My all-time favorite full-auto "fun gun" is the Browning Automatic Rifle (BAR).
Problem was, when I was younger, I was a "light-weight"...tall and thin.
That BAR would scoot me backwards along the ground in prone full auto fire!
I had to crook my right knee and gouge a hole in the ground with my heel to "anchor" me while I fired.
With AP rounds that BAR would just eat a hole through masonry.