I found some wino that was unloading about 100 of them, literally, on Craigslist. The guy drank nothing but carlo rossi rhone wine. Maybe one every two days. He sold them for $1 each. They will hold pressure without exploding. I did a starter but put the lid on tight instead of using tinfoil. Don't ask why. I doesn't make sense.
Next day I'm looking at it thinking, "man this is some sh!t. it didn't work. No krausen, no nothing." Then I popped the cap. It EXPLODED. Everywhere. Gusher for 2-3 minutes non stop. So some decent pressure capabilities.
I think it's a number 4 or 4.5 stopper for the next.