shot my MCX Virtus pistol yesterday and had a few issues with one magazine. Ive only put 60 rounds through this gun previously with no failures. Yesterday while my 12 yr old was shooting there were several FTF’s. i couldn’t duplicate the failures when I shot it but the bolt failed to lock back on the last round when I shot it. I only had 1 magazine ( which had been loaded for 3 years in a chest rig. sounds like a bad magazine right?

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Since you only had the one magazine, who knows.
You might check the gas key to make sure it is not loose and be sure to lube it well.
Buddy brought me his AR that was not feeding properly, slathered on the lube and did the oil dip for the BCG and it ran fine afterwards.
How many rounds through it? might be teething problems.

F-ire

or

F-eed

Failure to lock back on last round not likely a mag issue.  More likely a gas issue or lack of lube, or just new parts getting used to each other.

What weight buffer and spring?

my issues were failure to feed into the chamber.  I've only shot 60 rounds before the range session last weekend but those previous rounds fed,fired,ejected without issue.

If the bolt fails to lock back on last round, then the problem is you aren't getting enough gas for your current setup. You either need to increase gas (by opening gas block up more), or decrease the reciprocating mass (bcg, buffer), or reduce buffer spring stiffness.

The failure to feed rounds is also a result of this same issue. If the BCG isn't going back far enough to lock back, then it's not going back far enough to strip off a new round from the mag either

Sounds like it's running on the ragged edge of not enough gas, or too heavy of a BCG/Spring, or weak ammo, but it really needs to be able to cycle even the weak ammo within reason, so I'd look at increasing gas, or decreasing spring tension or BCG mass. Also, this will likely settle a bit with time and round count as things wear in and slick up and the springs settles in.

Being an adult, you have more mass to push against the recoil which is letting it go back far enough to feed, but as you found, sometimes not enough to catch a bolt catch. Your 13yo son is letting the recoil push him back a little more, which will take momentum out of the BCG and short stroke the action.

I would bet you can replicate the issue if you softened up your shoulder on the rifle and let the recoil move you a bit more.

Virtus pistol? Or Virtus upper on AR lower? Weak ammo like Tula? Sounds like short stroking which I’d say weak ammo, adjust the gas block to “+” setting.