Every drop chart, ballistic app and legal energy figure rests on one number: the speed your bullet actually leaves your barrel. Most shooters have never measured it. Here's why the printed figures mislead, what the law expects you to know, and how to measure the real thing in seconds.
The velocity printed on an ammunition box was measured in the manufacturer's test barrel, which is often 24 inches or longer with no moderator fitted. Your stalking rifle probably runs a 20 inch barrel, or shorter. As a rough rule each inch of barrel is worth somewhere between 15 and 40 feet per second depending on the cartridge, so a real-world rifle can easily run 100 fps or more below the box figure.
It doesn't stop there:
The only number that counts is the one measured from your rifle, with your ammunition, in the conditions you shoot in.
Muzzle energy is calculated from bullet weight and velocity: energy (ft·lb) = weight (grains) × velocity (fps)² ÷ 450,240. You cannot know your muzzle energy without knowing your muzzle velocity.
Deer. England and Wales require at least 1,700 ft·lb for all deer. Scotland requires at least 1,750 ft·lb, from an 80 grain or heavier bullet at 2,450 fps or more, for deer other than roe. Take a 100 grain .243 load with a box velocity of 2,960 fps: that is about 1,946 ft·lb, comfortably legal everywhere. The same load leaving a short barrel at 2,800 fps makes about 1,741 ft·lb: still legal in England and Wales, but below Scotland's minimum for red, sika and fallow. The box says you're fine. Your rifle may disagree. The full requirements are in our deer calibre guide.
Air rifles. The limit for an air rifle held without a Firearm Certificate is 12 ft·lb. An 8.44 grain pellet at 795 fps is about 11.8 ft·lb. The same pellet at 820 fps is about 12.6 ft·lb, and the rifle is then a section 1 firearm: an offence to possess without an FAC, even if you didn't realise it had crept over. Springers can speed up after a service, and regulated PCPs can drift, so a periodic check is cheap insurance. Air pistols are limited to 6 ft·lb.
Inside 100 metres a velocity error hides. At distance it doesn't. Feed a ballistic app a muzzle velocity that is 100 fps off and the error at 300 metres is already a few centimetres; by 500 it can be the difference between a clean hit and a wound or a miss. Every drop table and turret label you make is only as good as the velocity behind it.
Measured velocity is the first input a ballistic solver asks for, and the one most people guess. Measure it, and first-round hits at distance stop being luck.
A chronograph tells you more than the average. Over a 10 shot string it shows extreme spread (fastest minus slowest) and standard deviation (how tightly shots cluster around the average). A load that averages the right number but swings 60 fps shot to shot will print vertical stringing at long range however well you drive the rifle.
Chronographs used to mean a fragile optical gate on a tripod downrange: fussy about light, sitting in the line of fire, and unpopular with range officers. Doppler radar ended all that.
The Garmin Xero C1 Pro is a palm-sized radar unit that sits on the bench beside your rifle. Nothing goes downrange. It reads every shot instantly, works with moderators, indoors or out, and handles centrefire, rimfire, air rifles, shotguns and even bows. Shots log to the Garmin ShotView app on your phone, so velocity, ES and SD for every load are recorded without a notebook. The newer Xero C2 sits above it in the range.
Garmin Xero C1 Pro › Garmin Xero C2 › All Garmin ›
A good first session: shoot a 10 shot string per load, note the temperature and the ammunition lot number, and keep the logs. Re-check when you open a new batch, change anything on the rifle, or the seasons swing.
Multiply the bullet or pellet weight in grains by the velocity in feet per second squared, then divide by 450,240. A 100 grain bullet at 2,900 fps: 100 × 2,900² ÷ 450,240 is about 1,868 ft·lb.
Usually a little, in either direction, and occasionally enough to matter near a legal threshold or a carefully built zero. Re-chronograph after fitting one. A radar chronograph reads normally with a moderator fitted; optical units often struggled.
Chronograph it with the pellets you actually use and apply the formula above: pellet weight matters as much as speed. Check again after a service or a change of pellet. If it reads over, stop using it and speak to an RFD before anything else.
They are honest for the test barrel they were measured in, which is usually longer than yours. Treat the box figure as a ceiling, not a promise.
Almost always. It sits beside you on the bench, nothing goes downrange and there are no cables or screens in the line of fire. Check your club's rules, but radar units removed the usual objections.
Last reviewed 7 November 2026. Spotted something out of date? Tell us and we'll fix it.