- By Wheelwrite
- 0 Comments
- Last updated: 15/12/2016
Most of us assembled our first rounds of ammunition using either the tried and tested recipe from a shooting colleague or cautiously constructed a standard load from our first reloading manual. Hopefully the exercise was supervised by someone with experience of ammo manufacture, but nonetheless the process was probably still an act of faith. How did you know what would happen when you pulled the trigger – how far, how fast, how accurate?
Whilst the use of a manual gives you a set of safe working parameters and specifies the ‘performance’ of the round, this data could be significantly at odds with the results that you actually achieve. If the cartridge is accurate, consistent and comfortable to use then does it really matter? Well yes, sometimes.
Real Time Figures
Let’s assume that you’re going deer hunting somewhere in England. The manual quoted muzzle energy of 1750 ft/lbs for your chosen recipe, which is 50 ft/lbs over the legal requirement to shoot larger deer in the UK. So everything seems to be in order, but is it? Let’s take a random sample of .308 Winchester data from a number of manuals for a 150-grain jacketed boat tail bullet. Hodgdon tells us that 44-grains of IMR4895 gives us 2794 fps, Nosler needs 45-grains to get 2790 fps whilst Lyman (45th edition) reckons that 45-grains will give just 2777 fps. Same calibre, same powder, same pill… different results! These variations may be relatively small but they make the point. By calculation we have almost 3% variation in charge weight for the same claimed velocity. So let’s reduce that load giving 1750 ft/lbs by a pessimistic 3% and guess what, it’s illegal!
Last month we touched on the effects of changes in case volume. A reduced capacity such as that found in many brands of military brass creating much higher pressures. Indeed, some manuals recommend a blanket reduction of 2-grains for their published rifle loads when used in such cases. So, the publisher’s choice of brass could have an impact. As we have also discovered, choice of primer, neck tension and crimp could also. However, the variation in manual data often has more to do with their choice of barrel. Most employ a device called a test receiver. This is a precision barrel/chamber assembly made to comply with SAAMI dimensional specifications. Usually they are a minimum of 26”, which is a bit more than your average hunting rifles. The variation will be in the selected rate of twist and the overall length of the barrel, none of which will be fitted with a moderator. The test loads will then be fired indoors in controlled environmental conditions. So how fast is your reload?
We’ve now justified the need for a chronograph. From the Shooting Chrony to the Oehler 43 Mini Labs the choice is quite wide. Basically, all are posh stop watches measuring the time of flight of the bullet over an accurately controlled distance. This is achieved by two, light sensitive sensors that the bullet passes over so switching ON then OFF the stop watch. By knowing this they do a simple calculation to turn this data into speed; normally fps. The more advanced machines then offer a suite of extra features such as the storage of a data, the ability to isolate the highest and lowest velocities from the shot string, calculate the average velocity and also the standard deviation (SD). This can also be known as extreme spread (ES).
Put simply, SD is determined by first calculating the average velocity of the string and by then working out the average amount by which the individual results of the string vary from this datum. It is a valuable indicator of the consistency of the shots in the string. A few models allow the input of the bullet weight and calculate the muzzle energy. As with all handloading kit, buy the best that you can afford, allowing for the portability of your choice, its ability to work in artificial light, the vulnerability of the vital parts to accidental bullet strike and the operating range of the unit (you may also wish to measure arrows or air gun pellets).
Calibration of your chosen unit is the next task. Since there is no actual means of adjusting most chrono’s, this must be of a comparative nature. That is, comparing it with a known and reliable datum. Contracting your local Proof House to test fire a batch of your ammo through your rifle would be the ideal solution since they use calibrated equipment. However, using the law of averages is almost as good. If other members of your shooting colleagues have chronographs then there is mutual benefit to be had by conducting a comparative test. If all the units are in good condition and unmodified then it is reasonable to take the mean figure as being true.
In use it is important to place the sensors at the specified distance from the gun as muzzle blast and debris can cause anything from false readings to actual damage. If you have a forward-mounted LCD display I would suggest you protect this area too. Rain and dust on the lenses of the sensors can cause defective readings as can ‘glint’ from stray light on the projectile. The ‘flicker’ from 50hz indoor lighting has a range of effects, from total failure to read to consistently high or low readings – depending upon the make and model.
So what do we do with the data we’ve collected? E=MV²/450380 is the formula we need. Where E is muzzle energy in Foot Pounds, M is the mass of the projectile in grains, V is the velocity in FPS and the constant 450380 is made up of 7000 (the number of grains in a pound) multiplied by G (acceleration in feet per second squared due to gravity) of 32.17, all multiplied by 2.
Not only can we establish that our prize load will be legal on deer but we can confirm that the effects of our consistency engineering have paid off. A Standard Deviation of less than 15 fps for ammunition for most modern centre-fire calibres should be considered the benchmark. Good shooting.
• Pro Chrono – Tim Hannam, 01977 681639
• Shooting Chrono - firstname.lastname@example.org
• Skan - www.skanar.co.uk
• Oehler - www.oehler-research.com
• Pact - www.pact.com
• Competitive Edge Dynamics – Sportsman Gun Centre, 01392 354854