This is an interesting question and nice to see each persons observations. I have never thought about calibration of a chronograph, but my observations over the years would make it impossible to calibrate with a "standard load".
I agree that to check to see if your chronograph is in the ball park of correct would be to purchase 22lr target ammo that has the speed on the box.

The problem with that and other standard rounds- The rimfires appear to have a larger variance than centerfire rifle rounds as you heat of the barrel. The bench rest rimfire shpoters know that you have to heat of the lube in the bore before good groups (same point of bullet impact) becomes the norm. Do this some times over a chronograph - the velocity of those first 5 to 10 rounds are never the same and can be a long way from those groups shot later as the lube warms up. Also one gun does not produce the same velocity as another due to bore dimensions and barrel length, chamber specifications, etc. Then you get into temperature variations in both air and the ammo, which causes different velocities. Point is - impossible to have a standard round to check your chronograph. Which brings up the question - why does it need to be calibrated ?

IMHO- the chronograph is used to 1) develop loads with a low SD for the target uses (high SD - big group, low SD, tight groups, always). 2) verify velocity so you can uses a ballistic table for drop and wind drift and calculate terminal energy for those inquiring minds that want to know. 3) know the velocity of your load so you can discuss with other shooters on what they are doing with a certain powder and bullet.

Now for the last question, why does anyone care on the velocity of factory ammo vs factory propaganda? I have been reading shooting magazines for over 50 years now and all I have every read for 50+ years is that factory published specs are more that what people see in real life situations. If you load does what you want it to do, then who cares what the speed of a factory round produces? M2CW. (Due to inflation, that is now worth on one cent.)