I was working on an AK and just couldn't figure out why it wasn't shooting well...(it was a mess to begin with and came baggies of parts...
). After I went over every screw and nut and I thought that I had it done, I chronied and it shot 290fps (this was with a M120 spring). WTF!??!?! Then I noticed that the hopup was almost all the way on...turned it all the way off and it immediately shot 420fps. Turned it on just a bit to stop the bb from rolling out and it was 390-ish.
I had a tiny AK Spetznas that shot 540fps with an M120...put in a M110 and it shot 458fps...put in a M100 and it was 410fps...crazy. It was a magic gun.
As a general rule I chrony newly assembled guns with the hopup off....or at least to on enough to stop the bb from rolling down the barrel on it's own. I do chrony at least 10 shots and do a rough average. That FPS result will be the hardest it shoots ever...and the FPS will likely be less when the hopup is set....but at least I know it's ok when it leaves the shop.
If it's shooting 401fps...it'll likely drop anyways as the spring settles in. Run 500 rounds through an see what it shoots afterwards.
The blue madbull chrony really likes a fresh battery and it's not a bad idea to clean out the sensors every so often. Obviously...you've got to be shooting straight through it. Shooting cockeyed through it will give you wild readings.