My M&P-10 will not lock open on an empty magazine. I have mags from Magpul, DPMS, ASC, and Brownells, and it the same with all of them. If I manually press in on the latch portion, I can (sometimes) get it to catch, but it is a weak hold and it dislodges very easily of it catches at all. Any ideas?

It's most likely your ammo. I bought a whole bunch of freedom munitions 150gr. Fmj rounds and they won't cycle. I'm having my gas port opened up and having an adjustable gas key installed by a gunsmith. Running me about $90 so I can run any ammunition. I haven't had any problems with federal, Hornady or Winchester 150gr. Rounds.

I do not have an AR10 type rifle, but I do have extensive experience with the AR15 type rifles from years of high power rifle competitions.

As you describe the condition, I don't believe that you have an ammo problem. It sounds more like there is an issue with the bolt hold open latch. Have you had the bolt lock open with any magazines? If not, it sounds like the tab at the base of the lever is either worn or broken, and needs to be replaced. If it does work with some magazines, but not others, it sounds like the follower is your culprit ... needing to be either built up or replaced.

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There has been an issue or two in the past with misaligned bolt hold open latches. There is a thread here somewhere with pictures that might help you.

If the latch looks good, I would look a magazines. If the magazines look good and function fine with .308 ammo.

It's the action short cycling due to not enough gas. You then either have to go with only .308 (the hotter powder charged cartridge) or look at opening your gas port up as mentioned above.

Sounds like a mechanical issue. If you are manually holding the bolt catch and it still doesn't offer a positive lock its not ammo. Can you take a pic through the ejection port with the bolt pulled back and an empty mag?