Tubb 42 coil spring in 5.56 guns and why it runs almost everything

[17K]

Find even one A Class or better shooter who says that stiffer springs track flatter than lighter...

[DG23]

With respect, these are not 1911s where the reciprocating mass is above the bore axis. It might be still above a the CoG, but the moment of inertia of a 1911 and an AR aren’t comparable. The reciprocating mass of a 1911 slide is a very high percentage of the gun’s mass relative to an AR.

To be clear, making the gun softer shooting was never the goal. The goal was to be easier on brass. The softer shooting and broader apparent operation envelope were serendipity.

A simple experiment you can do at home with no ammo expended at all:

With your bolt locked back on an empty mag and nothing in the chamber look through your sights and hold as steady as you can while you hit the bolt release. Observe what happens to your sight picture as that bolt slams home...


Repeat experiment with a regular weight spring and chime back in with your findings.


Same sort of experiment can be done at the range using paper targets and live ammo. Send two or three rounds downrange as fast as you can with the heavy as hell spring and then swap paper and try same test with the standard weight buffer spring.

I completely understand where you are going with your thoughts about going easy on the brass. 100% with you on that being a good idea and if you can tune your gun to plant that brass exactly where you want to (without sacrificing too much elsewhere) then power to you.

[lysander]

However, we aren't compressing the springs the same distance. Let's say the compressed spring length inside a carbine RE is 7 inches. (I don't know what the spring length is inside an RE, I'm just pulling 7 inches outa my butt.) That means when the BCG is on battery, the 12 inch spring is compressed 5 inches and the 24 inch spring 17 inches.

You are only off by 1/4 inch. Your butt is pretty good at estimation.

Why is a flat rate spring a better than a rising rate spring for this application?

It isn't, necessarily. Flatter spring rates are desired for main/recoil springs to keep the force required to hold the bolt open (maximum compression) with reasonable limits while the force holding the bolt closed (minimum compression) enough to reliably strip and feed a round. These should not be more that 6 to 12 pounds. If you keep the force produced by spring to 7.5 pounds through the whole compression, you actually may while up with a bolt carrier bottoming out with higher velocity, as the energy absorbed by the spring is the area under the curve.

This ignores basic physics. Objects at rest tend to stay at rest. Objects in motion tend to stay in motion. It takes more force to start and stop objects with greater mass.

No free lunches in physics.

If you want more closed bolt dwell time, a stiffer spring is far more effective than a heavier buffer because the latter can only be relevant AFTER the bolt has started moving.

The buffer & carrier are in motion before the bolt starts unlocking.

NO. You are not going to put a spring in an AR strong enough to alter "dwell time". The force from the pressure on the piston pushing it open is in the neighborhood of 400 pounds. the mass of the bolt, carrier, and buffer is about a pound, calculate the initial acceleration of the reciprocating mass if the spring load is 6 pounds or 12 pounds . . . it's about 1.5% difference, that is not going to make an appreciable difference in time to move the mass 0.325 inch . . .

In fact, this statement is flat wrong: "a stiffer spring is far more effective than a heavier buffer". Changing the mass makes a greater impact on opening time than spring rate or preload. As we have seen, doubling the spring rate (100% increase) reduces the initial acceleration by only 1.5%, but a 10% increase in total mass (going from a standard buffer to an H2), decreases the initial acceleration by 10%.

Physics shows that inertia is much better at stopping things than springs, at least spring that people can manipulate.

Ejection angle affected by extractor/ejector setup. Carrier speed has little impact on ejection angle if the extractor and ejector are healthy.

As long as it is not throwing spent cases down your shirt, does ejection angle even matter?

If anyone has experience tuning motorcycle racing suspension, the analogy here is: lower spring rate, but more preload. Kinda like A5 vs carbine...Little known thing about springs is that removing coils increases, not decreases, spring rate. Given identical space to work with, that means the shorter spring (of same material and diameters, etc) has less preload, but higher rate. Something to consider if you are burning brain-calories comparing carbine to rifle to A5 REs.

There is a difference. If you cut a couple of coils off a suspension spring, the preload is the same but the ride height is lowered. Let's say the suspension spring supports 250 lbs of the motorcycle's sprung weight- that is, the spring has a 250 lbs preload. Two coils are cut from that spring. It's still supporting 250 lbs of sprung weight but it lowers ride height- that is, the length of the spring with the same pre-load is shorter.

Cut two coils from an AR recoil spring, "ride height" remains the same (spring length under preload is the same), but preload is reduced.

Actually the analogy is totally wrong, the two systems compared are completely different.

If you cut a coil or two off a vehicle suspension, you have shortened the working room for the spring, assuming you haven't moved the travel limiters, and changed the spring rate.

Springs can change how recoil feels, but they cannot lessen recoil (unless they increase the overall weight of the firearm).

Something a lot of people don't seem to understand.

How do you know? How much change in gas flow does each click make in the gas system you plan to test?

And, how much gas flow is required to make a noticeable difference?

[lysander]

There is another issue with the very long 42 coil spring.

A very long spring with a few coils has a very high helix angle. Compare the angle off the axis of wire on the two springs



As the spring compresses the wire twists, the more the wire twists, the higher the stresses are in the wire (see below)



Optimally spring should be kept to a working length of not less than 50% of the free length. The standard AR spring has a free length of 10.70 inches long and the compressed length (bolt closed) of about 6.75 inches, and 3 inches fully compressed (so, a range of 41% to 71%), if the 42 coil spring is 24 inches long then the working range is 71% to 88%. Even for a flat spring, I would consider this a bit high.

What you really want is a long spring with a lot of coils, this gives a low helix angle, so low stress, a fairly flat spring rate in the middle range, but the down side if the solid length is long.

[HKGuns]

I'll stick with my A5 buffer and springs, thanks very much.

[lysander]

If I double the spring force holding the bolt closed, then the initial acceleration of the bolt to the rearward would be cut in half.

NO.

The acceleration of the bolt group is the total force of the bolt, divided by the total mass.

The total force on the bolt is the piston pressure minus spring force:

A = (Fp - Fs)/m

So:

A2 = (Fp - 2Fs)/m
A1 = (Fp - Fs)/m

Where:
A2 is the new acceleration
A1 is the acceleration with a standard spring
m is the bolt/carrier/buffer mass (constant)
Fs is the standard spring force

We can see the m will cancel out, yielding:

A2 = (Fp - 2Fs)
A1 ... (Fp - Fs)

Let's put some numbers in the equation:

The piston force (Fp) is around 400 pounds, and the standard spring pre-load (Fs) is 6 lbs, so doubling the spring load gives 2Fs as 12 lbs:

A2 = (400 - 12)
A1 ... (400 - 6)

or,

A2 = (388)
A1 ... (394)

a little math and A2/A1 = .98478, or 1.5% drop in velocity.

If the bolt has half the acceleration rearward, then the "apparent weight" of the buffer is less. Let's say for illustrative purposes that in one case, the bolt accelerates rearward at 20g and in the other, it heads rearward at 10g (where "g" is acceleration of gravity at ~ 32.2 ft /sec^2).

NO, just no.

The mass of the buffer is unchanged, therefore its inertial response to acceleration is the same. The buffer is not forcing the movement but resisting it.

The acceleration is different because the force is different (by a whopping 1.5%)

[TomMcC]

So I understand that a spring change from a standard spring to one of these extra long flat springs won't change the amount of recoil, so my question is, will it change the feel of the recoil to something better?

[17K]

So I understand that a spring change from a standard spring to one of these extra long flat springs won't change the amount of recoil, so my question is, will it change the feel of the recoil to something better?

Depends on what you think better is.

Personally I absolutely can’t stand the way a heavy buffer setup cycles. Seems like the dot dances forever and I have to work a lot harder to keep it on target. It feels smoother, but targets and timers DO NOT lie.

[TomMcC]

Depends on what you think better is.

Personally I absolutely can’t stand the way a heavy buffer setup cycles. Seems like the dot dances forever and I have to work a lot harder to keep it on target. It feels smoother, but targets and timers DO NOT lie.

I'm with you. I shoot 3gun and some USPSA and for my pistols I run lighter springs trying to flatten out the recoil impulse. Looking for the straight back and straight forward effect. On AR's I run them fairly stock. H1 buffer in my carbine and midlength. Standard rifle buffer in my 3gun rifle. Nothing fancy. Just wondering if these flat springs would benefit me in anyway.

[Disciple]

What you really want is a long spring with a lot of coils, this gives a low helix angle, so low stress, a fairly flat spring rate in the middle range, but the down side if the solid length is long.

Academically would conical springs be useful? If the coils nest the solid length of each one is only the thickness of the wire. A stack of these would fit more coils in the same length I think, but it would consume the internal space used for the buffer. If that could be addressed somehow, perhaps tungsten weights inside the carrier, would it be a more ideal spring system?