By John Barsness, GUNS Magazine
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.
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.
Next, visit a ballistics lab