Did you make an adjustment between your 10 and 5 rd 100m groups?
You should expect that 95% of your shots from this system will fall within around a .45MRAD circle due to dispersion, based on your 100m mean radius. It will be a little smaller or larger as a 10rd mean radius measurement isn't exactly precise. You should expect a group of any number of shots at 300m to have an extreme spread of anything up to 13.5cm purely due to dispersion, although most of them will be smaller. The ability to centre that on a target, or any wind error, ranging error, additional shooter errors off the flat range, etc are of course additional issues to deal with.
The 3 shot group at 300 of course tells you nothing informative.
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