For all you and I know...the guy had blanks!
You may be right.
But if he did it would be obvious to the guys on his lawn.
To fire blanks from a semi-auto rifle you have to do one of two things.
One; manually cycle the bolt for each shot.
Two; install a special attachment over the muzzle of the rifle to provide the backpressure necessary to cycle the bolt automatically. (Typically this attachment is about three times the size of the muzzle of the rifle and painted orange so that the training officer can see that it is installed)
Either one of these things makes it obvious to anyone familiar with guns that you are firing blanks.
If you have a recoil operated semi-auto you just have to manually cycle the bolt.