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Thread: How does the CFW bolt work? Is it just heavy or also springs involved?

  1. #1
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    How does the CFW bolt work? Is it just heavy or also springs involved?

    Since not CFW 45 bolt appears to be coming, Iím eying tungsten, my existing Lage bolts, and wondering if it could replicated with just replacing several strips of steel with tungsten.

    After the weight is added do the springs need to be tweaked? As a non-engineer, Iím trying to understand the relationship between ROF, bolt weight, springs, and the ability to cycle.

    Thanks

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    It's way heavier. It almost feels weird in your hand.

    People have been adding all sorts of weights to m11 bolts for years. The CFW seems to be the most elegant solution I've seen.

    Coming soon: People drilling holes in the CFW bolt to speed them up.
    Last edited by greenpeas; 07-24-2020 at 01:51 PM.
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    Quote Originally Posted by greenpeas View Post
    Coming soon: People drilling holes in the CFW bolt to speed them up.
    I could see it happening!

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    I believe it has been said that a m10 version would cause damage to the gun due to the weight going back and forth.

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    Greenpeas got it- it works by being heavy. Seems ridiculously simple, but that’s physics!


    Your solution could be done- drill out a cylinder and fill it with a tungsten piece of the same shape.
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    The mass of the bolt and the recoil spring are a system. Newton's second Law of Motion is F=ma or the force equals the mass multiplied by the acceleration. The force is the gunpowder in the middle, the bullet going down the barrel and base of the shell casing. The shell bears on the the bolt face and pushes it back.

    Newton's first law says that a body at rest tends to stay at rest and a body in motion tends to stay in motion. So the bolt in an open bolt gun is accelerated forward by the recoil spring. When the bolt fully chambers the round, the protrusion in the bolt face sets off the primer, igniting the gunpowder while the bolt is still moving forward. The force of the gunpowder acts on the face of the bolt through the shell casing. So both of mass of the bolt and the strength of the recoil spring resist the force of the gunpowder. In a perfect world the mass of the bolt and the resistance of the recoil spring should be just enough resistance to fully cycle the bolt then drive the bolt back with enough speed to strip the next round, chamber it, and fire it.

    The short answer:
    If the only variable that is changed is the mass of the bolt is significantly increased but the recoil spring remains the same, then the length of the stroke would be shortened. This would tend to cause the bolt to short stroke. The recoil spring would need to be diminished in strength, if the mass of the bolt were increased, the length of the stroke was longer, or the energy in the round becomes smaller.

    There are limitations of function. If the bolt were made from Titanium, so it had significantly less mass, the recoil spring would need to be so strong it would be difficult to overcome the recoil spring to lock the bolt onto the sear. By the same token, if the mass of the bolt were significantly increased, then the recoil spring would need to be so weak that the bolt might need to bounce back off the buffer covering the back of the receiver to get extra momentum to go back into battery. I would think that the all tungsten M10 bolt had that issue.

    Disclaimer: This is just my understanding from my own experience. Others could have a different opinion. Your Milage May Vary.

    Scott

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    A&S is correct. The momentum of the recoil is constant, so a heavy bolt and a light bolt both have the same momentum during cycling. but when the bolt mass is increased, total cycling energy is reduced. Compressing the spring a fixed distance requires a specific amount of energy. The solution is a weaker spring.

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    Speed buffer + CFW bolt. The latest cool toy!

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    Quote Originally Posted by woodenword View Post
    I could see it happening!
    I do actually want to get another original full size cfw and do this to try and end up in the 900 rpm range.

    600 is kinda slow

    1200 is kinda fast

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    Quote Originally Posted by Mackjack View Post
    I do actually want to get another original full size cfw and do this to try and end up in the 900 rpm range.

    600 is kinda slow

    1200 is kinda fast
    Coffee’s hybrid gen 1 full size CFW with suppressor is like 850

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    Quote Originally Posted by Mackjack View Post
    I do actually want to get another original full size cfw and do this to try and end up in the 900 rpm range.

    600 is kinda slow

    1200 is kinda fast
    Gen2 will get you there.

    This timed at 852rpm with 115 S&B.
    Started with a new spring and wanted faster than 650.
    New spring wouldnít cycle. Went I believe 4 coils cut, and it was 100% reliable suppressed. Installed a stronger extractor spring and it is now 100% reliable unsuppressed too.

    852rpm

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    Quote Originally Posted by Hey... View Post
    Gen2 will get you there.

    This timed at 852rpm with 115 S&B.
    Started with a new spring and wanted faster than 650.
    New spring wouldn’t cycle. Went I believe 4 coils cut, and it was 100% reliable suppressed. Installed a stronger extractor spring and it is now 100% reliable unsuppressed too.

    852rpm
    Awesome!!!

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    Quote Originally Posted by Hood886 View Post
    I believe it has been said that a m10 version would cause damage to the gun due to the weight going back and forth.
    I donít know about that. The problem w the 45 bolt is that itís very large piece of tungsten that costs a lot.

    I think if you have a lage upper and bolt, you could get a slab of tungsten and replace the steel or aluminum slab and bolt it back on the bolt to increase its weight a little.

  14. #14
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    Quote Originally Posted by TheColtCollector View Post
    Coffee’s hybrid gen 1 full size CFW with suppressor is like 850
    I have a Gen 1 (hybrid), a Gen 2, and an A-bolt. They are *all* great, but I do have a special affinity for the Gen 1. It has simply always worked with no tweaking. Second to that would be my A-bolt that lets me run 9mm in an M11A1! More controllable than a full auto Glock while about the same size, and doesn’t have the post-sample restrictions.
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    I think regardless of the bolt weight, the receiver + springs are absorbing exactly the same amount of energy from a given cartridge.....

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    Quote Originally Posted by JohnFreeman View Post
    I think regardless of the bolt weight, the receiver + springs are absorbing exactly the same amount of energy from a given cartridge.....
    Well yes and no. The coefficient of restitution governs how much energy gets transferred into the receiver. The ability of a spring to overcome the kinetic energy of the rearward traveling bolt, the spring length energy asorbtion rate decreases with the square. Your ability to add mass will never be able to keep up with the rate your cutting spring. That’s the problem with the CFW45

    1/2*m(I)v(I)^2 - 1/2Kx^2 = 1/2m(f)v(f)^2

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    Ah those pesky "coefficients" ( unit-less numbers that make the calculation match the experimental results ) :-)

    For a given cartridge, the same energy goes into accelerating the bolts, heating stuff up and moving the receiver (and shooter), and I suppose noise too.

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    Quote Originally Posted by JohnFreeman View Post
    I think regardless of the bolt weight, the receiver + springs are absorbing exactly the same amount of energy from a given cartridge.....
    Nope. A lighter bolt will have more energy transferred into it. For example, if the bolt was extremely light, as light as the bullet, the bolt would recoil backwards at 1000 FPS, with as much energy as the bullet has. The heavier the bolt is, the more energy gets transferred to the bullet and less energy into the bolt.

  19. #19
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    The energy from the bullet is opposed ("equal and opposite") by the kinetic energy created in the bolt as you note, AND the potential energy in the compressed spring. (Plus noise, heat, light, etc..) and any motion imparted in the receiver.

  20. #20
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    Quote Originally Posted by Fishman View Post
    Nope. A lighter bolt will have more energy transferred into it. For example, if the bolt was extremely light, as light as the bullet, the bolt would recoil backwards at 1000 FPS, with as much energy as the bullet has. The heavier the bolt is, the more energy gets transferred to the bullet and less energy into the bolt.
    Isaac Newton is spinning in his grave on that one.

    Newton's 3rd law (https://byjus.com/physics/laws-of-motion/) says that the bolt and the bullet get the same force imparted onto them, his 2nd law says that the lighter bullet will move faster than the heavy bolt but THEY WILL HAVE THE SAME ENERGY. Energy and velocity are NOT the same.

    It's sometime hard to believe that the recoil going into a bolt, or your shoulder on fixed bolt gun, is the same as what goes into a bullet that is traveling faster than the speed of sound and can do so much destruction to a target but it is.

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