What is the muzzle velocity of that shell? That question, of something very much like it, is raised quite often on Internet forums. Invariably, someone will respond about chronograph testing of one load or another and then a discussion will ensue and sometimes those discussions become a bit heated.
Exactly how are shells tested for muzzle velocity and what industry standards are used? To set the stage just a bit, it must be understood that there must always be standards or baselines for any evaluation. Without those standards or baselines, there would be no way to scientifically compare anything. I say that because at times very serious “discussions” can take place when some of those standards are discussed.
Recently, such a discussion took place when a 30” barrel for testing 12 gauge shotshells was mentioned. There was a comment that few people shoot 30” barrels currently, so in effect, that testing was irrelevant.
The reality is that the 30” barrel establishes the baseline for testing. By using the same length barrel for all 12 gauge shells, the muzzle velocities of the various shells can be compared.
The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) is an association of the nation's leading manufacturers of firearms, ammunition and components. SAAMI was founded in 1926 at the request of the federal government and tasked with:
Creating and publishing industry standards for safety, interchangeability, reliability and quality;
Coordinating technical data;
Promoting safe and responsible firearms use.
SAAMI supports science-based solutions to the many issues related to firearms, ammunition and components.
In this brief article, I will discuss the “basics” of how shotshells are tested for muzzle velocity. I will also include some information about “barrel and choke strain” and the service life of barrels. It is by no means an exhaustive narrative of the subject but may provide a glimpse into what can some complicated procedures.
The appropriate test barrel must be properly serviced and the chronograph set. Then a
cartridge should be seated in the chamber.
The rate of fire should not be rapid enough to cause excessive heating of the barrel. The time
between rounds depends on the equipment, as the barrel may be cooled by a constant stream of air on the outside or by directing air through the bore after each ten rounds.
A minimum of one and up to three warming shots should be fired before firing each series
for record. The velocity and/or pressure of these shots may be recorded, but should not be
included in the record of the sample.
Ammunition shall be conditioned for a minimum of 72 hours at 70° ± 2°F (21.1° ± 1.1°C) with relative humidity of 60% ± 5% before firing.
Tests should consist of ten (10) rounds for velocity and pressure fired during a single day.
Test barrels shall conform to SAAMI specifications for internal dimensions, length, and transducer location.
Counter-chronographs and inductance sensors or photoelectric screens (depending on the type of load) shall be used in velocity measurements.
Only an approved transducer shall be used in pressure measurements.
All loads fired in test barrels with a choke constriction of .031 ±.005 (0.79 ± 0.13)
SAAMI Definitive Proof loads are shells commercially loaded by SAAMI member companies which develop pressure substantially exceeding those developed by normal service loads. The pressure levels are designed to assure gun safety when using ammunition loaded to service pressures in accordance with accepted American practices.
Proof loads are designed to stress firearms components which contain the cartridge in order to assure safety in the recommended use of the firearm during its service life.
It is important from the safety standpoint that Definitive Proof loads be used only for the proof of
firearms. Adequate precaution must be taken to protect personnel performing firearms proof testing.
The supply of Definitive Proof cartridges will be the responsibility of the company that first
introduced that particular shotshell to the Institute. Definitive Proof Cartridges should be loaded with the heaviest shot charge used at the time of introduction and the slowest powder which will meet the pressure values indicated for that particular shotshell to maintain effective pressure-distance relationship. Once established, the shot charge weight for the proof load does not change unless the charge weight becomes obsolete.
It should be noted that all barrels are not necessarily suitable for use in determining pressure or velocity levels. New barrels may require a number of rounds to be fired to remove sharp corners or burrs resulting from the manufacturing process. Barrels in service do not have an unlimited life and may become unserviceable from wear and erosion. There is no predictable number of rounds to which a barrel should be exposed before use for pressure and velocity determinations, nor is there a predictable round life for such equipment.
Barrel length may vary depending upon the gauge of the shotgun. A 12 gauge gun will be tested with a 30’ barrel and a 20 gauge gun will be tested with a 26” barrel. All velocities will be taken at 3’ from the muzzle.
The general type of equipment used to measure the amount of dynamic choke strain (or
stress, if the modulus of elasticity of the barrel is known) in a shotgun barrel is the
resistance strain gauge. The principle of the strain gauge is based on its ability to
deform exactly as the choke does under firing conditions. Deformation of the choke
produces like deformation of the strain gauge. With the proper electronic interface,
dimensional changes in the choke are converted from resistance changes in the strain
gauge to strain measurements of the choke.
The subsequent gauge set-up will allow for the measurement of residual choke
deformation as well as maximum dynamic choke strain for each particular shot.
Residual choke deformation indicates the cumulative effect of deformation. At the
conclusion of the test, residual choke strain will monitor permanent deformation.
Dynamic choke strain will define the maximum strain in the choke (at the gauge
location) when the shot is being constricted through the restricted area. Strain will be
present both before and after the shot passes the gauge, but will be a maximum only at
one point in time.
Okay, if you’ve read this far you have shown that you have an interest in the proper testing method for determining the muzzle velocity of a shotgun shell. I would like to emphasize a few points:
SAAMI is an organization tasked by the Federal Government with testing all things firearms related. This is one area of life in which the government is not involved, mainly because the industry has regulated itself well.
Determining the muzzle velocity of a shotgun shell requires some baseline information and involves a very specific procedure under controlled conditions. Simply shooting a shell or two through a chronograph does not do the job.
Barrels and chokes do not have an infinite life. Older barrels and may not show signs of strain to the eye of the shooter but firing many loads may produce strain which can make the barrel and or choke unsuitable or unsafe. All shooters who fire high-powered shotshells and those with hard shot, should take this into account.
The more we know about our firearms and components the better we are equipped to shoot safely and have fun shooting. We can also pass along our knowledge to our fellow shooters and help to create a more knowledgeable shooting public.