Howdy ??

I’m for availability or info on hammer catch cut in the BCG? I have seen a couple archived posts, but I’m not seeing a way to reply to them or PM the creator or the folks that commented.  

I know there was a Dude that signed the bottom of his posts with Scott. And he had parts and a bcg he was getting the dimensions and heat treat in to make some parts.  Does anyone know how to get ahold of him or know if that project turned out? I think his posts were in January of ‘21…

I see Tactical Innovations AM15  has a fcg offered on there website, but it’s out of stock.  Does anyone know experience with that kit?

Thank you for any advice and help….

We are actually using a modified version of the Colt open bolt system for another machinegun platform. I ended up with two sets of the fire control from Tactical Innovations that have been modified for use with another firearm transferable registered receiver. We have sent a complete set in an adapter for the M10, into the FATD of the NFA Branch for determination.

The biggest difficulty of the project is the sear notch on the bottom of the carrier. I managed to find an original Colt open bolt, bolt carrier assembly. It was $1,000.00 plus shipping. We had a local EDM shop profile the notch in the bottom of some carriers. We had some 5.56X45, 7.62x39, and CMMG rotary delayed blowback carriers modified. We modified the closed bolt system to be able to be used with both closed and open bolt versions.

The other issue was the hammer release trip in the open bolt auto sear assembly. The Tactical Innovations auto sear assembly is missing the hammer release lever. It doesn't need the lever for the AM15 upper as the bolt in that system has a fixed firing pin  protrusion in the face of the bolt. The Tactical Innovations auto sear only needs to release the bolt, not release the hammer. The original Colt open bolt system was designed to release the bolt carrier to strip and chamber a 5.56X45 round, then release the hammer just as the closed bolt system does it.

Since this was for another machinegun receiver, these modified fire control parts were not going to be installed in a M16 receiver. It is my understanding that the standard open bolt fire control can be very hard on the M16 receiver. Please let me know how I might be able to help you.

Scott

Just the man I was looking for! Just my luck. Thank you for responding sir.

How were you able to modify the bcc’s to work in both open and close operating configurations?

To be clear, for duel use (open and closed bolt operation) of the same carrier, I have found that the top of the closed bolt hammer needs to be ramped. What I mean by "ramped" is, I left the spot of the hammer that strikes the firing pin. Above that point, like the ramp on the bottom side of the carrier.

The factory rounded shape of top of the hammer is still too sharp for the notch in the bottom of the open bolt carrier. I simply shaped the closed bolt with a dremel. If we get a positive determination we'll be shaping the closed bolt hammer for both systems, we will be putting the ramp in with EDM.

Scott

I have owned and shot 2 open bolt Colt LMG guns and have never had an issue with wear on the trigger group holes in the receiver. This is been speculation on the internet about this ever since they were introduced but zero proof has been given. I run them with the proper green recoil spring and hydraulic buffer. The guns are always brought out to machine gun shoots and used heavily. Both of my guns were on NIB receivers and I keep a close eye for any trigger group hole wear and have not experienced any.

Do you use KNS or factory stainless steel fire control pins?  I am converting the open bolt system for use in another machinegun platform. I ended up spending $1,000 on a used Colt open bolt carrier group. With that as a template, I have had several carriers modified for use with the open bolt system, including 7.62x39 and CMMG RDB. Hopefully the ATF will like them. I am glad to hear of your great experience with the Colt open bolt system. I had great luck with the Colt 9mm blowback system but I used KNS pins and ramped bolts in my Colt M16  and stainless steel pins with my DIAS in a Colt 9mm lower with the larger 0.169" fire control pins.

Scott

nope, no need for them since they do nothing.

Would you please clarify your answer? Apparently, Colt seemed to think that stainless steel fire control pins were worthwhile in their 9mm guns. I have owned three Colt 9mm carbines. All three had stainless steel fire control pins. The pictures I have seen of factory Colt 9mm subguns all have had stainless steel fire control pins. With the Beta-C mag installed on the gun you posted the picture of, I could not see the fire control pins. So I wondered if Colt also used stainless steel fire control pins with the open bolt mechanism. There is also the issue with the Colt 9mm system of the bolt catch being damaged from the 9mm bolt being shorter than the 5.56X45 bolt group. It looks to me that the notch in the carrier catching the notch in the top of the hammer would put that force squarely in sear to the hammer pin. Could the internet lore of the Colt open bolt system being hard on hammer pins be true? I have no idea. Thank you for posting your actual experience.

It is great to hear from someone with that direct experience with the Colt open bolt system. You are the first person that I have seen post with that first hand experience. Thank you for posting. I would appreciate any details of your experience with the open bolt mechanism. Our system of open bolt operation is based on that system. My search prowess is lacking. I have found many articles on the principles of function but very little of practical use/experience. Thank you for sharing.

Scott

Well I was being a little flippant with my answer but its the truth too.

Let me first say that every thing you hear about the Colt open bolt firing group on the LMG wearing out receivers is internet rumor. When Colt first released the open bolt LMG kits I got one immediately. Every one said they would wear out my lower receiver. My first open bolt LMG was installed on a brand new Colt factory M16A2. The gun had never been fired and did not have any wear on it. I kept track of the wear on the receiver and I could not measure any appreciable wear. The gun had about 40K rounds on it when it was stolen. I replaced the gun, since they are incredible to shoot, with the flat top version shown in my previous post. The receiver was once again a new never been used A2 that had never been assembled into a gun. Its one of the receivers that Olympic Arms replaced with new ones before ATF changed their ruling and they had to stop. I've had many rounds through this one as well without any noticeable wear and I do keep a close eye on it as well.

The key to the open bolt fire control group is the recoil spring and hydraulic buffer. Never use an open bolt group without them. The hammer/sear will not catch the bolt group when using the charging handle until the hydraulic buffer compresses. What this does is reduce the bounce back from the buffer hitting the end of the recoil spring tube to near zero. This allows the bolt group to settle onto the sear with very little forward momentum. This keeps the wear to minimum as well as impact on the hammer pin. The 9mm guns suffer from excessive bolt group velocity causing the hammer to impact the disconnector/trigger breaking pins. Excessive bolt group velocity is one thing the open bolt LMG does not suffer from.

I would suggest that the receiver including all the trigger pins be hard anodized. If you have a converted receiver that has raw holes they might wear because they will be softer.

Thank you very much for sharing your experience. My application does not allow the use of a rifle length buffer and spring. I think that I might be able to use a carbine style hydraulic buffer. Since this is for another firearm platform, I have more flexibility with how the open bolt mechanism is used, but not installed in the registered part. We got the system to function using a JP Enterprises PDW captive spring system and a PDW H2 recoil/buffer system. We might be able to adapt a Kynshot hydraulic buffering system. The recoil/spring system only has a PDW length. We need a positive determination to move forward with development of the recoil/buffer system. Again, thank you so much for sharing your experience with the Colt open bolt system.

Scott