Broken Charging Handle


@Kona @Johnksg

To be fair, my original idea was for a layer of Delrin or similar material bonded to my redesigned steel charging handle to minimize the friction resulting from the now tighter tolerances of that design. But if the material (Delrin) is capable of handling the job there is no reason not to make the entire CH out of it. The paintball experience suggests it may be as the recoil of those devices actually can be more punishing to equipment than a real firearm.

Regarding @Kona’s point, the only thing holding the CH in place and the only thing that can hold it more snugly against the bolt tube is the recoil spring and the associated notch in the CH, the design of which differs in the recoilless CH from other versions of the CH. See photo below. That fact alone could explain the difference seen in @Kona’s photo - and why a better comparison would be between two CHs of the same type - but his point remains a valid one. If the notch tolerances vary, so will the snugness with which the CH is held against the bolt tube by the recoil spring. There is little that can be done about the OEM CH tolerances, but those of the three MCarbo CH variations are completely within its control. If it maintains consistent (and tight) tolerances, each and every CH, revised or otherwise, should ride along the bolt tube at the same distance from the tube without regard to the firearm on which it is installed.

That leaves as variables only the dimensions of the bolt and bolt tube. While some variation in the bolt tube is possible, any substantial variation of the diameter (which is what mainly matters here) is mathematically improbable; at least to the extent it would significantly impact the snugness we are seeking to achieve. As a practical matter, if KT allowed substantial variation in this parameter it could not sell a replacement heavy bolt with any assurance it would fit all of the firearms sold. And if the diameter does not vary, neither does the circumference.

I do not have enough knowledge to comment specifically on the bolt itself, but relevant deviation would result only to the extent the recoil spring channel was drilled off center or oversized. We do not know the acceptable KT tolerances for that operation but I would expect them to be fairly strict.

Despite the foregoing I am concerned about @Kona’s comment that it works because the tolerances aren’t tight and will stop working if you make them tight. Do you stand by that in light of the above Logan and would you please explain if you do?



@JoeFridaySays Let the user sand the shaft to fit. If the radius matches then you will have a tight fit which a crucial element. :+1:

Heck of a solution Brian! (Wish I had thought of it.) :grin:

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@JoeFridaySays I honestly can’t say one way or the other whether a CH riding the receiver would work or not. An example would be the performance trigger bar MCARBO makes. Sure it technically can go in any Sub 2000 but the tolerances are tight and each one even though they fit, act completely different. My original sub has a very short wall and a rock hard crisp break. My second sub has a little take up still and a good amount of mush with no crisp break. Both have the performance trigger bar and both sit completely different against the sear.

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Thanks. The truth is I know very little about firearms compared to you guys and am trying to learn. It is clear to me that there is a lot sloppy in the tolerances of the S2K so I went through the exercise in that post and determined that MCarbo could probably control the ones that really mattered. But as you say, just because it technically should work doesn’t necessarily mean it will. But am I missing anything obvious beyond that?

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@JoeFridaySays i don’t think you’re missing anything. It’s definitely all valuable stuff to talk about. I don’t think iterating the CH will ever stop until they consistently stop breaking.

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Only problem with delrin is it’s brittle. It shatters very easy when it’s small/thin pieces. I’ve used it in a ham radio go box, it worked good but when it gets thinner it shatters.

How about making the shaft of the CH out of durable steel and thread the bottom half so a delrin CH can be screwed on and mounted and locked with a set screw. Thus much more options or diy styles/sizes and a much lower fulcrum center.


@catfood, et al. We need to be careful to not outthink ourselves or exceed our expertise here. Your design sounds logical, but I lack the education and knowledge base to fairly evaluate it. The bottom line is that if we choose a design that rides closely and distributes stress over the radius of the bolt tube a friction reducing material will be required. It can be bonded or otherwise integrated with a steel CH or the CH conceivably can be made entirely of that material. The particular choices may dictate the material used. For example, nylon is often used in such bonding applications where reduced friction is the goal but you would never build an entire CH from the material. But nylon and Delrin are hardly the only choices and may not be the best ones.

We have presented and discussed the ideas and should continue to do so, but unless we are prepared to return to school or do far more research than I am prepared to do the actual testing and choice of materials may be better left to the experts in material sciences and MCarbo. I do not know the probability of a piece of Delrin flaking off and blinding a user as the bolt reciprocates but that is what experts in materials science are paid to know and to advise companies like MCarbo about before they incorporate such materials in their products. The same goes for its tensile strength, deformation properties, fatigue resistance, etc. and how those compare to alternative materials like ABS, acetal or polyester resins.

I freely acknowledge that I am out of my league when it comes to recommending a particular material. That is why I said in my original post that my suggestion would require bonding “a Delrin or similar surface” to reduce friction because I lack the expertise to know what particular material would be best.

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@JoeFridaySays, I catch what your throwing. I’m no metalurgist or material scientist by any means either lol.

With that being said, nothing hurts throwing ideas out there, if no one speaks up then great ideas my be passed by. With ideas everyone is throwing out there might be a few worthy of actual testing.

My cz scorpion uses a polymer bonded charging handle, though it’s not reciprocating it takes a beating getting hk slapped ect. I am going with my personal experienced with delrin, it’s a great material and is self lubricating and resists heat. So is cutting board material though a lot softer. Cool thing about delrin is it’s super easy to machine. Just my .02 cents worth.

On a side note, I might actually build the CH with delrin lower half, seems easy enough.

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Let us know how it works out. And be sure to always wear your safety glasses in case it shatters or separates at the join.

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@JoeFridaySays will do,

Gearing up as I speak lol.

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@Dane @Johnksg This is actually what I was referring to. Would the tube help reduce the amount of force on the CH thus helping to eliminate the breakage. Again, is this an overall design flaw that they can’t seem to compensate. They went as light as possible but making it a bit more fragile. That goes with a of things that sometimes shouldn’t be combined.


@Aravena Sorry if I didn’t catch your drift the first time. That is the value someone like @JoeFridaySays brings to these conversations is that he can make an illustration.

@CatFood while a bonded CH using a combinations of materials may be the way to go I would really like to see a single milled piece of something like delrin or similar material tried first for ease of manufacture and cost.

It would also be beneficial in my opinion for the exact reasons @Kona points out. KT is always making small changes in design, materials, etc in a somewhat random fashion. If MCARBO is going to make a replacement part it must be one that can be easily modified by the user to give the best fit to their individual weapon.:grin::+1:

The key I believe in using a dense plastic is that it must be resistant to friction and heat. If the shaft could be sanded/filed down by the user to bring those radiused outriggers into contact with receiver then you would have a much greater surface area and not be placing the stress of every single impact upon the point of contact of the shaft and the handle.

It would be worth a try.



You have mentioned user customization of the shaft by sanding, etc. in a couple of posts and I am confused. The CH is held in place by the recoil spring. See illustration below. As I note in the post above, I would expect variation in dimensions of the bolt and bolt tube would be minimal – correct me if that is wrong. The problem in achieving a tight and consistent fit then would be variation in the dimensions of the CH shaft which MCarbo can control. If the notch that engages the recoil spring (shown in blue) is consistent and the length from the notch to the base of the CH (shown in orange) is also consistent, then each CH released by the Company should fit the same in each firearm without the need for modification. The only thing that would alter that result would be variability in the bolt tube or bolt which again I think is unlikely to be substantial for the reasons previously stated. Of course, the slack inherent in using the recoil spring itself as the method of holding the CH in place introduces some variability. That also could be minimized by adding a solid end piece to the recoil spring where it engages the CH.

What am I missing?


@JoeFridaySays As you pointed out the recoil spring provides the tension to hold the CH in place. As @Kona mentioned there are some variances in tolerance between different S2Ks.

For the tabs on your charging handle to work they would have to ride over the receiver, to actually be in contact.

The radius of the receiver is a constant. That is the one part that will be the same on every S2K!

So if the charging handle was made out of a material that could be sanded/filed at the base (the part in black on your illustration) that could allow the user to eliminate any “gap” that Logan was pointing out. As the CH would ride slightly deeper in the hole in the counterweight.

We are talking about a small fraction of an inch.

The other possibility is to make the shaft of the CH one solid piece and make a corresponding “u-shaped” tab on the end to engage the CH shaft.

However, back to your point. I think you are correct. The tolerances or the receiver, counterweight hole, and point of contact for the spring are the same. I think what Logan is referring to is their are differences in all of the various CHs out there and how deeply they ride in the hole!:grin:

I hope my explanation makes sense? Lol


@johnksg seems like a lot of work on the user end. You have to make parts to the lowest common denominator. When you create a part that requires work on the user end, you immediately lose a significant portion of the consumer market. Consumers will take the path of least resistance and if that means buying a new CH that is “plug and play” over sanding and filing and manual labor to get a stronger CH working properly, people are just gonna buy a “plug and play” part instead.


Agreed. And that is a problem that is totally within the control of the CH manufacturer. My point is that once it is addressed, the concern about fit and variability largely vanishes and customization by the user would not be required. We are talking here about a specific CH design to be made by a particular manufacturer who would have absolute control of the critical tolerances affecting its fit and function.

Also, since the CH would be unidirectional a u-shaped engagement slot is the way to go for lots of reasons. It is more secure and removes less material leaving a stronger shaft.


@JoeFridaySays and @Kona I agree with both of your statements.

If the CH is made to consistently seat into the depths of the hole so the “wings” are in contact with the receiver tube then no modifications would be required.

But if for some reason it does not seat properly, then a piece of sandpaper flat on a table and draw the base of the shaft over it a few times should rectify that.

I think all of the variances in tolerances we are seeing are in the CH itself.


It occurs to me while reading this that these MCARBO parts are still fairly new. Going by the YouTube dates of the installation videos, the double finger CH is about a year old; the RCH about 7 months. The data is really just now coming in and we’re the data! We represent a very concentrated user group of these products - we buy’em, use’em, break’em - and talk about it here.

We have shooters here putting thousands of rounds down range with these products, a vital proving ground for reliability. And we’re seeing it first hand: shoot the snot outta the S2k, it goins to break! Shoot it once a year it probably ain’t gonna break.

Don’t forget MCARBO’s lifetime guarantee if it does.:+1:


@JoeFridaySays also makes another interesting point regarding the spring.

When the US Army wanted to slow down the cyclic rate of the Thompson in 1923 and again in 1941 they primarily accomplished this with a heavier spring versus modifications of the counterweight.

All S2K mods I see seem to revolve around changing the weight and not the tension of the return spring.

An MCARBO heavy spring with the u-shaped engagement hook would allow for a thicker CH shaft without adding extra weight to the S2K.


@Boomchucker We are the beta testers! (Especially @Kona) :+1: