To keep costs down I’m thinking of using 21grams 7½ shot clay cartridges over decoys. Will they kill cleanly and will I have to use tighter chokes than my usual improved cylinder and 1/4?
A successful pigeon shooting load is always dependant on a mixture of shot size, choke, and the range at which you expect to kill your birds.
The same criteria applies to whatever choice you make, and that is, any bird must be hit with enough pellets of sufficient energy to be reasonably sure of killing it at the range at which you have fired at it.
The pellet size you’re thinking of using will start to lose killing energy at about 45 yards, but more importantly, with 21gms.
The pattern density will be too thin with anything less than 3/4 choke, putting a premium on your accuracy at birds in the 20-25 yard bracket.
It is no coincidence that after more than 100 years of shotgun technology, the perfect combination of 30 grams of No.6 shot fired from a 1/4 choked barrel, has never been improved upon for general decoying.