M*CARBO Brotherhood

First Look! New SUB-2000 Hinge Optic Mount - Prototype


My Gen2 has the metal glued on Castle nut and I use the MCARBO sling mount without a problem. The sling mount actually helps hold the castle nut in place. I use a quick detach single point sling on mine and it has never come loose.


Same exact setup here and it works great.


If you needed to open the reciever to do some work, wouldn’t the glue be a problem? My castle nut came loose after my first range trip, 130 rounds in. Do you folks think the sling would be enough to hold it without glue?


It’s not an issue, especially if you have the MCARBO armorer’s wrench. It cracks that nut right off very easily. Clean things up and apply some Locktite 380 Black Max, which is what Kel-Tec uses, and you’ll be fine. Not sure about that last part, though. I’d always use the Black Max stuff when reassembling.



If your castle nut does not have a couple slots in the side then the sling mount over it will not be able to clamp the castle nut down to the receiver.


@Soundave If you don’t have MCARBO’s receiver wrench I recommend you buy one, it makes removing the nut so much easier. Otherwise it’s hammer and punch time. When regluing you have to remove most of the black Loctite because it won’t adhere to itself. Some have cut 2 slits 180 degrees apart in the nut and then used the single point mount to help secure it. Saves having to buy their expensive one. I use the same as @Wedge and @Texprep, no problems here.


Actually last couple of times I did not clean things up and I did not apply Loctite.

This way loctite mass creates sort of grooves, which allow to screw the castle nut on and tighten it well with the MCARBO armorer’s wrench - and it holds pretty well. Especially with the sling mount.

I know it sounds weird, but somehow it works for me. Maybe because I was extremely generous with loctite the last time I used it.


I don’t have slots in mine and with the sling mount clamped down on it, it won’t move. I mounted it towards the front of the castle nut and it doesn’t move. Even just dropping my S2K to my side on the single point sling while transitioning to my pistol the crown nut has never come loose. I did use a little glue, but didn’t get carried away. Probably about the same amount that was used from the factory.



I use the PSM slotted castle nut with PSM single point sling mount/clamp, not cheap but beefy and well worth it. If your castle nut stays put than it is probably from the Loctite/glue you use and not the sling mount you are using. The sling mount/clamp needs to cinch down on the castle nut and it can not do that without slots in opposite sides, or maybe one slot might work. None the less, your clamp is not coming loose and that’s good.


I can take my sling mount off and remove the nut with little effort, metal does flex. I used to think the same way you do until I saw first hand directional drilling. To see a 4" diameter section of drill pipe flex enough to directional drill in a shale formation amazed the s**t out of me. If you look at the MCARBO sling mount even when it’s tightened down on the castle nut, there is still 1/8" more adjustment. The heavier sling mount actually squeezes the castle nut slightly locking

it in place. Also, because I placed the mount at the end of the nut closest to the receiver, it actually crimps down the end of the castle nut. I’m very confident it would hold without any glue.


I’d be careful with that. The steel fastener will easily strip out the much softer aluminum sling mount if tightened down too much.


I will admit there was a little pucker factor the first time I tightened it down, but T6061 aluminum is pretty good stuff. I have machined many aircraft and helicopter parts from that material. Figured if I stripped it out I could just get a slightly longer Allen screw and nut.


@Texprep Hey Mike,if that works for you then that’s great but I would agree with what Scott is saying…I don’t like little pucker factors, mine would have stripped out. :wink:



I understand completely, to each their own. If it hadn’t worked for me, I was going put the castle nut on my mill and slot each side of it like those I’ve seen offered by aftermarket companies.
To give a little background of myself I’ve owned and operated a machine shop for 28 years. I do manufacturing for oilfield, aviation, power generation turbines, and custom tooling. I work closely with OEM metalurgists and design engineers. I do have a pretty extensive background in materials. I have had failures, but from failures have come greater successes.
If I was making that sling mount, I’d make it from titanium. It’s lighter and stronger but would be cost prohibitive to market and honestly overkill. Material is more expensive and machine time would be longer.


One thing I have started doing when ever possible when working on firearms is to use nylock nuts where possible. They don’t back out. Lock tite has its pro’s and con’s but a plastic inserted nylock will hold and keep its torque. How well it would do handing near a hot barrel, I can’t answer that. Tab washers are also a good means of securing small nuts. Does it mean a harder install, most likely yes, but in the end it should be a better fix.


Nylock nuts are the greatest thing since sliced bread. I use them almost exclusively in the products I manufacture in my shop that require assembly.


And marine grade Stainless are the bomb but they cost, I use them on my kayak where possible. No rust, and no backing off, I use an oversized fender washer as a backing, it won’t pull through if I hook with something of size.


Sorry, I put this in the wrong place.


Ive been missing a good bit these past few months. Just ordered the new charging handle with the offset screw doo-hicky. Hoping for the new sticker, forgot all about that forum and i was pretty hyped on the design.
Can’t wait for the hinge mount!


So when are we going to get updated pics of hinged optics mount waiting to see updates