So I went to zero my scope down the farm today. Using SMK TH78D (.22) with 2 x fresh 12g C02 bulbs firing Umarex pellets and a Excelvan 4-9 x 40 scope. I wasn't expecting too much in terms of accuracy but used a gun rest to at least eliminate shooter wobble.

Shot groups of 5 over a measured 25 metres and generally managed a 3-4 inch grouping. However, every so often a pellet would drop alarmingly - sometimes 6 or 7 inches below the expected POI. I though at first it might be the CO2 getting cold but then the next pellets would be back 'on target'. Wondering if it could be the quality control of the pellets? Any thoughts?