Today for the first time I jumped at a firing range.  I was at a nearby indoor range test firing a Glock .45, another first for me.  Nice gun.  I can see why law enforcement types favor it.  I'm still a .357/.38 revolver guy, but I can appreciate the 13 rounds and the Glock fast reaction and safety features.
Each firing position is nicely separated from neighbor shooters by a concrete wall.  Nevertheless, all of a sudden WHAM WHAM WHAM!, so much noise and an actual felt change in pressure and some smoke that I jumped.  Just a little, but first time ever at a firing range.
When I looked around the wall, I saw a tall young guy with a big metal gun that looked like a lot of big military metal guns.  He said it was a .50 caliber that he built.  
Wow.  It was a blast, literally.  Didn't get any further details.  Too much power for me.  I went back to my comparatively tiny .45 Glock and then fired a couple .357 rounds from my Ruger Security Six for comparison.  
Later I retrieved a .50 caliber shell casing.  It's about the size of two .45 auto shells lengthwise and slightly wider, but nowhere near the size of a .50 machinegun bullet.