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Hello! New to Sig Talk and new owner of a MCX rifle.

Unfortunately, having an interesting issue. I will be calling Sig on Monday, but figured I'd see if anyone else had a fix.

I looked around the posts here and did some google-fu, but although some others had similar issues, no one really spelled out how they fixed it. Any help is appreciated.

First off, the MCX had the carriage assembly replaced at the dealer. Taking it home, I stripped, cleaned and lubed the rifle. The issue: I cannot physically move the bolt carriage back far enough (using charging handle) to engage the bolt lock on the bolt - instead it will engage on the carrier body. Obviously doesn't matter whether there is a mag inserted or not, and as far as I can tell it would appear that the hard rubber recoil buffer is not compressing/allowing the carriage enough rearward travel to properly lock the bolt (she's about 1/8" short).

Anyone else have this issue and if so, how did you fix it?

Thanks!
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Welcome to Sig Talk!! While I haven't had your bolt lock back problem, as you mention - several here have. You should make sure the carriage assembly and charging handle are properly installed - like the charging handle isn't somehow modified and crammed-in upside down!!??..

It would be difficult to install these wrong, but anything's possible. Otherwise, I really don't think this is an issue you can or should resolve on your bench. You should call SIG, they'll either ask you to send them your rifle, or at least the carriage assembly.

I've enjoyed my MCX since 2015, it's a fun rifle. Obviously, you've bought your rifle during this recall cycle, that's creating issues. Sig will get ya running, but you will have to work thru this process...

Cheers
 

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/Users/jasonmorneault/Desktop/Sig MCX/MCX Issue/IMG_4325.jpg

I had a similar issue before. Sounds like the bolt buffer in your lower is not interfacing with your new bolt carrier. You are going the have to replace the hard rubber "puck" in the lower in order for it to work properly.
 

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/Users/jasonmorneault/Desktop/Sig MCX/MCX Issue/IMG_4325.jpg

I had a similar issue before. Sounds like the bolt buffer in your lower is not interfacing with your new bolt carrier. You are going the have to replace the hard rubber "puck" in the lower in order for it to work properly.
I think this is what I m experiencing. I can't pull my BCG far back enough with the charging handle to allow the bolt catch to engage the bolt face. This is after getting a replacement gen2 BCG. It's probably why I m getting empty mag lock back issues. This was never an issue with the gen1 BCG. Probably because it was shorter. I may need the red rubber buffer replaced.

I guess I will need to suck it up and actually send the rifle in.
 

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Discussion Starter · #8 · (Edited)
Thanks for the welcome and for the suggestions!

Everything (carrier, bolt, CH) seems to be assembled in accordance with the MCX manual - with the exception that the recoil rods are not pinned.

I wonder if some of the similar issues people were experiencing were mis-diagnosed as a short stroking or magazine issue. I haven't shot the rifle yet, but I imagine this is what would happen:

1. Closed bolt, insert mag. Rack CH - rifle chambers since you can pull the bolt just far enough past the back of the mag.
2. Pull trigger, rifle cycles.
3. On empty, locks back on carrier instead of bolt.
4. Insert new mag, hit bolt release, bolt skips over top round since it is sitting above the mag - does not chamber.

I've let the dealer know of the issue and will definitely be calling Sig next week.
 

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Rutherford,

I had the same exact problem. You need to call Sig and have them send you the new BCG buffer pad for your lower. The problem you describe happens when you pair a low serial number lower with a newer upper... Sig never told anyone they changed the design of the first Gen BCG and lower buffer. I have many MCX parts and pieces and now have seen three different BCG designs.

If I could figure out how to attach pictures I could show pics of what I am talking about.
 

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Jmornoinfo and Rutherford,

Excellent explanation! This is exactly what is happening to my gen2 BCG. I ve been troubleshooting for the last couple of months. I can't believe it is that simple -- BCG buffer pad. When I charge the BCG it is "short cycling" and only the carriage body engages the bolt catch.

I m going to request a box Monday from Sig CS and send my rifle in. No more troubleshooting and trying to replace parts on my own.
 

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No. looks like your bolt's locked on the Mag's follower or the underside of the bolt carrier. That's at least an inch forward of the correct position behind the bolt catch. Here's what mine looks like, and it works fine.



The Lower's Buffer Pad may be something to check. Make sure it's fully screwed-in and sitting flush against the back of the lower. Here's the front of mine. It's an inch across



And, 5/16ths thick



Here's the back end of my upper with the Carriage Assembly installed



Hope this helps, but you should talk to SIG Monday.

Cheers
 

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jmornoinf,

Mine locks back exactly like your picture and with PMAGs it rest on the follower. It has to be the buffer. I m glad you shedded light on this. It was hard convincing the Sig CS rep. My bolt locked back with Lancers b/c the follower was different. However, the bolt catch was engaging the carrier body. This all happened after the BCG upgrade.

kansascity,

Appreciate the pics and thanks for all the other ones you posted. They were very helpful for troubleshooting. My buffer is actually red ring and juts out further than yours. I m tempted to shave it down instead of sending it in. The allen screw doesn't look very robust and I m nervous about stripping it.
 

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Gotcha, my buffer's attached with a 1/8th hex head screw. I use a key tool so as not to put too much torque on this screw. A dab of grease on its threads should hold it tight with no more than a snug hand tightening.

Cheers
 

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Discussion Starter · #16 ·
Awesome pics - my buffer is definitely a different color than the one in the photos.

I called Sig today and explained the issues. They sent me a return label and RMA number, so back to New Hampshire she goes.

I also noticed a small gouge in the cam pin track of the upper receiver just behind the steel insert. Also, the manual says the recoil rods should be pinned, but they are not on my carrier assembly. There is a small bit of wear on the firing pin retaining pin as well.

Sig quoted 3-4 weeks turnaround so hopefully all will be fixed.
 

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Yeah, my recoil pins aren't pinned either.

Sent back my rifle today after talking with SIG CS. He was super helpful and took down notes. He quoted around 3 weeks. I believe SIG is trying to make everything right. They want you to be pleased with a reliable rifle. I really hope all will be fixed.....
 

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Awesome pics - my buffer is definitely a different color than the one in the photos.

I called Sig today and explained the issues. They sent me a return label and RMA number, so back to New Hampshire she goes.

I also noticed a small gouge in the cam pin track of the upper receiver just behind the steel insert. Also, the manual says the recoil rods should be pinned, but they are not on my carrier assembly. There is a small bit of wear on the firing pin retaining pin as well.

Sig quoted 3-4 weeks turnaround so hopefully all will be fixed.
Good!! SIG well get ya fixed!! Couple thoughts for ya. Your lock back issue will be resolved, NO problemo!! SIG will probably consider the 'small gouge in the cam pin track' as normal wear though. They did replace my MCX upper receiver, but its wear was significant.

This cam path upper gouging is an issue with all piston driven ARs. I have 4 AR piston uppers and they all have it. In my SIG516 with 100 rounds, it's a small wear mark, and 516 with 1,000rds, it's a gouge along most of the cam path. PWS AR10 shows the same wear.

I've recently, as well as a bunch of fellow Sig Talkers switch all my standard cam pin to POF's Roller Cam Pin. This roll pin provides immediately smoother action you'll notice with your charging handle, and almost no upper receiver gouging. Sorry if I sound like a POF salesman, but these are pretty neat tricks

https://pof-usa.com/shop/upgrades-accessories/roller-cam-pin-p415/

Another point of confusion is the new Carriage Assembly's recoil spring rod's plate. Yeah, the original Dec 22 '16 recall flier did describe these rods as pinned into the end plate, and not a removable part. BUT, that's not what has been installed with these new Carriage Assemblies. There is NO change to this plate, and the rods are still just snapped-in.

So, be very careful when reassembling these recoil spring rods into this plate, to make sure both rod ends are securely snapped into place!!

Also, should mention that I've sent MANY guns back to Exeter - many times. I own ~20 SIG pistons & rifles, and can think of only 1 or 2 that haven't made at least one trip back to NH - although there are several that have made >1 trip too. Heck, my local Fedex folks probably think I'm the state patrol's armorer or something!!

It's probably my combination of frequent range times, and perhaps being a little OCD too. But it's nothing to worry about - They usually return these guns in better shape than I sent, and on a faster turn-around than I expected...

Cheers
 

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Discussion Starter · #20 ·
Hey folks! Got the MCX back today and figured I'd write a quick update.

Sig's write up stated that they replaced the buffer with the updated version and mentioned that the firing pin retaining pin wear was normal and would even out. So if you've got an orange colored buffer with the new BCG, it definitely needs to be replaced.

They didn't say anything about the receiver gouge behind the steel track, so I guess it wasn't significant enough. I went ahead and replaced the cam pin with the POF rolling version, so we'll see how that holds up.

The rifle locks back like it's supposed to so I think it's time to go shooting! I haven't fired a single round through it yet...
 
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