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New Member and MCX Issue

This is a discussion on New Member and MCX Issue within the SIG Sauer Rifles forums, part of the SIG Sauer Forum category; 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 ...


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Old 03-17-2017, 04:26 PM   #1
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New Member and MCX Issue

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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Old 03-17-2017, 06:13 PM   #2
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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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Old 03-17-2017, 06:21 PM   #3
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Welcome aboard.
Get a bigger hammer.

Good luck.
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Old 03-17-2017, 07:09 PM   #4
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Old 03-17-2017, 07:12 PM   #5
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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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Old 03-17-2017, 09:30 PM   #6
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Quote:
Originally Posted by jmornoinf View Post
/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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Old 03-18-2017, 04:12 AM   #7
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Old 03-18-2017, 05:25 AM   #8
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Maybe check the rods that the springs ride on and make sure they are placed into the ends correctly.
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Old 03-18-2017, 06:35 AM   #9
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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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Last edited by Rutherford2; 03-18-2017 at 06:44 AM.
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Old 03-18-2017, 07:35 AM   #10
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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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Old 03-18-2017, 08:50 AM   #11
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Old 03-18-2017, 10:23 AM   #12
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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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Old 03-18-2017, 11:18 AM   #13
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Does you bolt lock on the carrier like the picture? If so, you more than likely have an incompatible buffer pad. The rifle can fire in this state, but will not lock on a magizine when empty.
Attached Images
File Type: jpg IMG_4325.jpg (3.02 MB, 48 views)

Last edited by jmornoinf; 03-18-2017 at 11:40 AM.
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Old 03-18-2017, 12:44 PM   #14
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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
Attached Images
File Type: jpg SIG MCX Bolt Lockback with Mag.jpg (117.1 KB, 166 views)
File Type: jpg SIG MCX Lower Buffer Pad Front.jpg (196.7 KB, 173 views)
File Type: jpg SIG MCX Lower Buffer Pad Side.jpg (193.3 KB, 166 views)
File Type: jpg SIG MCX Upper Charging Handle Spring Rod Plate Back.jpg (124.9 KB, 166 views)
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Old 03-18-2017, 01:43 PM   #15
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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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