Crow hunting is something we do when nothing else is really in season, or we're too lazy to go call coyotes or coons. It's best done much like prairie dog hunting, sit far away, use a none-too-loud rifle, and blow up dozens of them as they repeatedly come into your decoy set. Done right, it's fish in a barrel.

As the others have mentioned, 20yrds will be your Achilles heel, they're far to wary to land that close. If you conceal yourself incredibly well, you might get lucky, get a few to land nearby, and get a shot off, but likely just one shot because they'll bust you there after.

Where you place your decoys has to make sense to a crow (as odd as that sounds). I've known a few guys that would just put crow decoys out in a field, clustered together, then wonder why nothing came in.

Here are some tricks:

Place 2-3 decoys on the ground around your "carrion pile". The carrion pile can be a black trashbag or grocery bag with part of it cut to ribbons so it'll blow in the wind a bit, as would animal hair on roadkill. I have used black trashbags with white grocery bag inside it, cut to ribbons to look like black and white fur. Either put small rocks on it, or stake it down so it doesn't blow away, and stays sprawled out on the ground.

The movement of your pile decoy will catch their eye, and seeing other birds feeding 1) builds confidence, and 2) drives them to think they are missing out.

Place other decoys outside of that cluster, up on a fence post nearby, in a tree, etc. They're "at the party", but they're just confidence builders. It takes a lot to bring a crow clear into a ground decoy field. They'll often land nearby and investigate before they commit to the main density of decoys.

It's nice to have an electric call, or buddy to play a mouth call, when you're crow hunting, because 1) crows never F'ing shut up, and 2) it rattles them when the call STOPS, for example, when you put down a call and move to your rifle.

Best option you have is to call over a REAL carrion pile. Either grab a shovel and 'harvest' some roadkill (I hear banjos playing every time I do it), or try to find a carcass naturally somewhere you can hunt. One trick I use: it's deer season, when I fill my tag during the day, I leave the gut pile in the open. I go back that night and call coyotes over it, then the next day, try calling crows over it if anything is left.

The very best thing you can do is just sit and watch crows some time (best thing you can do to learn how to hunt ANY game), observe how they move, communicate, interact as a murder (flock of crows = murder). They're a very visual animal, and very social. But you won't see them gathered together in ONE spot like ducks on a pond, they'll be spread out and always moving.

A spring mount for one of your decoys with a wind-bob or even a well concealed string can draw their attention to your ground pile.

Good luck to you hunting them with a BB gun though. 20-30yrd range is going to be rough. At that point, I might just spot and stalk them, because you're only going to get ONE shot before all of them bail, even over a decoy field or bait pile. Trick to that is focus on one bird at a time, stalk up to a shooting position without the others busting you, and take the shot. Sometimes with crows, the more "sneaky" you act, the more alerted they get. Walk in like you own the place and they'll be more confident.

A buddy of mine, that I tend to have good faith in, claims that research has proven that crows have great long term memory, and have the visual acuity to recognize human faces (supposedly even discerning one person from another), so concealing your face can be important as well. He also takes that a step further to say that they'd recognize a rifle, so keeping your rifle on the DL while you move into position might not be a bad idea.