The chronograph as a sort of polygraph for ammo manufacturers

[stubbicatt]

I haven't shot factory ammo in a long while now, except for the odd combloc stuff. Yesterday I took a box of Hornady manufactured Vmax ammunition to the range with me to compare to my handloads when shot over the sky screens of my CED chronograph.

The Hornady ammunition was approximately 100 fps slower than advertised on the box, and 99 fps slower than my handloads, which featured the same bullets. Each showed relatively the same ballistic uniformity. The two data sets follow:

Hornady 55 gr. Vmax ammo:
3131.0
3125.0
3150.0
3084.0
3148.0
High: 3150.0
Low: 3084.0
E.S.: 66
Ave.: 3127.6
S.D.: 26.6
95%: 36.9

My handloads
3262.0
3213.0
3218.0
3243.0
3198.0
High: 3262
Low: 3198
E.S.: 64
Ave.: 3226.8
S.D.: 25.5
95%: 35.4

Hornady has a label on their box which proclaims muzzle velocity of 3220 fps.

Mine is a 23" barrel, so the deficiency in velocity is not due to a shorter, carbine length, barrel. But their average velocity is right at 100 fps slower than they advertise.

All very interesting I find.

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Aside: 7.62x39 Ammo

For those who may be interested, the Hot Shot 7.62x39 ammo clocked as follows:

2261
2261
2294
2256
2261
High: 2294
Low: 2256
E.S.: 38
Ave.: 2266.6
S.D.: 15.5
95%: 15.5

I was surprised at how uniform that ammunition is. The kid who was shooting that rifle shot a 8" (roughly) group at 470 yards. So the stuff will shoot pretty good. It doesn't hurt that he is young and can see the sights well.

 

[outerlimit]

But you also have to take some things into account. What barrel length was the spec written for. What barrel length was it fired from. What type of rifling does the barrel have. Is it chrome lined. What were the conditions under which it was fired.

By the way, it is highly irregular that you would get three shots out of five (two of them in a row) with the exact same speed. Might want to check out your equipment.

 

[stubbicatt]

outerlimit: what sort of check of my equipment do you recommend? I am at a loss as to where to start?

CED is regarded as one of the best, if not the best, chronograph available here in the US. It is used by the the USPSA at their regional matches due to its reliability and precision.

I'm at a loss as to what I should check? Please advise.

If you look at the previous 223 readings, which were taken only moments before the 7.62x39 readings, they show a greater variation. If the device worked properly for the 223 readings moments before, I am content that it was working properly only a few moments later for the x39 readings.

I accept your skepticism about the 7.62x39 velocities, but I am going to accept them due to the validating circumstances.

And of course you are right about the potential explanations for the differences in velocities from the Hornady factory ammo and my handloads. I was shooting a 23" barreled rifle. Assuming approximately 30 fps per inch of barrel difference, I guess the Hornady ammo may have been tested in a 26" barrel to arrive at the velocities they published on their packaging.

 

[OurSafeHome.net]

ammunition was approximately 100 fps slower than advertised on the box

Two 'identical' rifles could have a difference of 100 f/s using the ammunition from the same box and the same chronograph.

One rifle, using one box of premium ammunition (home-made or store-bought) could easily demonstrate a 100 f/s change in velocity just by leaving the weapon, ammunition and shooting range sitting in the sunlight all day.

That you got within 100 f/s of what was on printed on the box was pretty good.

That the young man was able to shoot an 8" group at 470 yards with anything that chambers 7.62 commie ammo is truly amazing.

"A man who has one watch knows what time it is, a man who has two watches is never sure."

 

[USSR]

Every firearm is a world unto it's own. For example: I have two bolt action .30-06 rifles. Both have heavy contour 26" barrels. With the very same load, one does 2950fps, while the other only gets 2875fps. Anyone who believes they will attain the same velocity listed on an ammo box or in a reloading manual is kidding himself.

Don

 

[jfdavis58]

Question about results

What does the 95% number refer to?

A hundred fps difference between published data and chrono data is typical; as is gun to gun (same model) variations, I've got a number of gun 'pairs' all produce similar spreads but different average velocity. I've seen identical readings on many occasions, again not uncommon.

Added my $.02 mainly to get an answer to the question.

 

[Oro]

What does the 95% number refer to?

In statistics, that is called a "confidence interval." What it means is that based on the data collected, 95% of the results should fall within that deviation. Think of it as a broadened bell curve.

E.G.: 95% of all shots will be within +/- 35.4 fps of the mean for his handloads.

 

[buck460XVR]

Hornady has a label on their box which proclaims muzzle velocity of 3220 fps.

Mine is a 23" barrel, so the deficiency in velocity is not due to a shorter, carbine length, barrel. But their average velocity is right at 100 fps slower than they advertise.

All very interesting I find.

interesting? Maybe. Unusual? IMHO, not really. I've always assumed the velocities labeled on ammo boxes was approximate, were from a test barrel and are only given for comparison. Most factory rifle bullets are manufactured to feed reliably in any firearm and not to a specific OAL for an individual rifle as are most handloads. The main concern of most factory ammo is accuracy and terminal performance, velocity is relative. You said that ballistic performance was similar. Does this mean that both factory and your hand loads shot equally well when it came to accuracy? To me that is interesting.

 

[unspellable]

Factory numbers

Factory ammo is generally fired from a SAAMI test barrel which will have the chamber, bore, and rifling cut to the SAAMI minimum dimensions. This will produce the highest pressure for the load since the primary interest is to not blow up the consumer's gun. However I find the velocity numbers to be highly suspect.

I fired 7.65 mm Parabellum ammo from Winchester and Fiocchi from several pistols of two different types, three different makes, and four different vintages with various barrel lengths.

The original DWM specs call for 1220 fps from a 120 mm barreled pistol, NOT a test barrel. The measuring equipment of the day was quite accurate enough and biased a bit towards a low reading. Winchester claims 1220 fps from a 4.5 inch test barrel. Close enough to 120 mm. I fired the ammo from pistols with barrels of 100 mm, 4.5 in, 5 inch, and 150 mm. The velocity increases noticeably as the barrel gets longer. None of these pistols got even close to the advertised figure. Not even the 150 mm (6 in). The chrono ain't lying either. The recoil was not sufficient to provide reliable cycling. Fiocchi makes no claim to velocity but their ammo was even slower.

It's my firm belief that at least some factory numbers are inflated.

 

[Lone_Gunman]

Mine is a 23" barrel, so the deficiency in velocity is not due to a shorter, carbine length, barrel. But their average velocity is right at 100 fps slower than they advertise.

All very interesting I find.

Maybe you have a slow barrel... At rifle velocities a 100 fps difference between barrels not uncommon.

 

[RonE]

I shot some CCI .17 magnum that advertised 2,550 fps on the box and it chronographs at 2,545 which amazed me.