Matt said it for me, grab yourself some .36 (9mm'ish) lead.You may even be overpowered... Though sounds kinda in the expected power range. You could go down to 22-18 maybe...
I'd probably go with smaller ammo as well - 9.5 or 8mm minimum. Or .36 lead ball
That sounds like quite the setup mate, I'll have to give that a try. What type of elastic are you using?I'm assuming you have short draw? I think what Matt said is sound advise, I narrower cut and 9.5's you should be able to get into the feathers. But pigeons can be very tough. My hunting setup right now is .65 elastic cut 5/8 to 3/8 (16mm to 10mm) with a 11"active length and 7/16 (11mm) steel, this maxes out at full butterfly and sends a sleeping pill. So far I've had nothing fly away from it after being hit.