The Basics
Most people think of chronographs as velocity measurement devices only. As such, let's look at that aspect:
Used properly, a chronograph will give you a real world look at the true velocity of your pet loads. Rather than blindly assuming that your particular rifle/cartridge/load produces the "book" velocity listed in your favorite manual (seldom the case), you can verify the load's performance (or lack) for yourself. After all, published data commonly varies in velocity levels with the same loads from manual to manual. Which one does your rifle (or handgun) produce?
Are the manuals wrong? No; their data is correct for their particular barrel/component/temperature/altitude combination. Data that is produced with a 26" test barrel can even be at variance with that produced by 26" barrels on field rifles. It is not uncommon to find actual velocity in a 22" barrel to be as much as 250 fps less than that listed in manuals that used 26" test barrels in universal receivers for testing purposes.
Why does it matter? A difference of 100 fps muzzle velocity translates to roughly .25" difference in impact at 100 yards with common .223 velocities. Not a big deal, but out at 400 yards (granted, stretching the barrel a bit for the .223 on game) the best you can hope for is an error of 1". Still not a lot but when added to wind drift and shooter error it can mean a miss or, worse yet, a wounded animal.
Varminters have a reputation for actively seeking those real long shots. Few of us have access to ranges where we can verify hold-over or scope settings for extreme distances. Verifying the actual velocity for your rifle/load allows you to consult printed or software-driven ballistic charts to have a reasonable picture of your trajectory. Nothing beats actual shooting at those distances, but pre-planning helps immensely. Take the additional step of taping a trajectory chart to your rifle or scope indicating hold-over, or scope settings and wind drift data and you take faulty memory out of the equation.
As a Safety Valve
Everyone knows that velocity is a product of pressure. Each cartridge has its own pressure limits as set by SAAMI and should never be exceeded. What does this have to do with chronographs?
(1) A couple years ago I ran low on my favorite powder for .17 Remingtons. I ordered more and, upon receiving the shipment, noted that as expected it was of a different lot number than I had been previously using. Following normal safety practices, I loaded several rounds beginning 10% under my previous load and worked up to the level used before. Each load was shot into my backstop and the cases were checked for any sign of excessive pressure. No signs having developed, I proceeded to load 20 rounds at my previous powder weight and retired to my range. The results were puzzling to say the least. Groups that should have been well under .5" were randomly spread over a 4" area. Since I still had a few left, I then tried 5 rounds with my previous powder lot. Groups returned to normal for that rifle. (Records do help)
Since nothing but the powder lot had been changed, I began to get nervous. Setting up my Shooting Chrony, I proceeded to fire the new rounds to test for velocity. I soon broke out in cold sweat. The velocities were so far in excess of expected (and indeed the top velocity of the .17 Remington cartridge standards) as to be unbelievable. The pressure created had to be enormous and that the rifle didn't become a bomb was a credit to the strength of Remington rifles and brass and to the guardian angel that sits on the shoulder of fools. Further testing showed that this lot of powder achieved my previous, rather modest, velocity using nearly 8% less powder than my previous loads. In a cartridge operating at the pressures of the .17 and the tiny amount of this powder required to elevate pressure in cartridges of this miniscule bore diameter, 8% only amounts to a bit under 2 grains in weight.
(2) A friend brought me a .222 Remington rifle that he had recently purchased. He had started his reloading level at just under the maximum for his selected powder/bullet weight combination as listed in the excellent Hornady manual. (First big mistake-Never start at or near maximum) Though he had experienced primer cratering and piercing (major league indications something is wrong!) he had continued until he had finally stuck a case into the bolt face. Once that problem had been resolved (and he had to endure a friendly but rather pointed lecture on what he had done wrong), we loaded a few rounds starting at Hornady's suggested starting loads. Working over the chronograph, we soon established that maximum loads for this rifle/cartridge and component mix was 3.5 grains under the listed maximum but still approached the velocity listed for the maximum loads! Why? Hard to say. Chamber and barrel dimensions could have been exceptionally tight on his particular rifle.
The danger of burning-rate changes from lot to lot of the same powder can be alleviated by starting well under your previous load and gradually increasing the charge while shooting over a chronograph. Once you have reached your previous velocity you are done (always assuming you have not reached any excessive pressure signals first), regardless of what that final powder weight is. 99 times out of 100, your point of impact will not have changed either.
Changing to a new powder can be safely accomplished in the same way. Start with a safe charge weight for that bullet weight/powder combination and use the chronograph to clock velocity. Again, once you have reached your targeted velocity without encountering pressure warnings, you are finished. A little fine tuning of the load may be required for accuracy but in most cases scope adjustments required to zero this new load will be minimal.
Shooting Chrony Tips
I have used the Shooting Chrony chronograph for years. I am currently on my third one, which is not a condemnation of the Chrony. Rather, this is a result of my own carelessness. Chrony #1 died a valiant death when a .223 I was testing sent a bullet through the casing. I had neglected to note that the front scope rings had worked loose causing the scope/bore angle to be excessive and the bullet to exit far lower than expected.
Chrony #2 proved to be no match for a 45/70 load that crashed through it when a make-shift shooting bench collapsed just as I triggered the round. Again, my fault.
When using the Chrony it is best to use a sturdy, adjustable support such as a camera tripod. Carefully adjust height and angle until you have a clean shot through the sun-shade braces with a reliable backstop behind it.
Take care to ensure that the Chrony is completely unfolded. The distance between the 2 sensors is critical for accuracy. A partially folded unit will reduce this distance and create erroneous readings.
Place a strip of clear packing tape over the front and rear sensors. This helps keep dust out of the sensor openings and seems to help with accurate readings too.
Place a wide strip of black tape between the front sun screen braces 6" above the unit body. Never allow your cross hairs or open sights to drop below this line when shooting. Remember that most scopes average 1 1/2" higher than the actual bullet path from the muzzle and open sights run between .7 - .9" depending on type. The bullet will be traveling lower than your sights!
Make sure that you are using a firm, sturdy shooting bench (see the fate of Chrony #2, above) and shooting bags.
I never allow anyone else to shoot through my Chrony. If someone wants to check loads, I do the shooting. ( Of course with my previous track record, that doesn't necessarily mean that it is safe.)
If you want to adjust the indicated velocity for true muzzle velocity, add 1 fps for each foot that the chronograph is located from the firearm muzzle (commonly 15 feet, which equals an additional 15 fps, corrected muzzle velocity.)
Couple your chronograph data with a good, interactive ballistic software program ( such as PointBlank, available free at http://www.huntingnut.com/pointblank.html ) and you can alter variables to get a good idea of outside influences on your bullet once it enters the unfriendly world outside your rifle barrel. As you learn how wind, humidity, temperature and altitude all affect your pet loads trajectory, you are well on your way to becoming an expert shooter rather than just a shooter.
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