Does anyone have the specifications for the notch in the bottom of the Colt open bolt carrier? I  thought I had a line on one, but it fell through. I have all the fire control, but I don't have the carrier specifications. I understand that this system is very hard on the hammer pin. Any help would be greatly appreciated.

Scott

I had a bunch of specs and info on them a few years ago. I found the FCG for a good price and was going to convert my Shrike/MCR to open bolt. I’ll see if I can find the info. I doubt I still have it but I’ll look...

If you could find it, I would very much appreciate a copy.  I am willing to pay you or maybe sending you an item you are looking for if I have the specs my machinist could use to modify a carrier. This would be for a submission to the Tech Branch so I am not so concerned about buying a Colt made carrier as getting the specs to modify an off brand carrier.  Please email me if you can find it.

Scott

All I found was a bunch of pictures of the carrier. I was pretty sure I had specs but I can’t find them...

Oh well, I'll find or get the specs for the notch in the carrier somewhere. Thank you for looking.

Scott

Oh well, I'll find or get the specs for the notch in the carrier somewhere. Thank you for looking.

Scott

Here’s my advice, there’s TONS of pictures online of the notch. Buy everything and they put the notch where it needs to go. It’s not going to be that hard.

I agree. Use a cheap carrier so you're not out much if something goes south.

Update: I have found a Colt open bolt carrier. I plan to use it for the specs. I'll then modify some carriers. It was suggested to me that I also should send the Colt carrier to a heat treatment shop for testing to see  how hard and how deep and how hard the sear notch is so the modified carriers can be heat treated the same.

Scott

I would not cut an open bolt sear notch on a carrier with forward assist notches.

The forward assist is extremely dangerous for the open bolt guns since as soon as the bolt group closes and trips the auto sear it fires. The trigger need not be touched for it to fire. If the forward assist is being pushed the recoil energy is going to also be transferred into the FA plunger, though that is not as dangerous as the inadvertent firing.

I can't say as I follow what you are saying. The Colt open bolt system requires the trigger to be pulled to release the bolt/carrier group to come forward. If you where pushing on the forward assist when you pulled the trigger, releasing the bolt/carrier group, the forward assist would rub the carrier and interfere with chambering the round. The hammer would not release until the bolt is locked with the round fully chambered.

I would think that holding the forward assist forward when you pulled the trigger on a closed bolt AR would be very bad. From my perspective, forward assist would be useless with the open bolt system. It would very hard to time pushing the forward assist just as the bolt locks and the hammer releases.  I get that Colt didn't put forward assist notches on the open bolt carrier and there is no forward assist in the LMG upper.

So what am I missing. How would a forward assist on the open bolt mechanism be dangerous?

Scott

The drag wouldn't be enough to interfere with feeding or chambering.

It'll fail to cycle, but if it's being held firmly, it probably wouldn't hurt anything.

(Holding it only partially down would allow energy to build up when the bolt carrier moves aft, and the carrier and forward assist would then transfer that energy into whatever is doing the holding.)

You aren't missing anything. It'd only be dangerous to a special kind of user, and they'd be in almost equal danger with a closed or open bolt system.

@sgthatred's point is the "almost" part (like if they're trying to clear a feeding malfunction without anything holding the bolt in the open position).

Colt just didn't want it to be possible, since there's no need for it to be possible on an open bolt system, hence the slick carrier & upper.

Think of it this way. The trigger is pulled and the bolt group goes forward but fails to fully chamber the round. So its sitting there just out of battery and the "I've shot M16s before guy and know how to do this" hits the forward assist while no longer aiming the gun but most likely allowing it to set on the bipod and butt. The forward assist pushes the round home and the gun fires as soon as the round is chambered the way an open bolt is suppose to and the round sails aloft at a nice 15+ deg angle into the heavens.. Unlike many other open bolt rifle caliber guns where a malifunction like this would not have enough energy to fire the round, the open bolt M16 is still hammer fired and will go off.

I've owned 2 of these machine guns since the mid 90s.

And there's your special user, hence why infantry weapons need to be "grunt proof".

Thank you for your explanation. I am actually planning to modify a Colt style 9mm blowback bolt for use with the Colt style open bolt system. But to modify the bolt I need the specs of the carrier notch so that the blowback bolt will work with the rest of the system.

Scott

One day, once I will have finished the half-dozens of projects that I have already running and those for customers; I will try to make an open bolt conversion.

If anyone have the specs; I would be more than interested; even at a fee.

I have found a used carrier. I spent almost a grand on it. I need to have it tested to find out what heat treatment was done to the sear surface. Then duplicate the notch and hardening to the sear surface to the Colt 9mm bolt.

Scott