Need Chrono Formula

[JD45]

Does anyone know the correct way to figure muzzle velocity when you have the skyscreens 10-15 feet from the muzzle? This is for shotshells only.

A bunch of us set up the chronograph today and checked every shell we had, from all of our trap and sporting guns.

Our handloads were near what they should be, but factory shells were all over the place. One popular shell from Europe was listed as an ounce and an eighth@ 1250fps. It chronoed about 1060fps avg. from three guns.

We were assuming that a load going 1150 at 12 feet should be near 1200fps at the muzzle. Shot slows down really quick.

[Merlin Orr]

Add 4.4% to whatever it reads at 12 ft.??? Looks like there would be a ton of variables from different shot cups, chokes and etc.

[Irishlad]

I've read that you lose about 40-50% of the muzzle velocity at 40 yards, given variables as shot size, etc.

But, you lose 25% of the velocity in the first 20 yards and then the velocity loss tapers off a bit. So 25% of 1250 is a loss of @ 312 fps in a 60 foot distance, which is 5.2 fps "loss" per foot. I would expect the velocity loss to be greater the first several yards...but!

I would guess you have a "loss" of @50 fps standing 15 feet away, instead of 3-5 which is where most chronograhs are I believe, at least the ones I've used.

However, you may be standing too far away for accurate readings of any kind. Were your deviations high?

A couple more feet back and you may have been shooting the screen with the wad.

keep in mind, this is information I've "read".

[JD45]

They say you can't chrono a shotgun real close because it screws up the reading. That is why we put it 12-15 feet away.

[Irishlad]

I could be wrong on the 3-5 feet, but I would follow whatever instructions on the distance of the bbl to the screens.

Depending on the chrono, I understand it may read the first "pellet" that crosses giving you a higher velocity than advertised. Which, I think, is okay using the new shells as a control with small deviations. Then, when you do your reloads you will have a fair comparison at least.

The speed of the shell, within limits, was less important to me than the variations.

[sargenv]

I usually chrono my shotgun at about 10 ft and if I get odd readings, I move things back. Generally though the readings are right about where I expect them. If the load says 1330 fps, I am usually wihin + or - 20 fps or so.

Vince