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2/21/2017 12:38:46 AM EDT
Received my replacement assembly this week so I took the rifle to the range to give it a try.  The bolt would not stay open on either the unsuppressed or the suppressed settings after the last round.  Also, when the bolt is locked back manually it will not load a round when a new magazine is inserted.  Upon inspection, it can be seen that when the bolt is locked back the face of the bolt is over the round in the magazine, not behind it.  The only was to load a round on a new magazine is to pull on the charging handle.  This will then pull the bolt and carrier far enough to the rear so that it is behind the round and will then pick up a round when the bolt and carrier are permitted to move forward.  I tried about ten different magazines and it occurred on all of them.      

Has anyone else experienced this with the replacement assembly?  Is this the new normal for the Sig MCX?

It looks like it is time to call Sig.  I liked the rifle when I bought it.  I was OK with the never appearing additional parts of this allegedly component swappable rifle.  I was also OK with the whole recall and replacement thing because hey, stuff happens.  But after all of this to have a rifle that clearly has a problem is very frustrating.

Update:  The bolt catch is catching the carrier but not the bolt itself.  There is no way to get the bolt catch to catch the bolt.  I have tried all of the various brands of magazines I have at home.  No type of magazine, polymer or metal, will catch the bolt, and there is to way to manually catch the bolt with the bolt catch.
2/21/2017 7:16:58 AM EDT
[#1]
Do you have a red colored carriage bumper? If so, it will need the newer white one. Sounds like that is the issue.
2/21/2017 10:49:30 AM EDT
[#2]
Mine is doing the exact same thing as the OPs. I called Sig CS and was told to send the new carriage assembly back. Still awaiting their email with the RMA/call tag. I guess I'll be down another 3 weeks.
2/21/2017 6:31:17 PM EDT
[#3]
What bumper did you have?
2/21/2017 9:11:14 PM EDT
[#4]
The red one. I informed CS of this and about needing the white one and the guy I talked to never heard of such a thing. I will email them to re-iterate it.
2/22/2017 7:30:32 AM EDT
[#5]
The red one is close to 1/2" thicker than the newer white one. That is your problem with bolt travel, it will fire and do a functions check but not have enough travel to lock manually or via the magazine follower. They will require the rifle to be sent back as they state the screw can be hard to remove.
I don't know when the newer one was utilized but mine was under 1k ser num range and had to go back.

Which has to mean that the new carriage body is longer than the older one.
2/22/2017 10:37:55 AM EDT
[#6]
Unfortunately I am in California, so I cannot send the rifle to them and them send it back (according to Sig). I hope they send me the white bumper! Thanks for the help so far. I guess worse case, I can trim the red one..
2/22/2017 10:40:41 AM EDT
[#7]
What is this white bumper you are talking about? Far as I know, they are all still red and there is no new updated bumper. Whoever has a white one, it must be a factory flaw and discolored somehow.
2/22/2017 7:24:32 PM EDT
[#8]
Mine has the white one.  Serial number 00218x.
2/22/2017 7:41:26 PM EDT
[#9]
The newer ones are white. It's a machine screw holding it in, I'm sure a good quality bit and maybe some heat from a heat gun will allow it to be loosened. I would ask Sig if they use loc-tite.
2/24/2017 10:16:58 AM EDT
[#10]
Sig is sending me a white one and yes, recommended using heat to break the current one out.
4/16/2017 4:10:52 PM EDT
[#11]
Ran into this also after getting back my new carrier.  Have the original red bumper and it's definitely preventing the new carrier from going back far enough to get caught by the BHO.  Called Sig and they wanted me to send by entire gun in to replace the buffer, but the screw holding mine in came out fairly easily with a small Allen key.  I don't see anything on the screw threads that indicated any thread lock was used.  I've removed screws from other guns that used Loctite or Rocksett, and there was always some residue present, but nothing here.  I think mine was just lightly torqued down.

Now hopefully Sig can just send me the new buffer.
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