Anyone Ever Heard of a Carbon Fiber Op Rod?

[barnbwt]

Seems like it could be a potential solution for the various issues that historically plague the things, like getting bent & weighing a lot.

From what I can tell, the ideal op-rod for a short-stroke/separate piston design (let's not debate it) doesn't weigh anything at all, but rigidly delivers gas-port/piston pressure directly to the front of the bolt carrier further back. Carbon fiber is pretty stiff stuff when made right, and if capped with a durable insulative material (ceramic composite or hi temp polymer) would have little to fear from the barrel as far as low-intensity fire rates, and may well do better in more arduous roles if a good matrix polymer is chosen. My query concerns an accuracy-chasing project, so high rate of fire or thermal loading isn't the primary consideration

If a fiber reinforced barrel can even half-work it should have no problems on something subject to far less stress & temperature. At that point, you would ameliorate the worst aspects of piston guns as far as weight/balance, possibly improving their potential accuracy & recoil characteristics, and making longer piston systems more practical. Meanwhile, the positive aspects like chamber-area cleanliness & suppressed gas-containment would be unaffected, narrowing whatever real/perceived gap there is between the advantages of pistol and DI systems. Carbon fiber rod also doesn't cost that much, so the cost differential may be fairly insignificant vs the labor for making it from any material.

Surprisingly, I don't see any mention of even fiber-reinforced op-rods in the usual places, so I wonder if no one's looked into it just yet

TCB

 

[briansmithwins]

The FN SCAR and F2000 already use a short stroke piston and use the bolt carrier as the op rod.

BSW

 

[barnbwt]

Yeah, and as a result you either have a really long (i.e. heavy) carrier just like a long stroke's but with less blow-by fouling, or a really short gas system with increased dwell time & attendant operating force (recoil). I'm talking about the element between the piston and the moving carrier, as is seen on numerous short-stroke designs, though it could apply to a long stroke piston shaft or the integral op-rod shaft of a SCAR type system, as well;

These are for the G36, and functionally identical to the AR180, HK piston ARs, and many other short-stroke AR piston retrofit systems. Just a lightweight, narrow rod that delivers the force from a short piston at the gas block back to the bolt carrier. It struck me that their mass isn't really important to the equation, usually a detriment in the end by adding forward mass vs a DI system, and bending the typical failure mode. Carbon fiber excels in both those areas, so if sized to handle the compressive load & supported against buckling properly, it should outperform steel rods & reduce some of the drawbacks of piston operation.

TCB

 

[briansmithwins]

Separate op rods aren't that common anymore, the only modern designs I can think of (off the top of my head) would be the G36/HK416 series.

Adding the mass of the op rod to the bolt carrier can have the advantage of retaining more momentum to unlock the bolt and get the bolt up to speed as the action cycles. A light weight op rod that's separate wouldn't serve that function.

Having a short stroke piston up front does make for a cleaner running action. Both the FS2000 and SCAR stay remarkably clean, as does the AUG. The Tavor (long stroke piston fixed to the BGC, gas port very close to chamber) does get much dirtier, but doesn't seem to mind.

BSW

 

[barnbwt]

Doesn't change the fact that the forward op rod 'bonus mass' of the carrier, if you want to think of it that way, ends up making cyclic recoil and overall weight higher, and is the primary complaint vs DI (well, apart from cost). To be fair, the AR doesn't seem to mind not having that op rod mass for the most part.

 

[Tirod]

It's why DI is the next logical step in the evolution of gas operation. Instead of a divorced cylinder and piston that requires an op rod to transmit force to the bolt carrier, Stoner just put the gas cylinder INSIDE the bolt carrier where it directs forces coaxially to the moving parts.

The whole disadvantage of an op rod is that it transmits forces off axis, and the further away from the bore centerline, the more leverage it exerts on the bolt carrier. This is why the vast majority of op rod designs use long guideways to control bolt tilt. And the AR still requires a cam pin channel. All other bolts run on channels to keep the bolt carrier from tilting.

Yes, carbon fiber rods might be an incrementally better option operating a divorced cylinder design, but the cost/benefit ratio is simply excessive. It would amount to a $50 additional price hike for? Slightly reduced operating system weight and it would require special fittings on the ends as carbon fiber alone can't handle the impact loads.

A carbon fiber op rod is the same as carbon fiber crutches - having a divorced gas cylinder design is the problem which got fixed 50 years ago.

 

[barnbwt]

I wouldn't call it the 'next,' myself, any more than I would delayed-blowback being the 'next' step over gas operated actions. We just had a giant thread on why DI hasn't been able to make the leap beyond the AR's own DNA to another species, and I have a hard time accepting the platform --or any-- being the pinnacle (i.e. last ever) of firearms technology.

So, the purpose here is not to debate the 'need' for an op-rod, but rather whether they've been made of fiber composites or could be.

"A carbon fiber op rod is the same as carbon fiber crutches - having a divorced gas cylinder design is the problem which got fixed 50 years ago."
Plenty of new gas piston designs doing just fine --in fact most of the cutting edge ones-- to pick from today. Please to not thread jack to debate the merits of OP rod vs piston! We both know that both have advantages and adherents, so the only question is how to improve both. The DI is apparently so perfect as to not warrant any improvement, so it merits no further discussion here in a thread about op-rods.

BTW, if they're anything like carbon-fiber hiking or skiing or fishing poles, I'll bet carbon fiber crutches kick butt vs aluminum or wood ones (let alone steel ones ). What would be interesting is if a carbon-fiber rod ends up lighter for length than even a stainless steel gas tube...

"It would amount to a $50 additional price hike"
I actually question the huge increase which I'd also wondered about at first, which is why I thought the question was worth asking vs something like a titanium op-rod; we're talking about sourcing some cheap Alibaba carbon fiber rod in large quantities (were we talking about production) vs having to precision-machine the same from stock in-house. Since they wouldn't have to carry the bending loads that make them so unsuitable for op-rod material, I imagine the end caps & guide bushings could be made from something as pedestrian as aluminum or mild steel, with less concern for quality control as far as straightness/strength. Make the guide tube from fiber as well, and you're talking like a 1oz increase for the op rod scheme; certainly not enough to affect the balance of the gun.

Considering the very real advantages over DI like overall carrier/buffer tube length and the ability to run a forward-mounted return spring, it seems like the increase in cost could be worth it to some folks. And with less off-axis mass moving around during the dwell time, there should less barrel flex & muzzle deflection, along with less felt recoil. In fact, it may even be lightweight enough to make full-length gas piston systems palatable again; from what I gather, the ideal port location for accuracy is closer to the muzzle, but historically resulted in a far too front-heavy firearm.

I'm certain thermal loading is the ultimate Achilles's heel, at least with current technology (the fibers themselves are quite heat-tolerant), but for moderate/intermittent fire in an accuracy-minded design, maybe we can do even lighter than the Stoner gas system? At least until someone pulls off a composite gas tube

TCB

 

[rbernie]

Carbon fiber is probably the wrong material for that application; it does not do well in high vibration environments where the vibration mode is high in amplitude and low in frequency. It is very stiff but relatively unyielding - when it reaches its yield point it shatters completely rather than degrading more gracefully.

 

[lysanderxiii]

The problem is - you want some mass to keep the accelerations, and thus the forces, in check....

 

[briansmithwins]

The whole disadvantage of an op rod is that it transmits forces off axis, and the further away from the bore centerline, the more leverage it exerts on the bolt carrier. This is why the vast majority of op rod designs use long guideways to control bolt tilt. And the AR still requires a cam pin channel. All other bolts run on channels to keep the bolt carrier from tilting.

You have to control the bolt rotation to keep it from twisting while the stripping lugs are stripping the next cartridge from the magazine besides controlling the BCG from off axis forces. They are several ways to keep the bolt from rotating early besides tracks in the receiver: the AUG uses a bolt sleeve, the F2000 uses the spent cartridge rammer, and the Tavor uses a control rod.

There are lots of possible solutions to every problem of firearm design. All of them come with compromises.

BSW