember that the NRA stopped advertising for derringers because they had so many documented accidental discharges..

I have a 9m.m. revolver, a S&W 547 and really do not think it has any advantage over a.38 Special/.357 magnum revolver. I have also owned a pair of 10m.m. and another pair of .45ACP revolvers. I only kept the 625 because it works well with .45 Auto Rim.

Jim
 
How does the mass of the frame effect velocity? I think you mean perceived recoil force?
If both revolvers were immobile, clamped into a vice, velocity would be identical.

Now, if the vice were mounted on electromagnetic rails and free to move, the lighter revolver would yield less velocity because it had less static inertia to overcome as the gun flew backwards, in effect becoming a blowback slide, lol.

Another fun way to think of it is that the cartridge case is a really a pneumatic piston...😁
 
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I always get a kick out of these slo-mo pics. They're fun to watch.
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No 2 barrels will give the same velocity even if they are the same length and from the same type gun from the same manufacturer. Most of the time you're looking at 20-50 fps, but there are times when 100 fps or more could be observed. It's not unusual to see the shorter barrel shoot faster than another nearly identical, but longer barrel. That is a greater difference than the difference between revolvers and semi's.

Note that the 4" Smith was faster than the 6" Colt or the 5" or 5.6" Smiths with most ammo


Some data here comparing cylinder gaps in revolvers vs no gap with the same loads.


Polygonal rifling seems to add a bit of speed too.

 
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