Legal. But very frustrating and disturbing to shooter.
At just completed state side match WB, a shooter had a gun that would fail to slide lock half of his mags. I RO'd for him quite a bit, and between the two of us, we'd both yell to each other "LOCK SLIDE BACK" as soon as his fifth shot was fired. He was good at paying enough attention, never took more than one-step before getting slide locked back, and made it through the match without safety penaties, but I am also quite sure he gave up lots of time trying to manage that situation.
As mentioned, take a loaner offer from a pard. This is not IPSC or 3-Gun. Thank goodness.

Good luck, GJ