Question for those of you with long range experience, I had twenty minutes free time today which coincided with near windless conditions and finally got to have a shot at a 100 yrd biscuit tin which I'd set up a month ago. I fired 15 shots and never missed it which made me happy, when I had a closer look most of the pellets struck within a two and a half inch vertical strip but were evenly strung out over the eighteen inch height of the rectangular tin. So do I ...

A. Weigh my pellets and group according to weight.
B, Head size my pellets .
C. Do both of above.
D. Just be happy to be hitting the tin.

I'm using a HW77 SE ( which has never been touched) in .177 firing 10.3g Airarms field heavies at 10.4 ft/lbs