The Comp Works By.......

[old shooter]

While at a match yesterday, I was quizzing those around me about the powders they use and posed a question to a "seasoned" shooter about he powder he used to make major (we were at a minor steel match). We discussed loads and I stated that I was using a powder that needed 7 gr. and he used one much less. I said that I was trying to use a load that needed as much powder (as practical) to reach a specified speed reasoning that if I burned 7 gr, it might give me more gas to work the comp vs 5 grains. Anyway, for now, the 7 vs 5 is not the question.

This person stated that it was the volume of air in the bbl ahead of the bullet that caused the comp to work. Call me stupid, but I always thought that the expanding gas from burned propellent was what operated everything.

Have you heard of this, and am I indeed stupid?

Mike

[tightloop]

You are not stupid, the guy who answered as he did didn't know anything about physics..

[shred]

Yeah, while there may be some tiny effect from air in front of the bullet, it's the massive quantities of burned-powder-gas squished into a small space behind the bullet that do the real work.

[L9X25]

What Shred and TL said.

I will add that different people seek a different "feel" for the gun and the slower powder does not always achieve the best "feel" for everybody. As you go slower you have a larger volume of gas but it may be at a lower pressure too. Going with a faster powder can raise the peak pressure and make the gun feel quicker and more responsive, particularly with barrel ports in addition to a comp.

Leo

[old shooter]

What I was trying to do with this load was get the best response with a minor steel load. I know things don't heat up as much with minor, I was just trying to get the most mileage with a reduced load.

Mike

[Mad Scientist]

most guys use quicker powders with minor loads . try using lighter bullets as they will require more powder to operate at a given power factor.

I had a conversation with Max jr and he likes the lightest gun posible for steel

at around 120 power factor. He hinted at very light bullets 115 and less with quick powders.

by the way gas presure and volume are the key to comp effectivness.

Johnnie

[shred]

I like soft loads usng fast powder (currently toying with N320) for steel shooting-- Some people like more powder to 'work the comp', but a fair chunk of recoil comes from accelerating the powder gas up to escape velocity, and without a need for a speedy follow-up shot, a little bounce works best for me.

[Skywalker]

old shooter,

if your friend is so confident about what he said, ask him to put his weak hand over the compensator while firing: a blow from the un-heated air in front of the bullet won't harm him...

Joking, of course!

[schmitz]

On this moment, I am busy with the development of Bianchi-loads, PF 140, using Vihta N350 behind a 115 grain Fiocchi-bullet.

A slow burning powder witch enables my comp to work properly and gives a mild recoil (adiabetic expansion) but still produces the power to knock down those plates.

I am now at a 6.3 grains powderload at a seatherdepht of 31,8mm (=1.25") and it works well!

(even at 50 yards Practical)

Gun: SVI .38 Super Auto, Schuemann barrel 5.5"

DVC, Henny Schmitz, NPSA, the Netherlands (EU)

[wide45]

Gotta be the air in front of the bullet. Thats why .45 open guns shoot so flat.

[ErikW]

LOL, wide45, same reason comps and porting is so effective on 12 ga shotguns.

Arguable/debatable compensator trivia: a comp works by gas hitting its baffles, not by jet or rocket effect.

[BSeevers]

I thought it was the sound waves that did it. Man you learn something everyday

[ErikW]

Yeah that's why comps are so ineffective with Minor or steel loads, they need to go supersonic.

[DogmaDog]

Heh. The air pressure in front of the bullet probably doesn't rise appreciably at all (if it did, the bullet wouldn't go). It's the high pressure gas behind the bullet that the comp uses to reduce recoil. Those little gas molecules come out of the barrel at high speed, and bounce off the compensator baffles in an upward direction, carrying momentum with them, and imparting an equal and opposite downward momentum to the gun. They also lose some forward momentum, which is imparted to the comp, and tends to reduce the rearward movement of the gun.

Erik: is there a difference between a "jet or rocket effect" and "gas hitting the baffles"? I think it's the same thing.

DogmaDog

[ErikW]

Heck yeah. No baffles required for jets or rockets, they just squirt the exhaust gas straight out.

Maybe the addition of baffles is to simply to lower the chamber size and thus increase the velocity of the escaping gas?

[Gun Geek]

Hey Dog:

I'm with Erik - heck yeah there's a diff between the gass hitting the baffles and the jet effect.

The jet effect is what causes some (a good portion) of the recoil in the first place. The physics here are that the high pressure in the barrel, when uncorked, acclerates the gas out the muzzle which creates a force toward the shooter. That air hitting the chambers of the comp slows down the rearward movement.

Some comps (particularly rifle comps that are truly hated) actually then turn some of that blast back slightly toward the shooter. These are the ones that make your teeth hurt (the mini Y-comp on my bushy M4 does this). In this case think of a thrust reverser on a jet engine - the exahust is directed forward, so the plane slows. Once the pressure is built, by redirecting the gas, the jet effect produces force in the opposite direction of the flow, even if it is redirected (Think Harrier Jet, or thrust verctoring on the new generation of fighters). It is less efficient to redirect the flow, but it still works.

Next time you see a photo (or an actual) tank or large artillery piece, you can see that the baffles are cut to direct gas back on these things. These really make your teeth hurt!

Anyway - oldshooter, the gas (air) ahead of the bullet is not much of a factor. Not enough of it and not moving fast enough. I'm sure it gets compressed a little just becuase the bullet moves it, but there just isn't enough. Here's an example (and this will blow you away!). You're in a car and either have balloons floating around or you're smoking. You stomp on the gas. Which way do the the ballons or smoke move? You probably will say toward the back of the car - the slurpie on the dash does, right? Well, the ballons and the smoke will go forward. The acceleration of the car compresses the air so that it is more dense in the back of the car than in the front. The smoke and balloons are then pushed forward by the higher pressure in the back (same reason they float - air below them is less dense than the air above them).

OK, Highschool physics lesson over

Geek

[shred]

The basic source of both is the same reaction-- throw some mass one way and you go the other. Throw a bunch of stuff out the back of a rocket and it goes the other way. Throw a bunch of powder gas out the front of the gun and the gun goes backwards in recoil. Throw that gas upwards instead and the reaction pushes the muzzle down. Throw that gas against a wall and it tries to push the wall away. If that wall's attached to the gun, it ends up pulling the gun forward.

There are three elements to recoil-- accelerating the bullet up to escape velocity, accelerating the powder gas up to escape velocity, and the 'jet-effect' from uncorking the barrel after the first two do their thing.

The only way to directly affect the first two elements of recoil is to load less powder and/or a lighter bullet at the same velocity. The third element can be directly affected by redirecting the jet via upwards ports or trapping it in a suppressor.

With a comp we also try and compensate for the effects of the first two elements of recoil as well, which is why we usually shoot comped open guns and not suppressed ones.

[DogmaDog]

Oh, yeah...Shred is helping to clarify. I was thinking that the "jet effect" you were referring to was gas blasting out the top of the compensator to push the muzzle down, which, as Shred said, is physically the same as the molecules acting like little ping-pong balls bouncing off the baffles, and the upward momentum they acquire is equalled by the downward momentum of the compensator.

The "vectored thrust" analogy is a good one, though--un uncompensated pistol is like a traditional jet, while a comped gun is like a Harrier ("Scarier", as many Marines call them).

More trivia: A rocket engine is most efficient when the exhaust gas leaves the venturi (nozzle) at:

a) high pressure

0 pressure

c) ambient pressure

the answer is "c"--the gas has done all the work it can once it expands to the point it reaches ambient pressure.

DogmaDog

[old shooter]

Ok, I like the way this thread is going, but let me ask one more question.

Suppose I want a load of 124 gr @ 1200 fps and 30000cup....now

what if I had a magic powder that could do this with 1 gr of propellent.

If I also had loads with different powders that met the above with 5 gr,7 gr, and 9 gr, what could I expect with each load. I guess what I am looking for is the relationship between powder volume/pressure/powder velocity/gas volume/gas burning time, etc etc.

Since there is a lot of information so far on this thread, could someone try or guess the importance if these factors, and list any that I might have overlooked.

Maybe this could be the definative answer on comps and operations, at least until tomorrow.......

Mike

[DogmaDog]

Old Shooter,

The mass (weight) of the powder is equal to the mass of the gas that propels the bullet and then works the comp. Since momentum is equal to mass x velocity, and the momentum of the gas is imparted to the compensator, you want the gas to have more momentum in order to get more effect from the comp. You could achieve this by having a larger mass of gas (I can see the jokes coming ), or by making the gas go faster as it leaves the muzzle.

To go faster, the gas will need to be at higher pressure as the bullet leaves the barrel, which can be done by using a slower burning powder, and by using more powder.

To get more mass, just use more powder.

The momentum of the gas ALSO contributes to recoil, and not all of that momentum is redirected by the compensator. I don't know if there is some combination that would cause the largest fraction of gas to hit it, or how one would find that combination, but presumeably the compensator is not equally efficient under all circumstances.

Regardless, I would expect the heavier powder charges to produce the greater recoil mitigating effects in a comped gun, but suspect you could reach a point where no further improvement is achieved.

Based on all this, the "optimum" solution should be a load that propels the bullet to the minimum possibel power factor, using the heaviest charge of a slow burning powder such that the pressure when the bullet exits the barrel is maximized...I think that would happen if all the powder was just burned as the bullet exited the barrel, so I suppose you could load to the point you get unburned powder, and back off just a bit, and be there.

Of course, you could get to a point where the comp is working too much and the muzzle actually goes down when you shoot.

Clear as mud, I hope,

DogmaDog

[Gun Geek]

Shred and Dog:

The jet/rocket effect and the ping pong balls are two very different sets of physics.

The jet comes from the fact that pressure is mantained in a chamber that has a hole in one end. The gas is pushed out of the hole (accelerated) by the pressure in the chamber. Because of action/reaction (Newton) a force is generated in the direction opposite of the direction of acceleration. A balloon is the everyday example - blow one up and then let go and it flies around the room. This NOT because the air in the balloon pushes against the outside air, it is because the gas is accelerating out the back. This is why rockets work in space - where there is nothing to push against. To use this force the comp generates a pressure area in the chambers and squirts the gas out back toward the shooter to produce force away from the shooter and counters the recoil of the gun. These are the comps that hurt. They work best in high pressure rounds with lots of gas - that's why you see them in rifles.

Just for the sake of discussion, a rocket and a jet work the same way - accelerating gas out the back to create thrust. The difference is that a jet needs to take in air (oxygen) to mix with the fuel which igntes and maintains the pressure. A rocket motor (and gunpowder) have an oxidizer as part of their fuel, so they don't need air (yes a gun would work in space!). Also, sorry to disagree with you Dog, but a rocket is most effective when the exaust gas is as fast as possible when it leaves the nozzle. If you measure the pressure at the nozzel it will be very high as well. This means the gas was accelerated as much as possible, and with Newton again, F=ma, higher acceleration means higher force. The efficiency would be measured by expressing the ratio of the potential energy contained in the fuel to the energy (velocity and mass) of the gas being expelled. The loss is mainly through heat in a rocket - no moving parts, and through heat and friction in a jet.

The ping pong balls example is plain old momentum. Think wind. If you stand in a strong wind you feel a force. The momentum of the air molecules hitting your body cause this force. To use this force, the comp just sticks a baffel out in the gas flow. The fast moving gas hits it and creates a force in the same direction of flow - away from the shooter, reducing recoil. In this case the energy transfer is most efficient when the velocity of the gas is 0 when it leaves the comp - it means all of the energy was transferred to the comp.

Now to old shooter's next question. Comps generally work best with slower powder because the powder is still burning (creating gas) when the bullet leaves the barrel. With the powder still burning, more gas and pressure are around to be used per the above discussion. Unfortunately, we hate this situation because it makes lots flash and a loud report. A comp is most effective in a high pressure round with lots of gas around (magnum rifles). You don't see comps on black powder rifles because there is not enough gas and pressure around to justify it.

With a fast powder, the pressure peaks quickly and then starts to fall, possibly before the bullet leaves the muzzle. In this case there is little or no muzzle flash and a lesser report. There's still lots of pressure behind the bullet, it is just not increasing.

From the comps' stand point it doesn't matter if it takes 0.1gn or 100gns to produce the pressure. It just cares about the level of pressure and the volume of gas available. Requiring more or less powder just has to with the chemical properties of the powder.

You can't really say what the effect of different powders that produce the same peak pressure but with different charge weights would be. (Your 5, 7 or 9gn load example) The key is the shape of the pressure curve - does it peak quickly and drop off or continue to build over time?

Clear as mud?

[Gun Geek]

Dog we were writing at the same time!

[Andrew B]

From "Dogma Dog"

"Based on all this, the "optimum" solution should be a load that propels the bullet to the minimum possibel power factor, using the heaviest charge of a slow burning powder such that the pressure when the bullet exits the barrel is maximized...I think that would happen if all the powder was just burned as the bullet exited the barrel, so I suppose you could load to the point you get unburned powder, and back off just a bit, and be there."

I agree with the theory. When I first started using an open gun on steel, I loaded lighter charges of the slow powder that I use for major. It works ok, but has wide swings in velocity. I believe that at the lower power factor, you don't get reliable ignition of the slower powders and that the volume of gas produced really does not work the comp well anyway.

In practice, what works better for me, is a somewhat lighter bullet (115gr is what I have gone to for this) and a powder with a medium burn rate - SR7625 or Super Feild to acheive a consistent load with a light power factor. You end up with a gun that has less recoil, but a little more bounce - less jet effect from the front of the gun, but also less gas exiting the upper ports on the comp. I think this is why guns purpose built for steel have relatively small comps.

Since you are shooting at each target once, then have to more the gun to the next target, the small bounce is an acceptable tradeoff for the decrease in recoil and the increase in control.

Dog, see you Friday night.

[DogmaDog]

Geek,

[rocket drift mode on] You're right, rockets are most efficient when the exhaust gas is fastest, but that happens when the exhaust gas reaches ambient pressure! As long as there is greater pressure, there is still force pushing the molecules outward--accellerating them. They stop accelerating when there is no more pressure gradient, so the gas will have reached its maximum velocity precisely when it reaches ambient pressure. The rocket nozzles on first stage and second stage boosters at the Smithsonian have different shapes to account for the different ambient pressures they operate in (I saw them myself ). [/rocket drift mode off]

DogmaDog

[schmitz]

Andrew,

You can easily avoid ignitionproblems with slow burning powders by using Small Rifle Primers. (Federal 205 Gold Match/small rifle)

I use them in my 6.3 gr N350 Bianchi-loads using a 115 grains Fiocchi-bullet, producing a consistent PF of 140, standard deviation reduced to the minimum

On my gun I have a 6-chamber comp with small holes. Muzzle flip and recoil easy to control.

DVC, Henny Schmitz, NPSA, The Netherlands (EU)

In addition:

Case: Remington.38 Super+P Nickel plated

OAL/COL: 31.8mm/1.25"

Flash hole/primer pocket: venturied 90 degrees

(The Fiocchi bullets are similar to the Sierra's 115 FMJ Tournament Master #8115)