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Question for those more energetic than myself

effendude

Gold $$ Contributor
I have read with great interest all of the discussions regarding the new Magneto magnetic chronograph. Here is my question: When does a bullet stop it's acceleration down the barrel? I hear all the time from the tacticool crowd that 16" is all a bullet needs to burn all the powder, but their cartridges are more limited.

I know that powders and cartridges make a difference. Most F-class and LR shooters use as long a barrel as possible within weight and balance limits of their discipline. 34" barrels are becoming more and more popular. Does measuring the speed of the bullet before it leaves the muzzle leave some possible inaccuracies? As an engineering school drop out, I am theorizing at generally what point the bullets starts decelerating. It may continue to accelerate for a very short period after leaving the muzzle. No more friction of the barrel but it suddenly dies meet some air resistance. If that is the case, measuring the bullet velocity before it is done accelerating might not be the most accurate method of measuring the velocity.

I recall an ad for Marlin Goose Guns where Jim Carmichael of Outdoor Life cut off the 32" shotgun's barrel an inch at a time. The velocity dropped a measurable amount with each cut of the barrel.

Makes a guy like me wonder. Better than watching AGT with the wife....

Scott
 
I am no expert but here is my take.

Using QuickLoad, I can see that the full complement of powder does not always get fully burned before the bullet leaves the barrel. The amount of powder that burns differs depending on several variables mainly what and how much powder is being used. So the statement that a bullet stops accelerating after x number of inches of the barrel is just not true.

With the question of measuring velocity before the bullet leaves the barrel, I feel there will be some differences in the reading but how much difference would be hard to determine. Even if you could figure out exactly what the difference is, would it really matter? When I use the Chrono, I am looking for consistency and not to get the last available FPS out of the load. As long as the measurement is taken at the same place from round to round, it will show if the round is consistent. If you loose 50 or 100 or whatever FPS by using the Magnetic Chrono, that would be ok with me as I never load to the absolute max where it would get me in trouble from a lower reading.

I also believe that as soon as the bullet leaves the barrel and the gases escape, the bullet starts decelerating since it is no longer being driven and hits air resistance and that would be a very very short distance past the end of the barrel.

BTW, I did watch AGT with the wife so does that make me more energetic? ;)
 
Here's my experiance. Two 6mm-284's (1) 24" bbl the other is 29.5" chamber with the same reamer same Man BBL recieved the same day. The 29.5" is 294 fps faster than the 24" with the same load. There is also a thread on this subject on BRC.
 
Is not the column of air in the barrel at 14.7 psi, therefore wouldn't the bullet see air resistance from the git-go. My take on this: Once the bullet base leaves the muzzle, the expanding gasses dissipate to the surrounding atmosphere (pressure drop) instantaneously losing influence on the base of the bullet: I say peak velocity is at the point where the bullet base leaves the crown.
 
LHSMITH said:
I say peak velocity is at the point where the bullet base leaves the crown.

There is an area of expanding gas right after the bullet leaves the crown that will still have some effect on the bullet. Yes, it will dissipate quickly but I've some High Speed photography that shows this gas mushroom to extend a significant distance beyond the muzzle, inches though, not feet.

As I view the pic's of the Magnetospeed, the first sensor is not exactly at the muzzle crown and the mount appears to be shaped to allow this gas bubble to dissipate before the bullet reaches the first timing point.

That's how I see it but take into consideration my last nap in a physic's class was back around 1964 or so. 8)
 
My understanding is that the moment the bullet leaves the muzzle it is actually passed for a number of inches by the air pressure expanding and passing for a short (12-18") distance the bullet it drove down the barrel and the bullet is actually traveling 'DOWN WIND'. I have no idea where I got this info except it was more than 50 years ago, and surely not in a classroom. I thought that a chronograph had to be set a certain distance beyond the muzzle so as not to read this short rush of air, as opposed to the bullet itself.
 
When does a bullet stop it's acceleration down the barrel?
It doesn't -ever
I hear all the time from the tacticool crowd that 16" is all a bullet needs to burn all the powder
Bucket full of crap
Does measuring the speed of the bullet before it leaves the muzzle leave some possible inaccuracies?
How is anyone doing this? You do realize the Magneto does not do this, right?
 
I'm waiting for the day that someone develops a system using strain gauges (like Pressure Trace) that actually measures the speed in the barrel. Just think, you'd also have a direct readout of barrel timing as well.
 
It would be nice amlevin.
Pressure trace does show total time of event.
But barrel timing -w/resp to position/dimension of bore to bullet exit, would be more complicated than strain gauge measure.
You have bore twisting, whipping, vibration, and dimensions at pressures, variables like temps/fouling, plus the gun's movements.

I thought once that I could passively capture it with laser micrometers and DAS, but my sampling wasn't fast enough..
On reconsidering the endeavor; such a speed with needed precision and resolution is still too expensive for benefit (to me).
 
I don't own one, but from what I've seen of the magneto speed chrono it is shaped like a bayonet and measure the speed just after the bullet leaves the barrel. The part on the barrel is just an attachment device.

The barrel length required is based on the powder used. Faster podwers require less length. A .223 might get away with a 16" barrel but a 16" 338 Lapua would be a big disappointment.
 
(Optimum Barell Length) is thought to be at the point of diminishing returns re velocity, I was told by shilen barrels many year ago that it was 16 inches for 22lr, 24 inches for .223, and 28 inches for larger cartridges, now this can only be an approximation, I'm more interested in optimum barrel lenghts for accuracy which is harder to find, when accurate reloading did their tests by cutting down a .223 barrel the accuracy started to fall off under 20 inches, anybody got a quality larger barell IE; 6.5-7mm they would like to sacrifice for the sake of science ? obviously this will vary for different cartridges. dooley UK
 
FYI, Some information on velocity versus barrel length for various calibers. http://www.loadammo.com/Topics/October05.htm
 
thanks john henry, interesting article, I think that between 28 and 32 inches velocity drop gets even less, probably about 12 fps per inch of barrel
dooley UK
 
And barrels of 280"to 320" might only change velocity 0.12fps, but acceleration(however slight) will always remain.
The consideration here should not be about MV gains/losses though.
It should be about accurate balance for best tune and management of external ballistic realities(like wind).
Too bad there aren't any tables or formulas for this yet..

But accuracy-wise, you know a 300WSM won't benefit from a 20" barrel, and a 223Rem won't benefit from a 30" barrel.
Muzzle pressures would be a huge slap to boat tails with a 20" 300WSM, causing ugly release.
And 8-10" of extra whip and weight mis-balance, won't help a 223 with potential accuracy over it's normal 20-22" length.

Maybe in the future this will be defined. I believe it could be with enough resources.
 
so a 223 would not benefit from a 30 inch barrel, Pity our laurie holland does not know that, he uses a 30 inch barelled 223 with 90gr bullets in ftr comps with great affect at 1000yds and even further, and very often beats the 308s.
dooley UK
 

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