Why not to trust the MV on the Ammo Box

[DocUSMCRetired]

Why you CANNOT rely on the MV on the Ammo Box!

Here are # of reasons why you cannot trust the MV on the ammo box.

1. Their rifle and your rifle are not the same. Aside from having a different chamber, and possibly barrel length some other things are important too like the barrel twist rate, and how much wear was in the barrel. Was it just recently cleaned, has it ever been cleaned? You simply don't know anything about the rifle used in testing.

2. What were the conditions at the time of the test? Temperature has a physical effect on powder, which changes how it burns. Couple this, with the fact that different powders can vary in temp-stability quite a bit. You just don't know what the conditions at the time of testing were. Also a lot of factory ammunition is loaded with powder that is meter friendly. Meter friendly can often times be ball powder, which is less temperature stable than stick powder often times.

3. Unknown SD. You will often notice that while a MV is provided on the box an SD is not. It is not uncommon for factory ammunition to have an SD of 18 or higher. Sometimes as high as 40+. As such is the nature of metering powder. With marketing in mind, did they pick the high, low, or average end of the SD? We really don't know. You won't either until you test it for yourself. For hand loading, or to be considered a good SD most people look for around 10 or less. Having a high SD is often the nature of metered powder and factory loads.

4. What chronograph system did they use, and how did they back track to a muzzle velocity? A chronograph does not measure MV, it simply measures Velocity at the location it is sitting. As we covered earlier in the week, calculating MV is not as simple as everyone thinks. It requires a semi accurate BC. So whos BC was used to back track to the muzzle or did they even do that? Did they simply print the numbers off the chronograph? What kind of chronograph setup did they use? We know from Lab Testing, not all chronographs are created equal. Without knowing what chronograph was used, you have no idea the quality of the measurement. For more on that read here: http://www.appliedballisticsllc.com/Articles/ChronographChapter.pdf

5. Do they update that data every lot? Or is it the same data from 2 years ago? Some manufacturers rarely if ever re-test and update information. Some update it every lot (Some ammo like ABM Ammo is actually tested every single lot for 1% consistency). Without knowing this information, you could be using data for years ago.

So in short, never trust the SD on a box of ammo, and here is why:
1. You have no idea about the rifle used.
2. You have no idea what the atmospheric conditions were at the time, and yes it matters a lot.
3. You have no idea the SD, and where they pulled the MV from in that SD. (Marketing plays a role here).
4. You have no idea the quality of chronograph they used.
5. You have no idea if they used the raw velocity, or back calculated the MV. The BC used to back track that data is also unknown.

So in short, never trust the MV on the box of ammo as anything more than what the factory was able to do in best case scenario. Unless they are willing to provide you with specific details of the test conditions and equipment.

When you are using a Ballistic Solver such as the AB Apps or Devices integrated with AB. You need to know the MV to an accuracy down to 5 fps. Never use the printed MV off a box of ammo as anything more than a starting point, their are too many factors to account for. You must always either test for the MV with a chronograph, or use live fire carefully obtained data. MV on the ammo box, is simply a marketing tool, and not reliable information.

For more information on Ballistic Calibration, here is a free article: http://www.appliedballisticsllc.com/Articles/BallisticCalibration.pdf

 

[Corn-Picker]

You need to qualify your statement with "for long range shooting" IMO.

To play devil's advocate, a lot of people would do fine trusting the numbers on the box. I can never see further than I can shoot where I hunt; the only time I ever had a 300 yard shot was at the 300 yard range. I need to know that my copper bullet is going 1800 fps (otherwise it won't open up) and an approximate holdover if I'm nearing my max range, which is less than 300 yards for me.

I just ran a few scenarios with my 7.62x39. I decreased the MV by 100 fps and my drop and drift at 300 yards (which is 150 yards further than I'd shoot a deer with my 7.62x39) only changed by two inches and one inch, respectively, assuming a 10 mph cross wind.

I think it's a little too much to call the velocity numbers on the side of the box marketing, they are generally reasonable. The velocities in the airgun world are marketing, they will list a rifle that shoots 750 fps (with a lead pellet) as an 1100 fps rifle because they got some plastic pellet up to that speed once.

Edit: I hope I didn't come off as a smartass, just trying to point out that this only applies to shots past 400+ yards for your average 308 class of cartridge.

 

[DocUSMCRetired]

We actually generalize the statement because you never know what someone's hunting situation is. Someone might be hunting with 22LR,224, 243, 264, 308, 338. We just don't know. But what we do know, is that a lot of people put faith in to the those numbers on the side of the box. Which is generally a bad idea, without more information and maybe even verification. Aside from not knowing the powder used, which in many instances of factory loads has very bad temperature sensitivity. I would hate to see a user who is using 224 varminting that put faith into the MV end up wounding at 300 yards. It is just best to look at what some of those numbers like the MV and BC are. Which in a number of cases are just marketing information. Its too easy to get the right information, and to reliably measure in this day and age.

 

[denton]

The natural dispersion due to the characteristics of the rifle and the dispersion due to variation in MV do not add linearly. This is profoundly important, and often missed by people writing about the topic.

Here is some analysis from one of my Varmint Hunter magazine articles:

So assume that you’re shooting on a perfect, windless day, with your trusty 243, which never throws a flier, and shoots groups where the standard deviation of the radius of the group is .125”. At 100 yards, that will get you a situation where 95% of all shots will land within plus and minus two standard deviations, basically producing .5” or smaller groups most of the time. Also assume that you have a lab grade scale that lets you measure powder to the milligram, essentially “dead on”, and assume that the standard deviation of your muzzle velocity is 15 fps, owing to differences in bullet weight, case capacity, and rifle temperature. Further assume that adding a grain of powder produces an additional 75 fps, and that your average muzzle velocity with 75 grain bullets is 3400 fps.

What effect will imprecision in the powder measure have on groups at, say, 500 yards?

Your baseline situation, ideal conditions, and a very precise rifle, will produce a slightly vertically stretched group. Disregarding the vertical effect of muzzle velocity variation, we would expect 2.5” round groups at 500 yards. Adding the effect of muzzle velocity variation will stretch the group into an ellipse.

Standard deviations do not add linearly . So we have to do some squaring and square rooting to get the final effect, and it turns out that groups will be about 3.2” tall, by 2.5” wide at 500 yards, with a 15 fps standard deviation in muzzle velocity (our assumed initial condition). With a 24 fps standard deviation from the conditions assumed below, the groups would be 4” tall and 2.5” wide. You would have to shoot a large number of groups under ideal conditions to detect this effect.

Besides, you will never know your SD very precisely. In round numbers, if you shoot 10 shots from a lot of ammunition with true SD of 15, the estimate of SD that your 10 shot sample produces can easily be anywhere between 10 and 25. You have to quadruple the sample size to cut the uncertainty in half.

 

[taliv]

Long range starts at 800 yards Iirc

 

[DocUSMCRetired]

Technically, Long Range starts at Transonic. For instance, 300 yards with 22LR. I believe the community as a whole is best suited by not putting a distance on Long Range. 800 yards can be challenging with some 308 bullets that don't perform well through transonic. Meanwhile a 200.20X 308 or a 140 264 will do just fine at 1200 yards. The best thing to do is use the zones Super Sonic, Transonic, and Subsonic is determining factors.

Granted their might also be optical limitations for some shooters, but that will depend on the shooter (1000 yards on iron sights is great competition). You also have positional limitation factors.

I also have to disagree that we won't know our exact SD. In fact with tools like the Lab Radar we can now track every single shot with good accuracy. This of course might be dependent on how you shoot too. For us positional and PRS shooters its not easy to try track every single shot. Not really feasible either. But you can get a really good idea, in a non-invasive yet accurate way.

 

[Bart B.]

At 1000 yards, .308 Winchester rounds typically have a 40" vertical spread with a 100 fps velocity spread. That's 4" with a 10 fps spread.

Unless positive compensation of barrel whip launching faster ones at a lesser angle than slower ones. Then they'll all hit the point of aim. The Brits wrote a paper on this over a hundred years ago on testing their .303 cartridges in Lee Enfields which had huge muzzle velocity spreads.

If several people shoot the same ammo and rifle testing fps with a chronograph, there'll be a 50 to 100 average fps across all of them. Us humans don't all hold rifles the same way against our bodies.

In NRA high power rifle competition, long range starts at 800 yards and ends at 1000. 500 and 600 yards are mid range.

 

[plumberroy]

With the exception or 22 lr and 17 hmr every factory load I have seen run across my Chronograph has been 80 to 150 F.P.S. slower than what the box claimed

 

[Bart B.]

Plumberroy,

That's about the difference between factory velocity tests in a fixed barrel that doesn't recoil when shot and what folks get with rifles held against their shoulders.

If the ammo factory uses a SAAMI spec test barrel, most commercial rifle's bore and groove diameters are a few to several ten thousandths larger, so they'll shoot the ammo a little slower to start with.

 

[lysanderxiii]

Their gun:

Your gun:


BTW, their gun does move under recoil.....

 

[Howard Roark]

In NRA high power rifle competition, long range starts at 800 yards and ends at 1000. 500 and 600 yards are mid range.

Mid range starts at 300 yards.

 

[plumberroy]

Plumberroy,

That's about the difference between factory velocity tests in a fixed barrel that doesn't recoil when shot and what folks get with rifles held against their shoulders.

If the ammo factory uses a SAAMI spec test barrel, most commercial rifle's bore and groove diameters are a few to several ten thousandths larger, so they'll shoot the ammo a little slower to start with.

I know the reasons I was just stating my real word experience test guns are built to get every drop of speed out of a load including being lapped to as smooth as possible Even reloading data does this

 

[FL-NC]

I have seen M118LR (Us military sniper grade 308) from the same box fired through 4 identical and brand new M110 sniper rifles side by side and gotten consistent readings from the same chrono on each rifle but disparities between seperate rifles of as much as 50+ FPS. If this is happening with match grade precision ammo with identical sniper rifles all using the same chrono, then what is happening on a off-the-shelf rifle (which most of us use) with standard (not match grade) ammunition- in addition to all the other variables? The answer is, chrono your rifle with the ammunition you intend to use if you think you may need to fire at anything remotely resembling long range, or even shorter distances if more precise placement is required. And preferably under conditions as close as possible to those that will be present when its going to count.

 

[lysanderxiii]

I know the reasons I was just stating my real word experience test guns are built to get every drop of speed out of a load including being lapped to as smooth as possible Even reloading data does this

Actually, they are designed and built to eliminate all variables (at least as much and physically possible) inherent in the test gun. It is not intentionally done to inflate the velocity numbers.

After all, the reason they test fire rounds is for quality control, and if the test gun introduces velocity and accuracy variables, how will you ever know where the quality of your ammunition produce is?

 

[lysanderxiii]

I have seen M118LR (Us military sniper grade 308) from the same box fired through 4 identical and brand new M110 sniper rifles side by side and gotten consistent readings from the same chrono on each rifle but disparities between seperate rifles of as much as 50+ FPS. If this is happening with match grade precision ammo with identical sniper rifles all using the same chrono, then what is happening on a off-the-shelf rifle (which most of us use) with standard (not match grade) ammunition- in addition to all the other variables? The answer is, chrono your rifle with the ammunition you intend to use if you think you may need to fire at anything remotely resembling long range, or even shorter distances if more precise placement is required. And preferably under conditions as close as possible to those that will be present when its going to count.

Or zero at the range you are going to shoot....

Again, clocking ammunition is a quality check, not a guarantee that all things that shoot it will have the exact same point of impact.

Artillery, on the other hand....

 

[Bart B.]

Howard, if mid range for NRA high power competition starts at 300 yards, why does the NRA rule book say:

Note: The military target for 200 and 300 yards is known as “Target, Rifle, Competition, Short Range” and the target for 500 and 600 yards is known as “Target, Rifle, Competition, MidRange.” These definitions are abbreviated as “SR” and “MR” respectively in the descriptions which follow for the reduced targets.

I agree with lysanderxiii on fixed barreled actions being used for ammo velocity testing. They eliminate the variables in how the barrel's held. Otherwise, there would be a standard for free recoiling test fixtures in SAAMI specs. While some test fixtures are recoiling, all ammo should be tested on the same one else their differences will cause different muzzle velocities with the same barrel and ammo but different universal receiver mounting. Here's one spec'd by SAAMI:

http://www.ulyssesmachine.com/home-1

 

[stiab]

Why you CANNOT rely on the MV on the Ammo Box!

I've tested lots of modern ammo for comparison with the box or catalog, and for my purposes (self defense, plinking, & hunting), it's been really close.

 

[FL-NC]

lysander- yes, that is the old way- lots of shooting, lots of log books, etc. Using ballistic programs like Horus TRAG (which is what we used in the mil) is more efficient when the old way isn't possible. The root of the equation is knowing the MV of YOUR rifle. Some situations where logging data isn't preferred: Soldier at Ft Bragg (263 feet above seal level, 72 degrees NOW) zeros his rifle. Next day, gets on a plane to Shkin, Paktika, Afg. (+- 8000 feet above sea level, 53 degrees). The Soldier zeros his weapon at 100 yards (no KD range), inputs muzzle velocity and environmental data, bullet weight, etc. into his device, and can obtain a shot data card from muzzle to the max effective range of his weapon in increments of 100 meters or even less if he desires. The same process could be used by a hunter in Florida (hot as hell degrees always, sea level) going to the Canada (freezing cold always, up on some mountain) to hunt a record big game animal at long range. In either case, copious and diligent logbooks recorded in NC or Fl would be somewhere between inaccurate and totally worthless.

 

[Howard Roark]

Bart, if 300 yards is not considered mid-range then how is it part of the mid-range course of fire?

There are several references to 300 yard mid-range courses of fire in the rule book:

Rule 7.20: Mid-Range Prone Courses—

Note: The Mid-Range Course of fire may be shot in any combination at 300, 500, or 600 yds.

Rule 19.5.2 Courses of Fire used for Mid-Range Prone Classification-

300 yards – When not part of an NRA High Power Rifle tournament or is part of a Mid-Range Tournament Rule 7.10 metallic or any sights.

Rule 17.5 Courses of Fire for which National Records Are Recognized—

(ah) Mid-Range Course of Fire—Metallic Sights - 300 yds...
(ai) Mid-Range Course of Fire—Any Sights – 300 yds...

 

[denton]

I also have to disagree that we won't know our exact SD.

Let me try to be a bit more clear.

If you have a truckload of a particular cartridge, and you want to know the standard deviation of its muzzle velocity in a particular rifle, you have two choices.

1. You can shoot up all the ammunition, recording the MV of each. From this, you can calculate the true standard deviation of the truckload. But, at the end of the truckload, the true standard deviation is no longer very interesting because it's all gone, and you no longer have any use for the information.

2. You can draw a sample of a few cartridges and fire those. From the sample, you can estimate the SD of the truckload. This is the interesting case because you still have almost a full truckload left, and you have an idea of how uniform it is. That's useful. That's why we almost always do it this way.

The problem is that SD is not anywhere nearly as well behaved as means (averages). If you have 25-35 samples, your estimate of the mean based on a sample will be pretty good. However, your estimate of the SD will still be subject to a lot of error, just from luck of the draw in the sample you chose.

The point is not to know the standard deviation of the 10 or so rounds you drew as a sample. The point is to know how well it predicts the next 10 rounds and the next. The answer is, not very well.

The other point is that if you have several sources of variation (bullet weight, powder charge, neck tension, etc.) and if one of the sources is much larger than the others, the largest source will almost completely determine the total variation. You cannot shrink variation by working on the weaker variables.

If you want to test it, prepare five samples of five cartridges each. Shoot them and record the MVs. Calculate the SD for each of the five shot samples. See if they are close to each other. Unless you are pretty lucky they will vary quite a bit, something on the order of -50 to +100%.

 

[ironworkerwill]

The next question is, what does the factory test? Is it a full tilt production run or do they test handloads ? I suspect they test the handloads under controlled circumstances before they tool up production...and print ammo boxes with advertised velocity.

 

[denton]

When I was working on M855 problems at LCAAP, they constantly drew samples from production and test fired them in M16s and in test barrels. With a new lot of powder, they would tweak the charge weight to get the MV the Army required. I may have missed it, but I didn't see any handloading equipment in the place. BTW, step through the door of that plant and it's 1943 all over again.

 

[Bart B.]

Howard,

When courses of fire at 300 yards use targets for the 600 yard stage of the match, their scoring rings are reduced from the standard mid range (MR) scoring ring dimensions. Such courses of fire are called mid range. Here's the target used:

NRA No. MR-63—Reduction of the MR-1 target for use at 300 yards to simulate the 600-yard stage of the National Match Course.

It's confusing, I know, especially when short range targets have the same scoring ring sizes for 200 and 300 yard rapid fire stages.

ironworkerwill,

In my chats with factory reps from Federal and Remington, they use short sample runs of ammo from regular production lines to test for both pressure, velocity and accuracy. That's the only way they'll know if it meets specs. Problem is, they're pretty secret about those specs. They may quote accuracy from their rifles, but we don't know how they're held when fired; free recoil or hand held, a big difference in results.

SAAMI document on velocity and pressure standards says the following:

Ammunition tested subsequent to manufacture using equipment and procedures conforming to these guidelines can be expected to produce velocities within a tolerance of ±90 fps of the tabulated values.

Then there's this statement in SAAMI's info on velocity tests:

Due to the fact that sporting firearms for general distribution are typically manufactured to dimensional tolerances greater than those specified for test barrels, there should be no expectation that these velocities can be duplicated from any test utilizing firearms.

Lake City Army Ammo Plant used 270 rounds of 30 caliber match ammo testing loads for production runs. That gave a very high level of confidence that pressure and velocity specs were met. That ensured the rifle sight settings could be used for different ranges with different lots and be very close to exact. And that proper functioning was possible.

 

[jmorris]

Could also start a thread in "general" about why autos don't get advertised MPG....

Lots of variables that are not in consideration.

 

[Bart B.]

jmorris,

I've got higher MPG numbers that auto specs state for the last 4 new cars bought. But I don't care about MPG numbers all that much; I want to know how many miles per dollar of gas the car gets; that's fuel economy because it's got a monetary unit in the expression. MPG is fuel efficiency.