Johnson Semiautomatic Rifles
Threefeathers nailed it.
The invaders were supplied with weapons that were intended to appear to not have been supplied by the US military. The Johnson semiautomatic rifle was the primary long arm. The ammunition used was headstamped B N 43, though it was non-corrosive ammunition.
In the late summer of 1960, after plinking in an abandoned rock quarry where US 40 crosses the Missouri River near St. Charles, MO, a friend and I decided to hike south on the old MKT railroad, now the route of a bicycle trail. We carried rifles--imagine that!
After walking some distance, we heard gunfire west of the tracks and decided to investigate. We came upon some uniformed perimeter guards who did not speak English and who took us to an English speaking officer or NCO--I think he had three stripes--where men were being instructed in shooting at targets with Johnson semiautomatic rifles from prone positions.
I commented about the Johnson rifles, and my friend and I were each offered the opportunity to shoot one off-hand. If I recall correctly, and on this my memory is sketchy, we also fired a Smith and Wesson service revolver that our host carried in a holster. I picked up one of the fired cases from the rifle. My friend was carrying a handgun (probably unlawfully), and our host fired it.
We were treated very cordially and were escorted back to the tracks and allowed to leave.
The following spring, the newsmagazines had pictures of the men deploying from watercraft with Jonson rifles.
Some years later, a letter appeared in The American Rifleman with a question about the headstamp in question. The reply (could it have been written by Julian S. Hatcher?) stated that the ammunition had been loaded by Frankfort Arsenal in 1959 for use in covert operations. I later acquired a case containing a thousand or so rounds of the stuff.
About fifteen years ago, I asked a man who was working at a now-defunct gun store in Fenton, MO whether he knew of a Government range in that area. He replied in the affirmative, and told me that he had on occasion gone there as an FBI firearms instructor. When I then related the above story, he reacted visibly and said that and had been called upon to help train Cuban freedom fighters at that range before the Bay of Pigs invasion.