Slide Dropping in Mag Load

[GD2A]

Hi Folks,

I took three guns to the range today for function testing. One was a new to me Colt 1991 (ORM) that had seemingly minimal usage and is stock except for an aftermarket MSH. On three or four magazines the slide would drop forward when I inserted a magazine. It happened with both Colt and Wilson mags.

Any ideas about what gives?

FWIW, each time this happened it did properly chamber a round and the gun ran flawlessly otherwise.