It is quite important that the loading bolt has no slop and that the probe fits quite neatly inside the pellet cavity. This will, as much as possible, ensure that the piledriver, with its rear tapered body, is loaded as concentric as possible into the barrel. Good luck with the helium / air mix. You may need to clean the barrel perhaps every 10 shots or so. Even at 640 fps that does make a difference with these pellets.....H. Post script: I have gone back to read Hardair protocol which states that an FX Impact was used in testing. The Imact has an FX ST barrel which would not suit the Piledriver bullet. I suspect that to be the reason for the odd low 0.033 bc reported. Neither would the loading system of the Impact be a suitable match-up I suspect. My own bc test results have a generally close match up with Hardair data for those pellets for which I have data.