If you look at your reloading manual it will tell you that the loads that they load will vary in different rifles, and there are a lot of manuals that use test barrels and not actual rifles or pistole's. You should be more concerned with the pressure signs that you may see on your reloads instead of the velocity.
As an example of this last summer I was building a new load for my .340 Weatherby. The manual said that 96 grains of xyz powder was max with a velocity of 3390 fps. I loaded my cases up to 95 grains and obtained a consistent 3400 fps with 10 loads. I thought that I was doing ok with no signs of excessive pressure until the last shot. Then after examining the rest of the cases I saw the start of some primer cratering, so I backed off another grain and all is well. I would of hated to seen the pressure if I would of loaded them up to the manuals max load.

Critter,
Understand the pressure issue competely. So many things can make a difference. Even differences in powder or primer lots can make slight differences.
Also understand that more powder does not always equal more velocity. Finding best velocities with an "accurate" load is what really counts.

So the manual / data you were using showed the velocity of 3390 at the muzzle and you were getting 3400 with your load. Where was your chronograph reader set from the end of the barrel? Does your unit allow you to set it "at" the muzzle?
If your chrono was set 10 - 15 ft from the end of the barrel like mine has to be, that bullet was Haulin behind!

I usually set mine up at 5' from the barrel, I lost the manual so it may say 10' but 5' works.
When I set up these loads it was interesting that the manual said that I could load up to 96 grains and I stopped at 95 after seeing problems with pressure. I didn't say but the bullet was a Barnes 185gr XLC BT and yes I do believe that it was hauling when it left the barrel not to mention when I saw the 3400 fps on the crono. That is where I usually don't worry too much about the printed velocities in the books. If my load can take their max load I usually figure that I am close to where it should be. I will use the velocities to figure my bullet drop over different ranges.

Critter,
Hey, if 5 ft works, that’s cool.
I have a Beta Chrony and the read out is on the front of the measuring unit, I could try building a plate to cover the read out to protect it from blast shock & debris.
It would be interesting to know what velocity is lost at 1 ft, 5 ft, 10ft and 20ft. from the muzzle.
It may be so little that it's not worth consideration, I honestly don’t know.
I suppose one could figure it out using ballistic tables and come close, but considering a bullets actual flight path, I don't believe the velocity could be linier. All I know is the math that would be used in such a calculation evaporated from my brain a long time ago.

I set mine up 10' from the barrel. At first I tried closer but muzzle blast rendered the cronograph useless when it was to close. I would guess if you get within even 15' to 20' of the muzzle and you'll still get pretty good data. The bullet won't slow down that much in even 20' to make that much difference. The most important thing is screen spacing. Mine came on a bar that set that at the factory.

Don,
The Chrony unit I use folds in half and acts as a base, the screens are permanently set. There is a metal rod on each side of each screen, forming a V shape that the bullet must pass through. The rods are simply a reference and hold a sun shade in place on sunny days. I have replaced the Metal rods with cocktail straws procured from the local bar to replace the metal rods. If something goes haywire and you hit one of the metal rods, it might tear the unit apart. I rarely need the sun screens.

Next time I set it up, I will set it at 10, 15, 20 ft with my .243 and see what the differences are. May be a while before I get to this and report the findings.

Little tip. If you use it in the snow, the light bouncing off the snow can really sxdrew things up. I made a shelf that slips over my light screen and stops that.. Makes a pretty handy tray too!

Don,
Great idea, May build a tray for mine in the future.