Woops, for armors < 40 I accidentally multiplied by 16 pellets instead of 6. Fixed values:
On armor 4 (sectoid) : 21 per pellet, 127 for full blast. Chance of doing ANY damage per pellet : 92%
On armor 8 (floater) : 18 per pellet, 106 for full blast. Chance of doing ANY damage per pellet : 84%
On armor 20 (snakeman) : 9 per pellet, 54 for full blast. Chance of doing ANY damage per pellet : 60%
On armor 20 with 40% resistance (muton) : 1 per pellet, 10 for full blast. Chance of doing ANY damage per pellet : 33%
On armor 35 (ethereal) : 2 per pellet, 13.5 for full blast. Chance of doing ANY damage per pellet : 30%
Damage of a single pellet is extremely random, but for a full blast, the individual variations of each pellet are very likely to cancel each other, so the "full blast" average should be rather consistent.
I thought alien armor got increased on higher difficulty, which is why I checked armor levels 30 and 40, but I was mistaken.