I would suggest it is alignment of the combro when on your own rifle. if one sensor is seeing the pellets head and the other missing the head but getting the skirt you will get a difference this big as the sensors are only 50mm apart, so a 5mm discrepancy will produce the 10% error you are seeing. The combro is a great tool but very sensitive to alignment as it only has one a single light beam across each sensor.
They also prefere to be used on a silencer which helps to dissipate the air blast, under some conditions the combro will see the air and give a false reading.
As well as the normal adjustment of the mounting bracket you may need to pack between the gun and one end or the other of the mounting as I have seen ones that are quite distorted. A close fitting straight drinking straw inserted in the barrel or silencer can help as an alignment guide. McD straw is good in most silencers.
HFT, for people with only 2 buttucks!
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