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

[1168]

I absolutely agree that the 42 coil in a carbine seems a bit much on preload. But other than the subjective feeling associated with an “authoritative “ shall we say bolt drop, I see no evidence of it being excessive in terms of function. You absolutely don’t want your fingers in there when the bolt drops because it’s going home come hell or high water. It’s notably stiffer when charging, yet you can feel the linearity.

Your experience with the 36 coil spring would be especially noticeable with another 3# of bolt closed preload.

Did you observe any actual problems with the higher preload or was it a subject impression of it being excessive?

I initially got it to try to take better care of my match brass— less damage to brass and ease of recovery of brass was honestly all I expected when I bought it to try.

The dry film coating they ship with gets brittle and flakes off. I grease my spring and buffer lightly with synthetic grease. So far, so good.

In a lightly gassed carbine-RE gun, that was reliably able to eat and eat without cleaning while using a Colt spring, I experienced stoppages while dirty, using .223 ammo. In another carbine-RE gun, this one more heavily gassed, I noted excessive wear on the charging handle and wear near the cam pin relief, that seemed as though it might be accelerated, like on a 416. I don’t think it was at risk of imploding or anything, just something I noticed that made me put a Colt spring back in it and choke the gas down a bit, instead.

Both of the above anecdotes were with the older CS spring, which is stronger than the current SS spring. Both of those guns were my day-to-day beaters, at different times. Unfortunately I don’t have a personally owned carbine-RE gun right now to try the newer spring in.

I don’t think that there’s free lunch, but I do think that the Tubbs spring could possibly be part of the winning formula for a gun that is 50/50 suppressed/loud. I think that the weapon is more forgiving unsuppressed with a spring that is providing more force during locking and unlocking, but practically the same force at the rearmost of travel. I also think that suppressed guns benefit more from that force than the force provided by a stiffer spring, such as the popular Wolff, Sprinco, and Geissele offerings. So, again, I don’t think the lunch is free, but it might both taste better, and be more nutritious, in comparison to other options.

I’ve been working on, as funds allow, putting together an experiment to see if there is really any difference between carbine gas and midlength gas for a 50/50 suppressed/unsuppressed gun, assuming both are gassed lightly in the loud configuration. My hypothesis is the opposite of conventional wisdom. I’ll be using two 14.5” uppers, both with the same model AGB, same model mount, same 762 can, same BCG, same A5 SBR lower. I’ll open the gas block on both until they lock back with the can, then see how many clicks it takes for each to run without it. I was also planning on doing a follow-on where I also switch from H4 to H1 buffers when I pull the can.

After that, I’m doing a third pair of experiments to determine if Tubb’s 36 coil SS makes the guns more or less tolerant of can/no can. If it takes less clicks between min function with and without the can in comparison to the Colt spring, then thats a win.

I also have a SS 42 coil Tubbs, and enough parts to assemble a lower with a carbine RE.

If anyone would like to help me accelerate these experiments coming to fruition, shoot me an IM. I’m like 80% there in parts required to do all of them. The ammo bill is gonna hurt, though.

As for the dry film coating on the SS spring, I have not had any corrosion with mine, despite it chipping/flaking off. I did with the CS model.

[DG23]

I absolutely agree that the 42 coil in a carbine seems a bit much on preload. But other than the subjective feeling associated with an “authoritative “ shall we say bolt drop, I see no evidence of it being excessive in terms of function. You absolutely don’t want your fingers in there when the bolt drops because it’s going home come hell or high water. It’s notably stiffer when charging, yet you can feel the linearity.

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Sounds similar to when dummies stick an extra heavy recoil spring in their 1911's. Yeah, It can somewhat lessen felt recoil but at the expense of adding a lot more slide dip when that slide does go back into battery.

No thanks. You just made it more difficult to stay on target during a string of rapid fire (same as in the over sprung 1911 example).

[17K]

Long time 1911 USPSA single stack shooter here.

Stiffer springs increase felt recoil.

You want to run the lightest recoil spring you can reliably run.

I always used 15lb Wilson springs with factory loads and a 14lb with 185gr softball loads.

In an AR, stiffer springs and heavier buffers are the opposite of what you do make one shoot softer.

Look at JP if you want to see how to make one shoot soft.

[georgeib]

Long time 1911 USPSA single stack shooter here.

Stiffer springs increase felt recoil.

You want to run the lightest recoil spring you can reliably run.

I always used 15lb Wilson springs with factory loads and a 14lb with 185gr softball loads.

In an AR, stiffer springs and heavier buffers are the opposite of what you do make one shoot softer.

Look at JP if you want to see how to make one shoot soft.

So this is an interesting phenomenon. In motorsports the opposite can often be true. Often times cars and motorcycles are sprung too light, so the suspension bottoms out and gives a harsher ride. Going stiffer in these cases results in a "cushier" ride. I have found something similar in rifles: if the spring is too light, the buffer will smack into the back of the tube and feel harsh, whereas a heavier (but not too heavy) spring eliminates that harshness and slows the recoil impulse.

Obviously there is a balance that must be found here, or you get the bolt moving forward too quickly which also increases the tendency to move the sights off target.

[1168]

Like I said, lunch isn’t free. The energy used to compress the spring must come from somewhere, and must go somewhere. The Tubbs is not a stiffer spring. It is lighter, with more preload. In Hohn’s case, a eff-ton more preload.

My interest in this spring is not recoil reduction. I’d like to have a rifle function as close to the same as possible, both suppressed and unsuppressed. And I think this could be a part of that puzzle. Or not.

My plan for the experiments I posted earlier won’t work. I just measured a Sentry 7 and its .01” per click. Not precise enough. BRT doesn’t sell gas tubes with holes small enough to make a suppressed 14.5” carbine not run, so that won’t work, either. I might have to try something crazy like a 11.5” mid and a bushel of EZ Tune tubes.

@Clint, do you have any thoughts on this?

[Hohn]

Sounds similar to when dummies stick an extra heavy recoil spring in their 1911's. Yeah, It can somewhat lessen felt recoil but at the expense of adding a lot more slide dip when that slide does go back into battery.

No thanks. You just made it more difficult to stay on target during a string of rapid fire (same as in the over sprung 1911 example).

Long time 1911 USPSA single stack shooter here.

Stiffer springs increase felt recoil.

You want to run the lightest recoil spring you can reliably run.

I always used 15lb Wilson springs with factory loads and a 14lb with 185gr softball loads.

In an AR, stiffer springs and heavier buffers are the opposite of what you do make one shoot softer.

Look at JP if you want to see how to make one shoot soft.

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.

There obviously are ways to make a soft shooting rifle with LMOS, AGB and all those gamer toys. My rifles however are rifles that must work with all the ammo I might feed it. The sporty 3-gun rifles are hyper tuned niche guns with a narrow operating range. They are the opposite of my rifles.

I was a surprised as anyone that a spring I bought to try only for brass preservation during match shooting ends up showing potential as a duty acceptable enhancement.

It might not work for you. It might be awful in a short barrel. It’s a $20 spring. Try it if you want to add to the pool of experience, as opposed to the ocean of conjecture.

[MistWolf]

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.

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

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.

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

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!

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.

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.

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.

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

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.

...The appeal of the 42 coil is that it might be something of a free lunch— less recoil without a reliability penalty or margin reduction.

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

...My plan for the experiments I posted earlier won’t work. I just measured a Sentry 7 and its .01” per click. Not precise enough...

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

[Hohn]

A bit more speculation from me as to *why* the 42 coil spring works where it seems it shouldn't.

But before I do, I want to acknowledge that an 18" rifle gas might not prove much of anything. I can't say how it would compare to a 14.5 middy, for example. Lower port pressure for sure, but overall gassing I can't say how representative an 18" rifle gas DD barrel is on the spectrum of over/under gas. I have the impression that DD barrels are a bit on the gassy side for reliability?

Also, I want to point out that this observation was just on two DD barrels- a 16" mid and 18" rifle. It may not represent a nominal barrel or much of anything. So I mean to put a big asterisk by the "runs in anything" comment that I shouldn't have stated in the OP.


All that said, I'm not speculating as to whether they work well in my uppers-- they do, period. My speculation would be *why* they run when on paper they shouldn't, and if this perhaps means we don't understand something as well as we think we do. Or I should say that *I* don't understand it as well as I thought I did.


It did occur to me that the high preload at bolt closed might actually have the effect of raising port pressure a bit by allowing pressure to build a little longer before the bolt unlocks. I've seen testing of the SLR gas block showing notably different muzzle velocity when the block was in "restrict" mode vs "bleed off" with the bleed off dropping MV. I'm wondering if having the bolt held closed longer (bit more preload) might mimic the effect observed on the AGB testing where not bleeding off the gas as quickly showed slightly higher pressures in the port and behind the bullet.

We have to keep in mind that pressure in the gas tube and gas key rises on a curve, albeit an incredibly fast one. At some point on that curve, enough pressure has built to begin pressurizing the chamber of the BCG and push the bolt forward and BCG rearward. But this is NOT the peak pressure. The pressure will continue to rise until either the gas key is disengaged from the tube OR the bullet leaves the barrel.

So *if* the high preload of the 42 coil spring is holding the bolt closed momentarily longer until a point higher on the pressure curve is reached, this would cause the rifle to act as if it had more gas and *might* explain why the high preload seems to work even without an obvious over-gas situation.

If this theory is valid, I hypothesize that one might expect to see a difference in muzzle velocity between a high preload 42 coil spring and a standard carbine or rifle spring. I have not tested this. It would require probably 30 rounds of chrono data each for the 42 coil spring vs a regular spring. Even then it might not pass a t-test for significance.


Anyway, I appreciate your indulgence in my thinking out loud here. I'm not trying to sell anyone on anything or suggest this is the be all for every AR, and I apologize if my post suggested that. I reread it and can see how it might come across that way and I figured I'd rather post this mea culpa than edit the OP.

I get hung up on things that I don't understand (i.e. why this spring seems to work where it shouldn't) so group-processing it with other curious people is helpful to me.

Thank you.

[17K]

If you’re wanting to build a dead-nuts reliable duty gun, you build the gun around the ammo.

Trying to build a weird 1-off gun around ammo that you may or may not even have is starting from the wrong angle IMO.

[DG23]

Long time 1911 USPSA single stack shooter here.

Stiffer springs increase felt recoil.

You want to run the lightest recoil spring you can reliably run.

I always used 15lb Wilson springs with factory loads and a 14lb with 185gr softball loads.

In an AR, stiffer springs and heavier buffers are the opposite of what you do make one shoot softer.

Look at JP if you want to see how to make one shoot soft.

We will have to agree to disagree about how more spring increases perceived / felt recoil...


Either way though, that recoil is still there if the load remains the same. You can definitely change how the gun tracks as it cycles with springs / FPS's, etc. but you are not getting rid of the recoil without changing the load.


On my comped 1911's - Full power loadings are not going to cycle the slide at all with a 15lb recoil spring. That empty brass is staying in the chamber and you can forget about the thing called 'semi-auto'. LOL! With a normally radiused FPS and a standard weight hammer springs I need to drop to about 12lbs on the recoil spring to 'reliably' be able to cycle and lock back on an empty mag. If I up the hammer spring weight or go with a flat bottom FPS that recoil spring weight will need to be reduced even more to be 'reliable' and cycle properly.

Imagine full power loadings with a powder (like AA#7) that actually produces enough gasses to be able to 'work' a comp (and reduce muzzle climb) while at the same time being around 10 to 12lbs on your recoil spring weight affecting your slide dip when things go back into battery... That makes for one hell of a flat tracking 1911 my friend!