Sig MCX won't eject

Run it very wet for first 500 rounds or so.
Does it cycle freely by hand?
If so,
Check to see if the gas block is properly aligned and installed.

Picture of the cases would help.

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Run it very wet for first 500 rounds or so.

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Don't ever do this with ANY AR-type rifle, or semi-autos in general. An AR will run reliably bone dry if assembled correctly with the right parts, dumping oil on it only masks the problem and can result in catastrophic bolt failure. Take the rifle back to where you got it and let them check it over or send it back to SIG with some of the brass so they can fix it.

Did you get it new or used? What version? Has it had the recall on the bolt done?

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Don't ever do this with ANY AR-type rifle, or semi-autos in general. An AR will run reliably bone dry if assembled correctly with the right parts, dumping oil on it only masks the problem and can result in catastrophic bolt failure.

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This is SO wrong. Just so so wrong.

LINK

Don't take my word for it, here is a link from Larry Vickers. Love or hate the guy but he does know some stuff about the AR platform. I also was told (in-person) by Reed Knight, Sr. that the AR platform is best run wet. That guy DEFINITELY knows what he is talking about.

BTW, how the hell would lubrication cause a "catastrophic bolt failure"? I am really curious to hear that one.

Man you are correct; the one thing an AR platform DOESNT like is going dry!!! That’s pure unadulterated phooey. They are designed to be scuba’d up and swam with and the worst environment for them is the sandbox when they overheat with no lube.

I don't give a sailing shite what you or Vickers thinks. Neither you, him, or Reed Knight have seen the research conducted with instrumented rifles to measure and characterize bolt-face forces while these rifles are firing. The research was conducted specifically to find out what happens when these rifles are over-lubricated and nothing good comes from the practice. It's actually common sense why these rifles (any firarm except some protype Garands actually) aren't supposed to be run wet, that excess oil gets into the chamber, that interferes with how the brass "grips" the chamber as it obdurates during the firing phase. That grip transfers most of the rearward force of the cartridge case to the chamber instead of the bolt-face. Oil away that grip and the bolt goes from seeing somewhat less than 3,000lbf to nearly 6,000lbf. You just explain to me what you think is beneficial about that and how it impacts the fatigue life of the bolt. In combat, a wet rifle with the potential to have a bolt fail is one thing, that extra force allows the rifle to keep cycling even full of grit. Acceptable risk when considering what happens when facing incoming fire and no way to return it. None of you are running these rifles on two-way ranges though, not even Vickers these days. Lubricate according to the -10, there's a good reason why the Army explains exactly how much oil and where.

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Man you are correct; the one thing an AR platform DOESNT like is going dry!!! That’s pure unadulterated phooey. They are designed to be scuba’d up and swam with and the worst environment for them is the sandbox when they overheat with no lube.

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He's wrong, you're wrong, and every other retard that parrots this "run the AR wet" bullcrap is wrong.

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The case mouth pressure and resulting bolt force were measured for M855 and M855A1 cartridges as a function of the level of lubrication in a modified M16. The research found that the level of lubrication on the ammunition, and specifically the lubrication between the cartridge and the chamber of the weapon, greatly changes the level of force on the bolt face. The average bolt force difference between the conditions of heavy and normal lubrication was found to be 3000 lb. This force has a linear correlation to the pressure of the cartridge case and reaches a peak well before the unlocking of the bolt and projectile exit. The data showed that the effect of lubrication is not dependent on the cartridge type or temperature tested. The bolt forces increased with temperature; however, the increase was due to the associated increase in pressure caused by the elevated temperatures. These results provide direct data on the synergistic relationship between lubrication, cartridge pressure, and the force exerted on the bolt face.

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That bold part is the truth. Nothing you or any other AR15.com forum reject is going to change that.

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This is SO wrong. Just so so wrong. LINK Don't take my word for it, here is a link from Larry Vickers. Love or hate the guy but he does know some stuff about the AR platform. I also was told (in-person) by Reed Knight, Sr. that the AR platform is best run wet. That guy DEFINITELY knows what he is talking about. BTW, how the hell would lubrication cause a "catastrophic bolt failure"? I am really curious to hear that one.

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Ridiculous.
My ARs shoot thousands of rounds a year in competition. Very harsh environments. Know countless numbers of 2 gunners and 3 gunners at the local and national level. Almost everybody runs them wet. Have one custom gun that prob has 10k rounds through it. Besides a few sets of rings, only failure is a firing pin tip.
Running it wet for the first 500 rounds or so will help it break in.
Besides that, there are thousands of people that shoot these guns thousands of rounds a year that pretty much know what they are doing. They all run them wet. Some more so than others.

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My ARs shoot thousands of rounds a year in competition. Very harsh environments. Know countless numbers of 2 gunners and 3 gunners at the local and national level.

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And there are countless numbers than run their guns with only the amount of oil needed and zero problems.

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Besides a few sets of rings, only failure is a firing pin tip.

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Yeah, rings do suffer more when the bolt is being hit with a few thousand more pounds of force than it needs. Firing pin tips and extractors suffer because the brass case head and primer can be extruded back into those respective areas of the bolt face.

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Running it wet for the first 500 rounds or so will help it break in.

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Complete horsecrap. The bolt-carrier is hard chrome plated where the gas rings slide, they aren't "breaking in". Not a damned car engine with cast iron bore on cast iron piston rings and pressure fed, hydrostatic lubrication. Whatever oil left in there is blown out by the 2300 - 2600 psi of propellant gas flowing into that space and the little residue left behind just catches carbon. The only areas that are going to burnish in a little are between the bolt carrier rails and the upper receiver and that happens well before 500 rounds and soaking those areas in oil doesn't help. Breaking in, you just post up some data showing changes in the velocity of the BCG before and after this break in period because I have a hard time believing any human being possesses the awareness to detect changes measured in milliseconds...

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Besides that, there are thousands of people that shoot these guns thousands of rounds a year that pretty much know what they are doing. They all run them wet. Some more so than others.

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No shite, thousands of people bought off on bad advice and practices handed out by drill sergeants for decades to basic training recruits. I quite remember entering the firing line and having the DS hose down my rifle with a spray bottle full of CLP (that bottle somehow never appeared when the rifles needed cleaning). When I ran the ranges in the years after, no spray bottles of CLP. Rifles had a few drops on the rails put on them before the racks were loaded into the vehicles, that was it, and they ran. Some of those rifles would have over a thousand rounds put through them by the time everyone had qualified and the SPENDEX was over. No malfunctions caused by anything other than operator error at times. Civilians have choked down bad advice because they heard it from some grunt who heard it from his drill who heard it from the drill before him. Just because thousands of idiots are doing something wrong doesn't mean the empirical data is thrown out.