# Factoring Velocity Mathamatically

#### RVTECH

Does anyone know if there is an equation that can factor the additional velocity gained with a pistol round from a longer barrel? I realize a chronograph is probably the only way to obtain a true velocity but I was just wondering if there was any other way, possibly a 'rule of thumb' etc. Thanks.

#### Josh Smith

Hello,

While the pressure curve is not linear, you can usually count on an extra 50fps per inch of barrel length.

Josh

There are bunch of factors that control speed of a bullet. Bore size (the actual measurement, not caliber) can either be tight and create excess friction, loose and allow gas leakage, or just right and give optimum speed for charge, weight, and barrel length. Which describes your barrel?

A chronograph is needed to truly know the speed as there is no accurate way to calculate the speed from barrel to barrel.

I agree that most use the 50fps per inch of barrel but that's just a guess at best. All the factors I mentioned can change from barrel to barrel so unless you slug each barrel and only use barrels with the same, exact, bore diameter it's a new game with each one.

#### nrc

Chrony's are under \$100 nowadays. Everyone who reloads should have one. The empirical data you can get from having your own really adds to the enjoyment of making, shooting and testing you own ammo.

The x-fps per foot method has never worked for me. Bullet weights and the difference in pressure curves for powder (not to mention each bore being unique) will throw the curve.

Nate

Chrony's are under \$100 nowadays. Everyone who reloads should have one. The empirical data you can get from having your own really adds to the enjoyment of making, shooting and testing you own ammo.

Nate
If you are looking for just a "How fast did my bullet go" chronograph then a sub \$100 one may be OK. . If you are looking for good "DATA" and lots of it, then you need to consider one of the more expensive models.

My "sub \$100 chrony" worked fine as long as the sun was at the right angle, I was not using it under any cover, and I was perfectly happy to wait until I had fired a complete string before retrieving readings. It has no remote other than a reset, doesn't like low light, and like my ex-wife, hard to read and just as difficult.

I finally bought a Pact XP Pro with the readout and printer back at my shooting table. The skyscreens are InfraRed so I don't worry about shadows, low light, or color of the bullet. I can get all kinds of data from shooting strings as long as 100 rounds and it all prints out on a neat little tape I can staple to my development log sheets.

It all depends on what you want or need but don't expect an inexpensive chronograph to perform like the more "fully featured" units.

#### DoubleTapDrew

I got this one and it works very well and can be had for around \$100 when on sale.
Competition Electronics ProChrono Digital Chronograph - MidwayUSA
This is a great unit. One caution though is to make sure to take into account the "bore height", the difference in height between scope and bore. There are several of these where the "brains are downrange" that have met untimely ends. They quit working when they have a 30 caliber hole in them. That's why I prefer the "brains on the bench" types. Shoot a sensor and it's \$25 or so, not a full replacement.

#### DoubleTapDrew

One caution though is to make sure to take into account the "bore height", the difference in height between scope and bore.
+1 I am extra careful, especially with things like AR-15s with carry handle mounted optics that have a large scope/bore offset.

A simple trick if you have one of those inexpensive laser bore-sighters. Put your rifle on the rest and aim it at the target down range. An extra sandbag or two will hold it "on target". With the bore-sighter turned on, place your chronograph in the desired location and adjust the position so the laser beam passes between the uprights and at the desired height. This is a quick way to both make sure your rounds are passing through the "sweet spot" of the chronograph's sensor area and avoid shooting a big hole in it at the same time.

One like this is only about \$25

#### DoubleTapDrew

That laser boresighter trick is a great idea, I'll have to pick one up. Will save me time setting it up without having to keep getting up, move it, check again, move again, etc.

That laser boresighter trick is a great idea, I'll have to pick one up. Will save me time setting it up without having to keep getting up, move it, check again, move again, etc.
I use one and it saves me tons of time. At the range I shoot at it's also a bother to the other shooters because it requires a cease fire to go forward and tend to the chrony. I made some plastic "screens" that are held in place by the sky-screen supports. I put a couple of circles about the size of a quarter where the chronograph manufacturer said the "sweet spot" is. The front circle I left clear in the center and frosted around it with some real fine sandpaper (2,000 grit). The back screen's circle is frosted in the center so it shows the dot.

My rifle has a bipod on front and a monopod on the buttstock. I can set the rifle dead on a target, put the boresighter in place, and set up the chronograph in one step. For anyone that uses something similar, TWO IMPORTANT ITEMS TO REMEMBER. First remove the boresighter and second, take any aiming screens off the chronograph. I haven't made this mistake yet but it's obvious it could be overlooked if distracted.

I like to use a target while shooting over the chronograph because then I can evaluate groups as well as speed and deviations from the same rounds. Some will just set up the chronograph and shoot over it, usually into the ground or some random point in the berm.

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