The basic idea is, in fact, far from new: get an airsoft gun and transform it into the real McCoy, namely for a highly-illegal test project or for actual criminal purposes. The latter use was recently obvious when one such weapon was seized by officers of PMESP – Polícia Militar do Estado de São Paulo (São Paulo State Military Police). From the very superficial details coming from a low-quality video footage obtained by TFB – and from which the video grabs that follow were made – it was an M4-type airsoft gun transformed to fire 9x19mm ammo.

A pistol-type 9x19mm magazine was adapted to the rear end of the of gun’s STANAG-shaped unit.

By pulling the M4-type charging handle to the rear, the bolt is held in the open-breech position, the gun apparently being conventional blowback-operated.

Closer views of the upper receiver show the crudely-enlarged ejection window and the breech of the 9x19mm barrel fitted to the gun.

The modified gun is shown with the bolt in the forward (fire) position, the extractor being easily noticed.

Slightly higher-res (but not-too-sharp) images here: https://imgur.com/a/wlMKX