Some searching on what is obviously a do-nothing day led me to the world of miniature firearms. Here are two links.
https://minifirearms.com/miniature-guns-that-shoot/
Size Matters - The World Of Miniature Firearms
https://www.dailymail.co.uk/news/article-526655/Introducing-worlds-smallest-gun-fires-deadly-300mph-bullets--just-TWO-inches-long.html
It seems that while the 1/4 scale ones can actually be made to shoot 1/4 scale ammunition, smaller scale ones don't actually fire.
Oh, hey, I just noticed. One of the linked articles says that
when the miniatures get too small, the bullet velocity is too low for accuracy.
Hmm. Does that mean wildthing is right, the 1/6 scale M60 round will have a 1/6 scale velocity, and thus kinetic energy just 1/1296 that of the full scale bullet?
That would make his gun only a little more powerful than an Airsoft . . .
Do you remember the movie "Fantastic Voyage". And other movies where people are miniaturized. Wasn't there also a movie with miniature GI Joes (the smaller kind) running around a suburban neighborhood? I guess those movies aren't realistic at all :/)