Two related questions:
When ammo manufacturers label their shotgun shells as 1200 fps or 1300 fps (or whatever), is this measured at the muzzle? Some of the reloading manuals list velocities but start at 3 feet from the muzzle. Is there a difference? For most of us who do home chronographing, since the contraption is optical, the 3 ft from the muzzle is probably what we should compare to?
Now the second question: If one were to construct a lead model and for example compute the lead of a 90 degree crosser traveling at 59 fps (approximately the speed and angle of a station 5 high house) at 21 yards, should the distance measure be from the shooter or about 1 yard closer from the muzzle?
In the United States our normal method of chronographing is to use a 3 foot velocity. That means the center of the cloud of shot is measured three feet from the end of the barrel. Home chronographs measure the lead pellet, not the center of the cloud, so what you measure at home will be very slightly different. Velocity measurements from other countries use other methods. You have to be very careful when comparing printed US velocities to others. It is seldom apples to apples.
One more thing to consider in chronographing, be consistent in your choice of choke. Due to the Venturi effect, switching from a cylinder bore to a full choke, using the same barrel and shell, can raise your muzzle velocity by over 30 fps. I forget whether the SAAMI testing standard for 12 gauge is full or cyl for their 30″ barrel. I’ll have to go look it up.
Where do you get your 59 fps/ 40 mph speed for that #5 high house? I’ve been looking around for some good, reliable skeet velocity numbers.
As to working out your lead model using the shooter’s eye or the gun’s muzzle as the starting point for distance measurement, that’s up to you. There really isn’t a standard. I’d use the shooter’s eye. Then again when patterning, I use the muzzle. Perhaps it would be more consistent to use the muzzle for everything. “A foolish consistency is the hobgoblin of little minds, adored by little statesman and philosophers and divines.” I wonder if Emerson every did any patterning work?
I used a chronograph to measure the speed of a low house skeet target just outside of the window. (You have to reverse engineer the chronograph by putting the screens closer together to get a reading in the measurable range of the chronograph…since the instrument is designed to take readings with the screens 24″ apart, you can make a mount that has the screens 20% of 24″ or 4.8″ and then divide the fps on the readout by five).
The low house skeet target was about 75 fps at the window. I then moved the chronograph 10 yards further out from the window and read the speed there. Applying the appropriate physics formula to the two readings gives the appropriate constants to use in a formula to calculate the speed anywhere on the flight path (except at the tail end of the flight where the formula becomes a single degree equation instead of expotential). My formula gives a result of about 59 fps at the center stake for the low house target. I did this work three years ago and verified the initial speed with one of those radar guns used by some folks to set trap targets.
I never would have thought of using my chronograph to measure target speeds. Good on you. With my luck there would have been a gust of wind to send that clay target into my ancient “Chrony” and turn it into a cloud of dust and transistors. Congratulations on having done the work. I say this, of course, whilst sitting in my overstuffed office chair far, far away from the real world.
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)