Tubb 42 coil spring in 5.56 guns and why it runs almost everything
This posting is two parts. First, why I'm using the Tubb 42 coil spring in everything now, and secondly a bit deeper dive into the physics of what's happening that allow it to work well and why it might be a good choice for several applications. I'll assume you've seen Tubb's promo video on these flat springs comparing them to some mil spec and Sprinco springs. I have no affiliation in any capacity with David Tubb or any reseller of his products, FYI.
I've taken to using the 42 coil Tubb flat spring (his ".308 spring") in just about everything 5.56. I bought it initially to test in my #1 rifle, my DDM4 V11 pro (18" rifle gas), expecting to have to trim it back (to ~36 coils) to get it to function properly. After all, Tubb sells at 36 coil version of this spring he intended for carbine and rifle use in 5.56 guns. Turns out my 18" rifle gas upper loves it with all 42 coils in a carbine lower with an H1. Smooth and reliable. Turn out my 16" middy loves it with the same lower.
And it turns out that it runs great in my two different A5 lowers with either A5H2 or A5H3 buffers with the same uppers. How can the same spring work so well in such different applications? ("working well" here means it provides excellent power to cycle the gun, has a smooth recoil impulse without inducing functional issues or screwing up timing).
As many of you already know, it is a very long spring:
https://i.imgur.com/vy2JoZx.jpg
The question: why does this "too stiff" spring work so well in both carbine and A5 5.56 guns? Some basics about springs will help us understand. A spring works like a coiled torsion bar. You might remember torsion bar suspension from old Mopars in the classic muscle car era. Instead of the spring being coiled up, the torsion bar spring was essentially just twisting a solid steel rod. But if you were to wind this rod around a cylinder, you'd find you had a coiled spring that experienced stresses pretty much the same way. A coil spring works by inducing a twisting force on the wire of the spring.
A longer spring experiences less change in load with distance than the same spring at a shorter length-- this is because the same twist is distributed across a longer run of wire. So if I have two otherwise identical springs-- one 12" long and the other 24" long, we'd find that compressing them the same distance (say, 3.5" or so) would show the shorter spring to get "stiffer" at a much faster rate. In engineering terms, has has a different spring rate (F=kx, force as a function of spring rate and displacement/distance). They have different rates EVEN THOUGH the wire is exactly the same, the diameter is the same, and all other features are the same.
The comically long 42 coil Tubb spring ends up being very linear in force because of its length. This lets you have more force to hold the bolt closed, more force to strip a round from a magazine, and more reserve to cycle the gun but NOT have so much spring force at the rear of the gun that the timing of the rifle is affected and inducing short stroking or having the bolt cycle so fast it outruns the mag springs. It preserves the "dwell" at the rear necessary to let the next round present itself.
The Tubb video shows this rather strikingly. The 36 coil version of the spring has 10.5# bolt closed, 16.3# bolt open. This compares to a new carbine spring at ~8# bolt closed and 16.5# bolt open. The long 42 coil spring has 13# at bolt closed, but still just 16.7# at bolt open. Not even 4# difference between open and closed.
Takeaway: the 42 coil spring has a lower spring rate (change in load with change in distance) even at higher applied force. This is a good thing. We want the rate as flat as possible.
So now for the theory part:
While both buffer weight and spring force resist the rearward travel of the bolt, they do so very differently. The spring directly applies force, regardless how fast the bolt is moving (putting aside spring dynamics for the moment). It resists the bolt based on *position*.
The buffer, on the other hand, can only apply inertial force as a function of bolt acceleration. F=MA. No acceleration on the buffer, no force!
So when we start with higher spring preload holding the bolt closed, we not only gain more time in bolt closed dwell (time for blowdown and extraction), we also make the rifle less sensitive to differences in buffer weight because the bolt isn't moving to the rear as quickly.
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. 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).
In the 20g case, an H buffer that weighs 3.8oz can actually apply a force of (20x3.8 oz=) 76 ounces. An H2 at the same acceleration would appear to apply a force of (~4.6ox x20g=) 92 ounces. This is a difference between buffers of 16 ounces.
But if I slow the bolt down to 10g acceleration, now the apparent forces for the H1 and H2 are 38oz and 46 ounces, respectively, a difference of just 8 ounces.
By some simple math, in order to produce the same retarding force at bolt closed that the Tubb 42 coil spring does (13#), an H2 carbine buffer would have to be accelerating at ~ 45g. Obviously it will only exceed this acceleration value (if it does at all) very briefly because immediately after the gas key disengages the gas tube, the BCG is slowing very quickly.
So why does it work in an A5 setup so well? This goes back to the long length. Being just 3/4" longer, the A5 doesn't drastically change the operating environment of the spring relative to carbine. It will have slightly less force at bolt closed and bolt open, but still quite a bit more than an A2 rifle spring, and also be more linear in terms of spring rate (force gain, if you will). Because it's applying load to the BCG even at zero acceleration, it is making the apparent differences in buffer weights seem less.
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. By definition, this is after the "dwell" time is over and we've proceeded to unlock and extraction.
This approach -- more spring, more spring linearity, less buffer-- should have a broader operating envelope and provide better margin across loads and also suppressed vs not. My informal testing isn't really scientific, but it strongly suggests this is the case.
Remember, all of the energy available to cycle the gun comes from the spring. A buffer that's too heavy will deprive the spring of its full energy capacity by not compressing it all the way. A buffer that's too light will let the excess energy arrive at the shooter's shoulder when the buffer bottoms out hard in the tube-- it is attempting to deliver more energy than the spring can store.
This is why the big 42 coil spring can allow the same uppers to run great with something as heavy as the 6oz A5h3 all the way down to my 3.8oz H1 carbine setup.
Ejection with all lowers was pretty much the same spot at 4:00 with Geco .223 factory and even my weak 8208XBR hand loads, and it didn't seem to matter which lower or buffer weight was in use. Both carbine and A5 lowers ejected to the same spot and locked back on empty mags.
I point that out not that 4:00 ejection is evidence of perfection. Rather, the *consistent* ejection across buffer weights, across carbine vs A5, across factory load vs weak hand load, appears to confirm that the spring makes the gun much less sensitive and less sensitivity means a broader operating envelope.
This is why I think the 42 coil Tubb spring might be a good choice overall for almost any carbine or A5 setup. It should be kinder to your brass and enhance extraction by giving more closed bolt dwell, give you extra force to strip a round when its more difficult (debris, dirty mag) and overall have more effective operating envelope.
That has been my experience so far, admittedly with only basic function tests with A5 and about two years and ~1000 rounds in a carbine lower with H1. Perhaps a higher volume shooter has more insight? I think a lot of guys using the Tubb springs are using his 36 coil version and missing out on the virtues of the 42 coil one.
H
