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Velocity Myths Vs. Facts

Velocity Myths Vs. Facts
Chronographing Factory Ammo.

Quite a few shooters whine about factory ammunition not living up to its velocity specifications. Some even say ammo companies should publish their data as plot-free fiction. The primary reason for this general grumpiness is the availability of affordable chronographs. Before the 1990’s most of us guessed at velocity, but chronographs gave us actual numbers to complain about.

Unfortunately, chronographs can be part of the problem. I started using electronic chronographs in 1979, and my first came equipped with break screens—essentially foil circuits glued to paper. This cost money and time, since a pair had to be replaced after each shot, and to find the velocity I had to turn a dial through a series of numbered lights, record which lights went on, then look up the velocity in a booklet. But it beat guessing!

Later the manufacturer provided light screens, and when that chronograph eventually died I purchased a cheaper light screen model for $50 from the L.L. Bean catalog, of all places. Some .22 Long Rifle ammunition produced the same results as it did over my first chronograph, so it seemed like a really good deal. Over the next 25 years I purchased a few more chronographs, due to “accidents” (bullets) or chronographs going haywire from old age.

Eventually I upgraded to an Oehler, for many years the gold standard of not just handloaders but professional ballistic labs. I also got to know Dr. Ken Oehler and learned, among other things, why chronographs aren’t infallible. The big problem with most is the light screens. Their accuracy, unsurprisingly, is affected by both the intensity and angle of light. I first encountered this while using an inexpensive chronograph under varying conditions, finding the same load’s reading differed around 4 percent depending on whether the sky was clear or cloudy.

After buying the Oehler, I also found one inexpensive chronograph gave readings an average of 2 percent faster than the Oehler and two other Oehlers belonging to friends. While it’s theoretically possible for one $100 chronograph to provide more accurate readings than three $500 chronographs, my money would be on the more expensive models. While Dr. Oehler has managed to minimize the influence of light in his portable chronographs, the professional models used by ammunition companies are set up on indoor ranges with consistent light sources—one reason factory ammo may not match listed specifications in your testing.

Another reason is the distance between screens: The larger the distance, the less any tiny error is magnified. Not only is the distance between my Oehler 35P’s screen 2 feet—twice the distance of more affordable models—but there’s a third screen 2 feet behind the second one, providing a check of the first reading.

Also, while the average velocities recorded by cheaper chronographs may be close to the result from an Oehler 35P, the velocities of individual rounds are usually different. I’ve come to the tentative conclusion that screen spacing of a foot or less isn’t very useful for detecting extreme spread or calculating standard deviation in a string of shots. However, only a minority of handloaders worry about such stuff. Even fewer understand what standard deviation means, or how many shots need to be fired to provide a meaningful number.

Then there’s the difference between muzzle velocity and where the bullet passes over our chronograph. (In reality the chronograph velocity is at about the mid-point between the screens, but let’s not cut things too fine.) Many shooters set up their chronograph at whatever distance feels good that day. Most instructions, however, advise at least 10 to 15 feet, but I’ve seen cartridges from the .300 Weatherby to .416 Rigby produce erratic results at 15 feet, so set up the chronograph at least 20 feet in front of real blasters.
Some computer ballistic programs can calculate muzzle velocity from chronograph velocity. I usually don’t bother with most handgun and rifle cartridges, because the majority of both slow, blunt handgun bullets and fast, pointed rifle bullets lose around 250 or 300 fps in the first 100 yards. For practical purposes, adding 1 fps to the instrumental reading for each foot the chronograph is placed in front of the muzzle is close enough.

A weak battery also influences chronograph readings. Many chronographs indicate when a battery’s getting weak, but even before then an older battery will still give slightly wonky readings, especially on a cold morning. I do some chronographing during Montana winters, often deliberately on days down around 0 degrees F to see what happens, because I don’t want to be surprised by really low velocities, point of impact shifts or hangfires when hunting. All three have occurred during cold tests, but the chronograph results are only reliable with a fresh, relatively warm battery. This is accomplished by switching two constantly, with one warming in a pocket, or by using a HotHands chemical pack on the battery.

velo1

The published velocities of handgun ammunition will typically vary more than rifle
ammo because barrel lengths vary more. The cylinder gap in revolvers is also a major variable.

In fact most chronographs have to be kept warm at temperatures below about 20 degrees F, or their innards don’t work right. (And no, just chilling the ammo doesn’t produce the same results. For one thing, a warm bolt-face almost instantly warms up the primer, and a really cold bore has slightly different dimensions than a 70-degree barrel. The only truly valid test is with both rifle and ammo chilled by ambient conditions, just as they would be when hunting.)

The second major problem is the dimensions of our rifle or handgun’s bore isn’t the same as the test barrels used by ammo factories. Two main organizations oversee the manufacture of ammunition: the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) in North America and the Commission Internationale Permanente (CIP) in Europe. Both were organized so ammunition from various manufacturers would reliably work in firearms from various manufacturers, so their member’s ammunition is regularly shot in pressure-test barrels made to minimum SAAMI or CIP standards.

Most mass-manufactured rifle and handgun barrels have slightly larger dimensions in both chamber and bore, so pressures typically end up lower—and lower pressures mean lower velocities. Many custom barrels and chambers, however, have minimum dimensions, sometimes even smaller than SAAMI/CIP dimensions.

On a visit to one professional ballistics lab, I was told 0.0001 (one ten-thousandth) of an inch in bore or bullet diameter normally results in a difference of 800 to 1,000 pounds per-square-inch (psi) with the same load. This means a factory .30-06 barrel with a bore diameter 0.0005-inch larger than a SAAMI test barrel will result in a reduction of 4,000 to 5,000 psi, a 7 or 8 percent reduction of the maximum average .30-06 pressure of 60,000 psi. One general rule of internal rifle ballistics is that velocity changes at half the rate of pressure, so in our hypothetically oversized .30-06 barrel, a 180-grain load at the standard 2,700 fps would get around 2,600.

Barrel lengths are standardized for SAAMI and CIP testing. The SAAMI standard for most rifle barrels is 24 inches, but commercial rifles come with barrels anywhere from the legal US minimum of 16 inches on up to 26 inches. Most tests performed by cutting off a barrel then chronographing at each length indicate an average loss of 25 to 30 fps. Occasionally, however, the velocity stays the same or even increases, probably due to eliminating a slightly looser area in the bore.

Chambers in factory rifles and handguns also vary due to wear on reamers. The standard procedure is to start with a maximum-dimension reamer, then run it until it cuts a minimum chamber. The cylinder gap in revolvers creates yet another variable. SAAMI tries to simulate the cylinder gap by using vented test barrels, but some handgun cartridges are used not just in revolvers but semi-autos and single shots.

velo2

With most chronographs, results will vary according to light conditions, even with
overhead diffusers in place. Factory ammo will definitely produce more pressure and
hence velocity during summer varmint shooting on 85- to 100-degree days, even when
the ammo’s loaded with temperature-resistant powder.

velo3

This .300 Weatherby factory load with the 200-grain Nosler Partition chronographed very
close to the published velocity at 65 degrees, but landed lower than it should have on
a 320-yard shot during a cold elk hunt in Colorado. Was that due to shooter error, or slower velocity?

Then there’s temperature. SAAMI ammunition is tested at 70 degrees, but an outdoor range is rarely right at 70. Many modern rifle powders are supposedly temperature-stable, but temperature-resistant is more accurate. There isn’t a powder yet made that produces the same velocities at 90+ degrees F as at 70. While some powders do a reasonable job of maintaining their velocity down around 0 F, temp-stability also depends to a certain extent on the cartridge and bullet. Many powders used in factory ammo are not selected for temp-resistance. Instead they’re selected primarily by price, because like most manufacturers, ammo makers need to make a profit. Consequently, many years ago I started carrying a small thermometer in my range bag, and recording the temperature within 5 degrees for every chronographing session.

Occasionally some or all of these factors come together to create a “perfect storm” during chronographing. In the mid-1980’s, Browning sent me one of their brand-new A-Bolt rifles to test, a .270 Winchester. My wife Eileen had just started hunting the year before, taking a buck pronghorn with the Remington 722 .257 Roberts I’d inherited from my grandmother, but now she wanted a real elk rifle, preferably lighter than the 722 so she could carry it up mountains easier.

The A-Bolt was about as light as factory rifles got in those days, and arrived during the fall hunting season. There wasn’t time to work up a handload so I bought a box of 130-grain Remington factory loads. Eileen didn’t get an elk, but did get a deer on Thanksgiving Day.

That winter I decided to work up some handloads for the rifle, and while testing them I chronographed the remainder of the Remington factory ammo. The velocity averaged 2,683 fps, almost 400 fps slower than the advertised muzzle velocity of 3,060 fps. Unlike today, when I record the temperature, sky conditions, wind speed and direction of every range session, I just put down the date, February 14th. It wasn’t exceptionally cold, it was probably around 40 degrees F, the average daily high for that part of Montana at that time of year. I was also using the set of light screens on my first chronograph, and other than testing them against the old break screens with some .22 one summer day, I have no idea how they reacted to different sky conditions. They also didn’t feature overhead diffusers.

The low velocity of the factory ammo astonished me, but after far more chronographing over the next quarter century I slowly realized Remington’s listed muzzle velocity wasn’t a lie. For one thing, that A-Bolt proved to have a “slow barrel,” and not just because of the 22-inch length rather than the standard 24-inch test barrel.

Slow barrels do exist, as do fast barrels, due to throat and bore dimensions, though in factory rifles, slow barrels are more common. I’ve now thoroughly tested over 15 different .270’s, and that A-Bolt got the slowest velocities of any, even a couple with 20-inch barrels. The old Jack O’Connor load of 62 grains of H4831 with a 130-grain bullet barely broke 3,000 fps even on a warm summer day, while several other .270’s with 22-inch barrels chronographed 3,100 or more with the same load. (By the way, 62 grains is still listed in Hornady’s manuals so isn’t over the limit, even though H4831 has changed since O’Connor used the original mil-surp powder.)

After starting to record the temperature during every range session, I also realized another rule-of-thumb was about right for most rifle powders: They did lose about 2 fps per degree F.

Let’s add up all the negative factors, subtracting them from the Remington .270 ammo’s advertised velocity of 3,060 fps.

The 22-inch barrel accounts for about 60 fps. Now we’re down to 3,000 fps.

Next let’s subtract another 100 fps for the slow bore, since that’s the difference with the same loads between the A-Bolt and several other .270’s. The result is 2,900 fps.
Assuming the temperature was around 40, let’s drop another two fps for each degree under 70, for another 60 fps. Now we’re at 2,840 fps.

In centerfire rifle loads, SAAMI allows 90 fps over or under the advertised, standard velocity, and 2,840 fps minus 90 is 2,750.

Finally, we subtract 15 fps for the distance from the A-Bolt’s muzzle to the chronograph, and we’re down to 2,735. This is very close to 2,683, and the slight difference could have been due to light conditions or the temperature being only 30 instead of 40.

So no, Remington wasn’t lying about how much velocity that batch of .270 ammo developed in their test rifle, and neither are other ammunition manufacturers.
By John Barsness

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