I knew it. You live in the Romania/Hungary general area, right?
Don’t know about the efficacy of silver bullets on vampyres, which is that region’s specialty, but I have heard it rumored. I think we could pretty much all agree on their efficacy for werewolves. (The Invisible Pink Unicorn would nod in agreement, I believe.)
As far as a portable stop, a bullet of uniform shape goes through water pretty straight until it starts to slow down. It also keeps a round-nosed bullet intact enough to reload (in my experience).
The plastic (or even paper) milk cartons full of water seems like your best shot (no pun intended). It’s totally portable, and if you are concerned about deflection just lay out the cartons or jugs like bowling pins, but on about a three to one ratio.
1 carton in the first three rows, 3 cartons in the second three rows, etc. Yeah, it’s a lot of cartons, but you carry them empty and fill them from a hose (or stream) on site.
I’ve seen a .38 special penetrate about 7 feet of water when fired INTO the water. When fired FROM underwater, I’ve seen it go farther. Evidently entering the water has a major stopping effect. A .45 will have less penetration generally; the .45 was designed as a round that will not pass through a human body (which is some 98% water), but I’m not sure that this hold for water not encased in flesh.
Going from one bucket to another may increase the rate at which the bullet slows down, but it may also impact the deflection… so…
Oh, I got it! Get 20’ of firehose. Lay it out as straight as possible. Cap the far end, hold the near end up about waist high and fill it with water. Seal the near end with some heavy plastic and a hose clamp. Check the straightness of the firehose and fire away. If the bullet deflects within the water in the firehose the angle it hits the wall of the firehose will deflect it back into the water.
Good luck, and let us know how the test works out. We’ve got some pesky menehunes we need to take care of out here.