I built a 1/2 scale 6 pound field gun many years ago (my first lathe project) , if you load with tightly wadded newspaper you will get the full flash, boom and recoil without the projectile.
Nice, I did this one a while back, AR Lower with 22 conversion unit. Semi-auto Sterlings are lot harder to find than stripped AR lowers. Flash hider is made from solid aluminum bar stock.
Since 300 BLK will chamber in a .223 I painted the bottom of the 300 mags FDE and put a thin FDE band on the front of the handguard so there is no possibility of getting the mags confused.
We always qualified with the ammo we had been carrying for the last 4 months, then loaded up with fresh ammo. Being retired i only have to qualify once a year now, but i still continue the practice.