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  #1  
Old 02-19-2012, 09:30 AM
Bromel Bromel is offline
 
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Default How to match M2 ball velocity specification exactly?

I understand that the M2 ball velocity specification was 2740 fps measured at 78 feet from muzzle. What velocities does that specification correspond to at the muzzle and at 15 feet where I normally set up my chronograph?
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Old 02-19-2012, 01:57 PM
Mad_Gorilla Mad_Gorilla is offline
 
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Quote:
Originally Posted by Bromel View Post
I understand that the M2 ball velocity specification was 2740 fps measured at 78 feet from muzzle. What velocities does that specification correspond to at the muzzle and at 15 feet where I normally set up my chronograph?

It is virtually impossible to exactly match the velocity, and the fps you list wasn't always the same from lot to lot of ammo.

Most people would list M2 with a 147-155 gr. bullet at 2700 fps and call it the equivalent to WWII issue M2 Ball. Allowing for the difference in distance at which they are measured (about 60 ft.), if you get anything between 2700 and 2750 at 15 ft., you're good to go.

For example, in my rifle (a 1954 Springfield SG), 47 gr. Varget with a 150 gr. Hornady FMJ gives me 2700 +- 20 with excellent accuracy for that bullet (2-2.5 in. @ 100 yds when my eyes cooperate). However, I get better results with 46 gr. and the Hornady 168 gr. HPBT match at 2600. That is roughly equal to the older M1 Ball of WWI.
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Old 02-19-2012, 03:45 PM
colt100 colt100 is offline
 
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I always figured about 2800 for 150 or so and 2650 or so for 168/173 bullets. Of course, I rarely find a load that shoots best at these velocities. Most of the time, I'm back 50 fps or so from "spec" with my best loads. Since this is not that big of a deal, I load for accuracy instead of velocity.
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Old 02-19-2012, 07:40 PM
Unclenick Unclenick is offline
 
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Hatcher lists 2800 fps and later military manuals list 2740 fps. The number is measured at 78 feet from the muzzle. Hatcher's number may simply have been his calculating backward to actual muzzle velocity as closely as he could. I don't recall if he said. In any event, the 150.5 grain flat base FMJ M2 bullet that goes 2740 fps at 78 feet from the muzzle would be going 2798 fps at 15 feet from the muzzle according to a modern calculator.

Please note that velocity is only valid for a velocity test barrel with a minimum dimension chamber and very tightly controlled bore dimensions and length. Most actual production guns will shoot slower, so your best bet for copying is to shoot some M2 side-by-side with the test loads in your own gun to match their velocity. Even then, you need to be sure you use one of the powders the military uses for the purpose. IMR 4895 should work fine.
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Old 02-19-2012, 09:03 PM
mousegun mousegun is offline
 
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It should be noted that the velocity as measured at that time by JCG was not "at 78 feet" but the middle of the distance from the first sensor (at three feet) and the last sensor (at 153 feet) plus the distance from the muzzle to the first sensor. The velocity at 78 feet was assumed to be a close approximation of the average between the two sensors. See "Hatcher's Notebook" page 252 and following.
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Old 02-20-2012, 08:12 AM
Bromel Bromel is offline
 
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Quote:
Originally Posted by mousegun View Post
It should be noted that the velocity as measured at that time by JCG was not "at 78 feet" but the middle of the distance from the first sensor (at three feet) and the last sensor (at 153 feet) plus the distance from the muzzle to the first sensor. The velocity at 78 feet was assumed to be a close approximation of the average between the two sensors. See "Hatcher's Notebook" page 252 and following.
Wow, now that is an interesting factoid. I guess that their chronographs had to be a lot bigger back in then.

Thanks for the feedback, everyone.
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Old 02-20-2012, 03:36 PM
mousegun mousegun is offline
 
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Bit more on the apparatus:
Le Boulengé Chronograph
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Old 02-20-2012, 04:17 PM
Unclenick Unclenick is offline
 
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Bromel,

All the early chronographs were electromechanical and clocked time coarsely by modern standards, so they needed a lot of distance to resolve bullet transit time with any precision. Even then, they had to average a lot of readings to get good accuracy from them. I've forgotten when the Army went over to radio frequency sensing of the bullet, but they still needed plenty of distance as the exact moment of passage wasn't nearly as well defined that way as it is by a bullet shadow.

The assumption average velocity over a distance well reflects exact velocity at the average distance between screens is based on the rate of velocity loss (deceleration) over distance being constant. It isn't exactly true. On the one hand, you'd expect drag to be proportional to the square of velocity. However, between just below the speed of sound and about mach 5, that square relationship is corrupted by the aerodynamic shape of the projectile and creating and shaping shock waves. In that range it the v˛ relationship has to be corrected by a drag coefficient whose shape depends on the shape of the bullet.

In the case of the flat base spire point M2 bullet in the velocity range we are speaking of, it turns out the drag coefficient correction keeps loss of velocity per unit distance close enough to constant to prevent a significant error. Between the 1st and 11th yard (the first 10 yards of the measurement) velocity loss is about 23 fps. Between yards 41 and 51 (the last 10 yards of the measurement), it's about 22 fps. As a result, the average velocity error at 26 yards (78 feet) from using the 1st and 51st yards as the measurement endpoints, is around half a foot per second at the center, and may be ignored for practical purposes.


Mousegun,

Page 251 of my copy of Hatcher's Notebook (3rd ed.) is the next to last page of the chapter on headspace effect on velocity. It makes mention of the Le Boulangé chronograph, but no mention of the spacing setup. If you don't mind, could you say what chapter of your edition has the chronograph spacing mentioned? I'd always remembered the operation of the Bashforth chronograph with its drum recording system being described by Bashforth with examples using 150 foot spacing, but didn't recall if the Le Boulangé was typically set up with the same spacing for its dropping zinc rods marking system. I'd like to reread that part. Just can't seem to find it conveniently using Hatcher's index. Perhaps your link will tell me when I've read it.
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Last edited by Unclenick; 02-20-2012 at 04:19 PM.
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  #9  
Old 02-20-2012, 05:51 PM
mousegun mousegun is offline
 
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Quote:
Originally Posted by Unclenick View Post

.......

Mousegun,

Page 251 of my copy of Hatcher's Notebook (3rd ed.) is the next to last page of the chapter on headspace effect on velocity. It makes mention of the Le Boulangé chronograph, but no mention of the spacing setup. If you don't mind, could you say what chapter of your edition has the chronograph spacing mentioned? I'd always remembered the operation of the Bashforth chronograph with its drum recording system being described by Bashforth with examples using 150 foot spacing, but didn't recall if the Le Boulangé was typically set up with the same spacing for its dropping zinc rods marking system. I'd like to reread that part. Just can't seem to find it conveniently using Hatcher's index. Perhaps your link will tell me when I've read it.
Yeah.. I vaguely remember there's mention of the spacing in Hatcher's book, but I'll have to dig. If I find it I'll post it here and PM you. As far as the specific setup of the hardware, I don't recall that except what's in that british ordnance book of 1877. Also, I found the range setup somewhere else as well, but although my memory is perfect, it's awfully short these days.

UPDATE!
I found the range setup and description of the equipment in chapter 19 ("The Relation Between Muzzle Velocity and Instrumental Velocity"), page 404 of the second edition. Hope this helps some.
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Last edited by mousegun; 02-20-2012 at 06:09 PM.
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  #10  
Old 02-20-2012, 07:14 PM
Unclenick Unclenick is offline
 
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Yep! I scanned right past it looking for it. Thanks for the update!
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