Funny story, I went over a friends house a couple years back to check out his new to him xd40, loaded up a mag, took it outside and fired them off. I pick up all my brass, so I proceeded to collect all the spent cases, then I started finding ballooned 9mm brass aswell.. I said to myself wait just a hot minute...
Went inside the house,
Hey this is your first handgun right?
"Yes" he replies.
.. Sooo you don't own a 9mm correct?
"Nope, why?" he says...
Any reason why there would be 9mm brass in your yard?
"No, there shouldn't be."
Did you get to fire this gun at all yet?
"I was shooting it earlier but it kept jamming so I gave up."
Apparently the previous owner had sweetened the deal by throwing in ammo, and my friend assumed all the ammo was for that gun...well he won't be making that mistake again. Got 2.75 boxes of 9mm for my troubles.
But to answer the question posed earlier, they pretty much looked like a reverse .357 sig with the bottle neck down to the rim.