Here is the thing, they aren't accurate. If you really know how ballistics works you would know this.
I created a thread a few years ago talking about this during a chronograph discussion. I will dig it up.
In a nut shell the bullet is still accelerating 10 feet AFTER leaving the barrel.
Found the thread,
viewtopic.php?f=19&t=16233&p=232254&hil ... ph#p232254
I can shoot the same lot of ammo through the same gun. Every thing can be identical including the shooter except for one day apart. Temp and humidity the same since I have that controlled in the shop and our indoor range.
You know what? There will be an 100 to 200 fps difference sometimes.
Also measuring at the muzzle, if you can get a reading, gives you lower velocities. The bullet is not done accelerating until well after it has left the barrel.
Also factor in the variance in equipment. A $300 dollar chrono is no where as consistent machine to machine as a $3000 chrono.
Now as to why the above difference of 200 fps or sometimes it's 20 fps?
I asked this myself and discovered the answer in one of my 100 year old ballistics manuals.
Barometric pressure. So now when we load ammo it's test lot has all the standard data including BAR written on the box. When the ammo is tested the BAR at the range is also recorded. I have started, mind you STARTED, working on the correlation in fps change and BAR. In fact when some of my 10mm was tested by that third party the velocities he got where consistently HIGHER then any I had ever gotten including with the test loads from that run of ammo his samples came from.
Now here some really good reading from the creator of my primary pressure testing system when we where discussing what a pain in the ass it is to measure 5.7x28 velocities accurately.
Understand rifle/handgun velocities listed as "Muzzle" are usually taken 10+ ft from the muzzle.
This is the industry standard and even described in SAAMI protocols. Shotgun data may
be 5 ft. and real big bores to 15 or 20 ft. The data is never actually collected at the muzzle.
Since the advent of good radar range instrumentation it has been known the bullet continues
to accelerate 8 to 12 inches after exiting the muzzle. More importantly, the bullet exits
with venting gas and does not begin to decelerate until it punches out of the gas plume as far
as 4 to 6 ft. from the muzzle.
The above shot taken in a vapor chamber clearly shows venting gas, the shock cone where air drag
starts, etc. Only when a bullet punches through the venting gas will air drag cause velocity
erosion. Those software products that attempt to back up velocity to the muzzle from
chronograph screens are bogus and entirely miss this physical phenomenon.
The bottom line is even if erosion starts at 4, 6 or 8 ft. with screens at 10 ft; the
slight error is less then the instrumentation error that can be expected from light trap
chronograph technology and certainly less then the ambient temperature affect on propellents
or normal shot to shot deviation. Since the point where velocity erosion begins can only be
learned on a radar range, the industry has adopted standard screen spacing.
The venting gas in the above shot contains allot of heat. All chronograph sensors read into the
IR spectrum so the hot gasses only serve to blind the first sensor.