VELOCITY OF FACTORY RIFLE AMMUNITION
Rifle shooters often complain about low velocities in factory ammunition. This does occur frequently, but obviously they don't understand why--and there are several reasons.
The major problem with producing factory ammunition that lives up to its so-called "advertised" velocity is that rifles vary so much. In fact, the organization commonly known as SAAMI (the Sporting Arms and Ammunition Manufacturers Institute) was started in the 1920's because of that very reason. So many companies made ammunition and rifles that not all ammo worked safely, or even reasonably well, in rifles chambered for the same cartridge. SAAMI was started companies could get together and set reasonable standards for cartridge dimensions, pressures and velocities, so somebody could buy a box of, say, .30-30 ammo and have it work reasonably well in any factory .30-30 rifle.
One of the problems with such a project is that any mass-machined product will vary slightly in dimensions, partly because of tooling wear--exactly why SAAMI lists both maximum and minimum dimensions for rifle chambers and bores. Factories start with slightly over-sized (maximum SAAMI dimension) barrel and chamber reamers, then run the reamers until bores and chambers meet SAAMI minimum standards. (The same principle applies to ammunition factories, which start with maximum-dimension case-forming and bullet dies.)
Minimum chambers and bores result in higher pressures, and hence velocities. The head tech at one of the major piezo-electronic pressure labs in the U.S. told me a few years ago that with the centerfire rifle cartridges most of us use up to .30 caliber, a difference of .0001 (one ten-thousandth) inch in bore diameter results in a difference of about 1000 pounds per square per square inch (PSI) in pressure. One general rule of interior rifle ballistics is that any change in pressure results in about half as much change in muzzle velocity, and factory bores can and do vary as much as one-thousandth (.001) inch diameter. Obviously this can result in considerable differences in both pressure and velocity.
The same thing can occur to a much greater extent with chamber throats. A shorter throat can raise pressures enormously, and can occur even in chambers for very common cartridges.
About 20 years ago I bought a used factory .30-06 produced by a major European company. These days there's considerable agreement between SAAMI and the C.I.P. (Commission Internationale Permanente), the European organization that performs the same industry standardization over there, but the .30-06 was apparently made before that cooperation began.
When I bought the rifle, a major American ammunition company had recently introduced rifle ammunition specially loaded to produce higher-than-normal velocities, yet with pressures at standard SAAMI levels. After mounting a scope and sighting-in the rifle, I tried some of the high-velocity 180-grain .30-06 ammo. Muzzle velocities from the 22-inch ran a little over 3000 fps, and the third round blew a primer.
I contacted the ammo company and they asked me to send the rest of the box back for testing. A few days later they reported pressures averaged 58,000 PSI, right in the correct SAAMI range for .30-06 ammo. I then measured the throat length in the Euro-rifle, through the simple but effective technique of seating a flat-based bullet backwards in a resized case, then chambering the round. It took some effort to close the bolt, and the shank of the bullet in front of the case mouth revealed a considerably shorter throat than SAAMI standard.
This is also why many custom rifles show higher velocities with factory ammo than many factory rifles: Many custom gunsmiths use "minimum" dimension reamers to enhance accuracy. Such rifles often get more velocity than SAAMI standards with factory ammo.
A good example is my NULA Model 24 .30-06. With the same 180-grain ammunition that turned my Euro-rifle into an unsafe .300 magnum, got 2950 fps from its 24" barrel. There weren't any evident pressure problems, but I would bet 100 pieces of Lapua .30-06 brass the pressures averaged over 58,000 PSI. All of which is why SAAMI allows their member companies 90 fps either way from their factory-ammo muzzle velocity standards.
Many companies seem to be happier with 90 fps less, no doubt due to safety considerations, but that's when fired at 70 degrees, the standard temperature for SAAMI test-firing, the reason modern pressure labs are air-conditioned. While today's rifles are very strong, there's no way an ammo company can predict what temperature a shooter in the Great Outdoors will encounter, whether when hunting or on a target range. And even the most temperature-stable modern powders will gain noticeable velocity (meaning higher pressure) at much above 70 degrees.
Ammo companies also can't predict the condition of an individual rifle's bore. It may be very dusty or damp, or even rusty. All three condition can raise pressures considerably.
Another factor is barrel length. Well into the 1980's most ammo companies quoted velocity figures from 26-inch test barrels, but by then relatively few factory rifles had 26-inch barrels, with most running 22-24 inches. The general rule-of-thumb is a loss of 25-30 fps per inch of barrel, but that can also vary considerably, due to the powder used or bore dimensions. Eventually SAAMI standardized most rifle test barrels at 24 inches, with some exceptions for rounds often fired in carbines. The standard for the .30-30, for instance, is 20 inches.
Whether factory ammunition chronographs the same in your rifle as the test barrel depends on many factors. The 100-grain Remington factory ammo turned out to be slightly faster than the listed 3230 fps from the 24-inch barrel of this Ruger No. 1 .25-06.
SAAMI also allows member factories to produce (and list) ammo with somewhat lower velocities than their standard for a given bullet weight, if the particular bullet results in higher than "normal" pressures. Fifty years ago almost all rifle bullets were cup-and-cores with very similar construction, so resulted in very similar pressures. Today lead-cored rifle bullets vary enormously in construction, anything from very thin-jacketed cup-and-cores to bonded bullets with much thicker jackets, often made of pure copper, which results in more friction than the gilding metal (a mild brass) used in most jackets.
Plus there are several brands of "monolithic" bullets, made of copper, gilding metal, harder brass or even bronze. These not only tend to be harder than lead-core jacketed bullets but can produce widely different pressures, due to the widely varying numbers and sizes of grooves in their shanks to reduce pressures.
As a result, not all rifle ammunition will be loaded up to the velocity standards suggested by SAAMI. Because of the maximum diameter of many factory rifle bores, it will tend to result in even lower velocities in those rifles.
Another factor often resulting in lower velocities is temperature, plus the chronograph used. While most shooters probably prefer going to the range at 70 degrees, obviously they can't control outdoor temperature, and often can only go on a certain day. In much of the country, range-testing gets done during winter, because summers are for family vacations or too damn hot. Fall is hunting season and spring frequently too windy for decent group-testing.
Typical winter days are usually cooler than 70 degrees over much of the country. While a bunch of cold-resistant powders are now available to handloaders, most factory ammo doesn't use them. Instead they load powders that we often wouldn't even recognize by their designation, though most at least resemble some handloading powders, because they come out of the same factories. At cooler temperatures many will lose around two fps per degree Fahrenheit, which means about 60 fps at 40 degrees.
Factory ammo will usually chronograph slower in cooler conditions. This isn't a winter day, but since I'm wearing a jacket it probably isn't 70 degrees.
Then there's the shooter's chronograph, which may or may not provide accurate results. While "personal" chronographs have improved considerably over the past couple of decades, partly through using something other than ambient light to trip their sensors, most shooters still buy inexpensive light-screen chronographs. While most are accurate, one of the most popular brands can vary considerably, as I found out while testing three of 'em over the years.
Right now I own five chronographs costing from $100 to over $500, including non-light-screen models, and test them for accuracy against each other. At least one of the inexpensive brands is consistently accurate, but even accurate chronographs can provide varying results when their batteries are low.
All these reasons are why factory ammo may not chronograph within 200 fps of the listed velocity in YOUR rifle--and will normally chronograph slower than listed, and rarely even slightly faster. But the major reason, again, is because ammo factories have to design ammo to be safe in minimum dimension chambers and barrels. If they "worked up" ammo in barrels with maximum chambers and barrels, their ammo could cause problems in minimum-dimension barrels, as we've seen.
The reamers used in factories aren't made of the same sort of steel used in custom reamers, so wear quicker As a result, the odds of any particular factory rifle having a minimum-dimension chamber and bore are pretty small, probably around 15-20%. As a result, your particular factory rifle probably won't shoot factory ammo to the listed velocity, though part of the reason may be a 22-inch barrel, rather than the 24 used in most factory testing.
Whether this makes any difference in most hunting is another question. My wife cleanly killed one of her first deer on a cold Montana day with 130-grain .270 factory ammo that only got around 2700 fps when I finally ran it over a chronograph that winter. Despite the low velocity, the deer flopped right over, because Eileen shot it in the right place at around 75 yards. Since her .270 was sighted-in two inches high at 100 yards, the same hold would have worked fine out to 250 yards, which includes over 90% of the big game killed every year.
Factory ammo that didn't meet listed velocity specifications was common even before we started buying our own chronographs. That .270 load was chronographed in the winter of 1984-5, but the very first handloading manual I purchased, Speer's #6, was published in 1964. It includes a list of factory ammo chronographed in their indoor lab, almost all rifle ammo or handgun ammo shot in rifles, like .44 Magnum in a Ruger semiauto carbine.
The 47 cartridges ranged from the .22 Hornet up, and included a total of 126 loads. Only 41 of those loads (32.5%) came within 50 fps of the listed velocity. Of those a mere 15 exceeded the listed velocity, and never by more than a few fps. Among the big losers were ALL of the five .243 Winchester loads chronographed, none of which came within 100 fps of listed velocity, with most more like 200-250 fps, partly because the test rifle was a 22-inch barreled Winchester 88, and the factory-listed velocities came from a 26-inch test barrel. The only cartridge where all ammo came within 50 fps was the .44 Magnums in the Ruger carbine.
My guess, based on considerable chronographing of factory ammo in various rifles over the past 40 years, is the same sort of test would provide similar results today, because even back then factories were obviously loading on the premise of never exceeding listed velocities in minimum-dimension rifles. If this upsets some shooters, perhaps they should heed Colonel Jessup's famous line from the movie A FEW GOOD MEN, delivered so memorably by Jack Nicholson: "You can't handle the truth!"