About flechettes, like someone else said, I read they were experimented with very briefly in the 60s but proved not to be ineffective to the point of uselessness.
The other story was that they were so effective that they were considered "cruel, and unusual" if you will, and were not allowed to be used on other people.
Go figure right?
One of the earlier posters seemed to have done convoy security in Iraq and wished he had a few shotguns. That was what I did in Iraq as well, and we had a few shotguns. Somehow they were left over, and somehow, we got them. There were 2 Mossberg 500's and 1 Benelli M4. They made vehicle searches a little bit easier, but really, I felt we didn't really need them, because of our role. I wanted them to go to our PTT because it would serve them and their needs much better than ours. I spoke up, but I was only an E-4, an NCO by USMC standards, but still a peon in reality.
An interesting story from way back was about the infamous "trench gun" and the tactics employed with it. I'm not sure if this was WWI or WWII, but the Germans definitely feared this weapon. From what I read, it had the ability to "slam fire", and the tactic was to rush in the trench with a fire team all armed with trench guns lined up in a single row rapidly "slam firing" all their rounds and alternating with the next guy and so on right down the line. By the time the last guy expended his ammunition, the first guy would have been reloaded already and it starts again.
I don't know the veracity behind this, but it was an interesting read nonetheless. It's got to be true, because if it's from WWII or WWI, you just can't make that stuff up.