Richard Saunders’ guide to chronographs

Richard Saunders gets fast, but not furious, as he puts four chronographs through their paces.

Ignorance, it is said, is no defence in law. Responsible airgunners know that without a Firearm Certificate (FAC), possessing a rifle that pushes out more than 12 foot pounds (ft-lb) will not only get you into trouble, but fuel the argument of those who’d like to see our sport disappear.

There really is no excuse for exceeding the legal power limit. Manufacturers go to extraordinary lengths to ensure their products comply, and licensed gunsmiths know that any breach would have serious consequences for their livelihood.

However, the reality is that even new guns can see power levels creep up over time. Personally speaking, everything I touch with a spanner explodes in a cloud of springs and cogs and can only be fixed with the skilful application of a credit card.

However, many of us like to tinker with our rifles – springer owners love nothing more than fitting tuning kits, and I’ve lost count of people who’ve told me about the ‘mods’ they have made to their PCPs.

Fortunately, there’s a piece of equipment that will help you stay on the right side of the law. I’m talking, of course, about the humble chronograph. 

That said, they have many more uses beyond keeping you legal.

For example, a ‘chrony’ will determine the sweet spot on your PCP – the fill pressure range where your rifle is most consistent. It will also help you identify potential problems, and for you tinkerers out there, it will validate the effectiveness, or otherwise, of your labours.

Crudely put, chronographs measure the time it takes for a pellet fired from a gun to pass between two sensors. To work out the energy output, the feet per second (fps) reading has to be multiplied by itself, then multiplied by the weight in grains of the pellet and divided by 450,240.

So, let’s say your .22 rifle gives a reading of 530 fps. With 16 grain pellets, that would be: (530 x 530 x 16) / 450,240 = 9.98 ft-lb. Fortunately, if maths isn’t your thing, plenty of calculators on the internet will do the number-crunching for you.

We’re putting four different chronos ranging from £49.50 to £265 on the test bench. So if you plan to carry on shooting your air rifle for plenty more years to come and not find yourself sat in front of a bloke wearing a funny wig, read on!

Combro CB-625 MK4


Small, light and convenient to use

RRP: £49.50

It’s diminutive size and Heath Robinson attachment method puts some people off this bijou chronograph, especially when the alternatives are usually impressive metal constructions. Don’t be fooled however, as the Combro cb-625 mk4 is a competent and effective tool.

Measuring just 150x49x23mm and weighing only 55 grams – including the four supplied Duracell LR44 batteries – the unit is designed specifically for airgun use, while claiming (a little confusingly) to handle a maximum of 2,000+ feet per second.

It attaches to the muzzle with a couple of elastic bands, and uses a yellow plastic tool to ensure alignment with the bore. You can also use the cb-625 on guns with silencers.

In fact, it will handle muzzle diameters ranging from 12-32mm. No doubt Combro has looked at more elegant methods of attachment, but the fact is elastic bands are easy to use and they do work. 

There are three buttons, and the instruction manual is excellent. The ‘RST’ button switches the unit on – there’s an automatic 60 second cut-off – and scrolls through measurement unit options and other menu choices.

By timing pellets through a 60mm, 4MHz trap, the FPS button provides a reading in feet or metres per second, and by entering your pellet weight, the PDS button will give you a power reading in joules or ft-lb. Holding down the buttons provides average, minimum and maximum readings. 

If you really want to nerd out, there’s a computer interface port that requires a cable that is not provided, and free Windows software, downloadable from Combro’s website. However, if all that’s too much, the cb-625 will also allow you to set a simple ‘legal/illegal’ indicator.

Being so light and compact, the cb-625 can be used anywhere it is safe to do so. It’s accurate too. In fact, Combro promises accuracy with a 1 per cent tolerance, but says it will only release a unit if it succeeds in recording the velocities of three consecutive shots within 0.5 per cent accuracy. Each cb-625 comes with a specific reference number which you can use to access the test data on Combro’s website.


AirForceOne Ballistically Brilliant Chronograph

Compact, tough and well-made

RRP: £119.99

The AirForceOne Ballistically Brilliant Chronograph from The Shooting Party may not win any awards for the snappiest name, but take it out of the box, and you’ll instantly appreciate the quality of its engineering.

Measuring 105x70x57mm and weighing 320 grams, it’s compact and light enough to go into a rucksack, thanks to its aluminium casing, which is finished in blue powder coat.

The unit comes with a lead to transfer information via a data port to a computer, and a charging cable which also attaches to a PC. The Shooting Party recommends the Ballistically Brilliant Chronograph be fully charged before use, and says it can handle velocities up to 2,789 fps.

There are two small buttons at the front. The top one scrolls through different modes, while the bottom one activates a backlight, and also enables you to move through the menus. The digital display defaults to a 12:00 time setting when switched on, and is adjusted with the mode and backlight buttons.

Pressing the mode button again shows the temperature in centigrade and Fahrenheit as well as humidity values. Holding it down will take you to four dashes, indicating you are ready to start measuring pellet velocity. Another short press will give you a rapid fire mode – presumably for semi-auto .22 LR rifles.

Thanks to a screw-cut thread, the unit can be attached to a tripod which will help you line up your rifle with the 38x33mm sensor aperture. The Shooting Party recommends shooting from a distance of 150mm (six inches) and each shot is measured in feet per second only, with a claimed accuracy tolerance of 1.3 per cent.

By using the backlight button, you can scroll through the last 50 shots. Note, though, that once it is switched off, the data is lost, and you will need to reset the clock.

There is no facility to enter the weight of your pellet and calculate power levels automatically. However, you can download free software to do the maths for you, and by plugging in the supplied data cable, you can also record and analyse the results.


Skan Pro 1 Series 3 Diamond Chrono 40th Anniversary Model

A series piece of kit

RRP: £199

The Skan Pro 1 Series 3 Diamond Chrono 40th Anniversary Model is another product that won’t win any marketing awards, but if you take testing your air rifles seriously, you’ll really want to take a look at this chrono.

According to Skan, police forces use a version of the product here to test potentially errant air rifles and pistols. Many airgun manufacturers are customers too. It’s easy to see why. Taking it out of the box makes you feel as though you should be wearing some sort of hazmat suit or a lab coat.

The powder-coated steel construction measures 213x120x160mm and weighs just over 1.2kg. It’s a precision instrument better suited to setting up indoors on a bench with a proper backstop, as opposed to use in the field.

In fact, although it can be used with two 9v batteries, which are not provided, the Skan comes with a mains power supply, the bare wires for which connect to two snap terminals. 

I have to say though, bare wires on the end of something that plugs into the wall doesn’t sit easily with me, and I’d rather see jack plug connections.

The 80mm wide shooting aperture is diamond shaped to provide a larger calibration area, and reduce the potential for shots to miss the sensors. A rubber surround protects the unit from errant shots, and will absorb any pellets to prevent ricochets. Skan recommends shooting from 10 inches, and a little further for FAC-rated air rifles. 

Although it will handle projectiles weighing up to 49.9 grains and 4,000 fps, the Skan is designed purely for air rifles. Don’t be tempted to use it on your powder burners, as it will damage the sensors. 

With four buttons, the Skan is simple and intuitive to operate. You can enter the weight of the pellets you are using, and the unit will not only provide a reading in fps, but it will provide an energy reading in ft-lb.

It will also store 50 shots and retain the data after the unit has been switched off. There’s no ability to port data to a computer, although it’s possible with an upgraded model for £339.

What you do get, though, is a five-year warranty. That said, there are plenty of examples of Skan chronos still in regular use after 20 years or more. Part of that longevity is down to the quality of the materials and design, as well as a patented self-calibration and diagnostic feature that ensures accurate readings within a 0.5 per cent tolerance, time and again.


Caldwell Chronograph G2

A comprehensive and smart package

Available online: £165 – £265

Unlike the other products, the Caldwell Chronograph G2 is also designed for powder burners and even bow users, capable of measuring projectiles up to 9,000 feet per second.

Presented in a carry bag, the G2 comes with its own tripod, which can be adjusted between 80 and 170mm high. 

There are a couple of plastic brackets that form a platform and contain integral LED lights, and some metal tubes which slot in at an angle to hold a unit that contains both the sensors and the other technical gubbins. 

There’s an integrated lithium-ion battery which will give a run time of between two and 12 hours, depending on how much the lights are used.

The G2 provides a large area through which to shoot, which is handy as Caldwell recommends standing 10 to 15 feet away, though I’m sure you could get away with being much closer when using an air rifle.

The whole contraption snaps together quickly and easily, and the tripod provides a stable and versatile platform. A screen at the top of the chronograph provides a basic velocity readout. Three buttons switch the unit on and off, enabling you to select the units you want to measure in, as well as operate the lights.

Where the G2 really comes into its own is the fact that it can be paired to a smart device by downloading a free app to provide much more detail such as energy readings, average muzzle velocity, weather conditions and barometric conditions, all of which can be stored for later analysis.

Caldwell claims an accuracy tolerance of just 0.25%, the lowest of all the products on test here, thanks to a 48 MHz processor. 

Though I have no basis to doubt that claim, the fact that the shot measurement area is so large, and the sensors themselves are 460mm apart, makes me think you’d need to be very precise in setting up your rifle. 

Being a little askew probably doesn’t make much difference when shooting a centrefire or rimfire, but the need to observe a strict 12 ft-lb limit in the UK means you’d want your pellet to travel the shortest distance between the sensors.


Key specs

Summary

In my opinion, every responsible airgunner should either own a chronograph or have easy access to one.

I used a .22 calibre BSA R-10 as a test rifle, and while I had no way of knowing which of the four chronographs was the most accurate, they all provided readings which were within a few feet per second of each other. 

The other good news was that my rifle is legal.

Which one you choose is likely to come down to your budget, and the frequency and environment you intend using it. For example, if you have plenty of rifles and spend time working on them in a workshop, you are probably going to opt for the Skan or AirForceOne Ballistically Brilliant Chronograph, whereas the Combro cb-625 mk4 is more suited to occasional use in the field.

Finally, the Caldwell is likely to appeal to those whose interests lie across different shooting and hunting disciplines. All four will cover the basics and ensure that you stay on the right side of the law.

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