I agree Shadow that the velocity should have been very much higher which is why I was so shocked to see the chronograph ring up 450 consistently over every shot.  The chrono had a brand new battery and it was at least 12" away, I seemed to be getting expected results with a few other different loads including a factory load.  Maybe it was some weird thing with the sun hitting the sensors at an odd angle just as I was shooting these rounds.

I'm pretty perplexed so I'll probably be reloading some more with original charge and some lesser measures, maybe I'll mix in some 350 CCI primers along with the 300s I was using.  I'll also have to experiment with the chrono at different distances and at a different time of day.  Gotta get to the bottom of this!

Edit: One other piece of data.  COL=1.26"

Edit2: Also interesting about the bullet color, this load was the only one I shot that day with a copper jacket, everything else was a dull hard-cast.

I've used BlueDot, decent powder but I've had inconsistent powder measures.  I always seem to come back to Autocomp and HS-6 as my go-to powder (I usually load 180g bullets or higher).  Of course there's tons of powders I haven't used yet, I'm in AK so sometimes you have to take what you can get.  On the flip side I've had bad experiences 800-X and Longshot.

On a side note you'll have the best accuracy with minimum or near minimum published load data, you can really tame 10mm.  Of course many will argue that's hardly the point of the 10mm so follow your heart. 

I made a small batch of 10mm shells that resulted in a low velocity projectile with a fairly hefty recoil. I used a 180 gr. Hornaday HAP bullet, 9.5 gr. Hi-Skor 800-X, CCI 300 LPP. The powder filled a lot of space in the shell casing and I'm sure it was compressed with the bullet seated, this was towards the high end of the published load data. When fired my chronograph was registering an average of 450 fps, much lower than I expected. I was shooting a Glock 20 gen IV with a stock 17lb spring, only mod is a Storm Lake barrel.  I had fired a few factory rounds with my chronograph just to make sure it was giving solid data, seemed fine.

I'm guessing the powder ignited slowly and pushed the bullet out at low velocity and then continued to burn, possibly at an accelerated rate, after the bullet left the barrel; as mentioned I was getting a good kick. Only guessing though, I'd like some analysis from more experienced loaders.

I'll probably make one more batch with the minimum published charge just to see what happens, but probably the last load with 800-x, what a pain that stuff is to work with, got plenty of other powders to use.