Lock Time?

[tobnpr]

Lock time is an important consideration, as with any aspect of trigger control as it relates to accuracy- it becomes more evident as range increases.

When you consider how difficult it is to achieve the "perfect" trigger press- respiratory pause, between heartbeats if you're good enough...the shorter the time you need to maintain that with no movement, the better...

 

[Scorch]

Now the Anschutz barrels are back to 26 inches. I've no idea why. Maybe it's an accuracy issue.

With match rifles that use iron sights, it is a sight radius issue, the longer sight radius gives better accuracy. In a match, that is important.

the spring, not hammer mass, determines the energy of the hammer strike

Kinetic energy= 1/2 mass X velocity squared
Momentum = mass X velocity

You may notice in both equations, mass figures prominently. The mass of the hammer is the primary cause of induced vibration. In older cartridge rifles, the mass of the hammer is a carryover from percussion rifles, heavy leaf springs, and thick primer cups. John Browning demonstrated that the heavier hammers were not necessary, his 1878 design that became the Winchester 1885 High Wall had a much lighter hammer, and dominated target shooting in the last decade of the 1800s, beating out the Sharps, Remingtons, Ballards, Wessons, and Marlin rifles.

I was trying to shoot small groups with a Ruger #1 single shot rifle from the bench and as I squeezed the trigger and the sear released, the hammer fell but the round did not go off.
The image in the scope jumped a couple of inches from the vibration caused by the hammer fall. I dry fired it on purpose again just to see that again.

Yes, Ruger #1s have a large, heavy hammer. Consequently they have a lot of vibration caused by the hammer fall.

The shorter the lock time, the less time there is for the gun to move off aim during the firing process.
Also, if you have a heavy and slow hammer, the reaction to the hammer fall can cause considerable movement during the hammer fall along with the sudden release of pressure on the trigger finger.

Do not confuse cause and effect. Slow lock times with heavy strikers causes large amounts of vibration, which translates into POI movement because they casue the barrel to move. You will have little time to move your muscles very far during hammer fall, but the movement casued by the movement of the heavy hammer is immediate and noticeable.

There's two "times" critical to accuracy.

Correct, Bart. I usually hear them referred to as lock time and dwell time, but terminology is irrelevant. Lock time affects accuracy by reducing the movement possible during striker fall, dwell time affects movement during bullet travel in the bore. You want both to be short and vibration-free. This is one of the reasons benchrest shooters want fast locktimes, light weight strikers, and fast cartridges.

 

[MrBorland]

Kinetic energy= 1/2 mass X velocity squared
Momentum = mass X velocity

You may notice in both equations, mass figures prominently. The mass of the hammer is the primary cause of induced vibration. In older cartridge rifles, the mass of the hammer is a carryover from percussion rifles, heavy leaf springs, and thick primer cups. John Browning demonstrated that the heavier hammers were not necessary, his 1878 design that became the Winchester 1885 High Wall had a much lighter hammer, and dominated target shooting in the last decade of the 1800s, beating out the Sharps, Remingtons, Ballards, Wessons, and Marlin rifles.

The hammer doesn't store any energy so it can't be a source of energy - it simply transfers the energy stored in the spring. The equation above is one means of calculating that energy, but it doesn't imply the hammer's a source of energy. If we knew the spring constant, we could use a hammer-independent equation to calculate energy. And since energy's supplied by the spring, it's constant, no matter the mass of the hammer - decrease hammer mass by 20%, and hammer speed increases by 12%. Energy remains the same, but power increases 12%, while muzzle-jarring momentum and lock time decrease by 10%.

At any rate, my experience with revolvers has convinced me there are manifold benefits to a lighter hammer. There is a practical limit, though, and not knowing enough about AR15 design, I understand those hammers may already be closer to that limit. The basic idea (and physics) is the same, though.

 

[Tom68]

You see, the 100 lb brains did chime in

So, for all the physicists out there... exactly how is the energy in a spring measured? for that matter, what instruments are used to measure lock time, and dwell time? I know this is really deep laboratory stuff, but I do find this sort of information interesting.

 

[Brian Pfleuger]

The force exerted by a spring is expressed as:

F=-kx

where F is the force, x is how far it's stretched and k is the spring rate, or spring constant.

The potential energy of a spring looks suspiciously like the standard Kinetic Energy formula:

PE=1/2kx^2

Instruments to measure barrel time are pressure/vibration strain gauges. More information about strain gauge instruments, barrel time and links to barrel harmonics and the like can be found here:

http://www.shootingsoftware.com/pressure.htm

 

[Bart B.]

Scorch comments on Anschutz rimfire barrels going from short to long:

With match rifles that use iron sights, it is a sight radius issue, the longer sight radius gives better accuracy. In a match, that is important.

True, but their short barreled versions had a long bloop tube on them so the sight radius was the same as their long barrel versions. That way, their rear sights would change point of impact the same per click regardless of the barrel configurations used. Here's an Anschutz bloop tube that adds several inches to the sight radius. A slot at its top back end aligns with a pin on top of the barrel behind the muzzle where it clamps on to. It' got a 3/4" diameter hole through it the bullets go through.

I don't think a longer sight radius gives better accuracy as the front sight still wobbles around the target the same amount angular wise regardless of sight radius. But the further away the front sight is when one's aiming eye is focusing on it, the sharper the target will appear.

 

[B.L.E.]

The force exerted by a spring is expressed as:

F=-kx

where F is the force, x is how far it's stretched and k is the spring rate, or spring constant.

The potential energy of a spring looks suspiciously like the standard Kinetic Energy formula:

PE=1/2kx^2

Since most hammer springs have some preload in the hammer down position, the formula should be.

PE= 1/2k(x1^2-x0^2) or the potential energy of the spring in the cocked position minus the potential energy of the spring in the uncocked position.

The formula should look suspiciously like the KE formula since they both describe a function that looks suspiciously like the area of a triangle.