FPS, joules and chronos — how site limits actually work

Bottom line

Sites don't actually care about FPS. They care about joules — energy at the muzzle. FPS is just the proxy they quote because chronos are cheap and joule meters used to be expensive. Hit the joule limit at the BB weight you're shooting and you'll pass chrono regardless of what the FPS number says.

What a chronograph actually measures

A chronograph (or "chrono") measures the velocity of a BB as it crosses two infrared sensors a known distance apart. It calculates feet-per-second from how long the BB takes to cross that gap.

That's it. That's the whole measurement.

It doesn't measure energy. It doesn't measure rate of fire. It can't tell you if your gun is over the joule limit unless it knows what BB weight you're using.

Modern chronos (like the Acetech Aurora or Xcortech X3300) calculate joules automatically if you tell them the BB weight. The marshal reads off the joule figure and waves you through or sends you to the bench.

FPS vs joules — the conversion you need to know

Energy in joules from a BB's mass and velocity:

"

E = ½ × m × v²

Where:

• E = energy in joules
• m = mass in kilograms (so 0.25g = 0.00025 kg)
• v = velocity in metres per second (1 FPS ≈ 0.3048 m/s)

You don't need to do this in your head. Standard ballparks:

The joule figure doesn't change. The FPS does. This is why a 350 FPS reading at 0.20g and a 296 FPS reading at 0.28g represent the same gun, the same energy, the same hit-impact.

Typical UK site limits

These vary by site — always check the site's page in the sites directory before turning up — but the broadly accepted UK skirmish limits are:

Sites in Scotland must additionally comply with the Air Weapons (Specified Maximum Kinetic Energy) (Scotland) Regulations 2016 — typically 1.3J full-auto, 2.5J single-shot — which is more permissive than most English site rules but still legally binding.

Minimum Engagement Distance (MED)

If you're shooting more than 1.14J — i.e. DMR or sniper — sites impose a minimum engagement distance. You can't shoot someone closer than this distance with your over-energy weapon. Below the MED, you either bang-kill, switch to a pistol, or back off.

Typical MEDs:

• DMR (≤ 2.32J) — 15–25m
• Sniper (≤ 2.55J) — 20–30m
• HPA over 1.5J — usually 15–25m, treated as DMR

This is where you need a sidearm. A bolt-action sniper without a pistol is dead at 10m.

How chrono actually goes at a site

1. You queue at the chrono station with your gun, a magazine, and a bag of BBs (often the site's own BBs).
2. The marshal asks the BB weight. Tell them. Lying is a quick way to a ban.
3. You fire 3–5 shots into the chrono cone. They check the highest reading.
4. If you're under: passed. Often initialled or tagged.
5. If you're over: you have options — dial the hop down (sometimes drops FPS), swap to a lower-weight BB, or take the gun back to the car and use a rental for the day.

Most sites chrono with the BB weight you'll be playing with. If you brought 0.32g BBs, expect to chrono with 0.32g. This is to catch joule creep.

Joule creep — what it is and why marshals know about it

The same gun fires heavier BBs with more energy. Not less. This is counter-intuitive and it catches people out constantly.

A stock AEG that reads exactly 1.13J on 0.20g might read 1.30J on 0.30g. The longer dwell time gives the system longer to transfer energy to the heavier BB. That extra ~15% energy is joule creep.

Joule creep matters for two reasons:

1. Upgrading your barrel and hop unit makes joule creep worse. A tighter inner barrel and a better hop pack accelerate the BB more efficiently, increasing the dwell-time effect.
2. You will fail chrono with a "passing" gun if you've swapped to heavier BBs without re-chronoing.

The fix is simple: chrono with the BBs you actually shoot. If you change BB weight mid-day, re-chrono before the next game.

"Hot" guns and what gets you sent home

A gun firing over the site limit is "hot". Site responses vary:

• Slightly hot (under 5%): usually a hop tweak fixes it. Marshal will often help.
• Moderately hot (5–10%): spring change. Most stock AEGs let you swap the spring without a full teardown. Some sites have spare springs at the bench.
• Substantially hot (10%+): gun parked for the day. Use a rental.
• Dangerously hot (over DMR limit on an AEG): site may refuse you future use of that gun. Don't argue.

Calling out under-chrono lies — telling the marshal you're shooting 0.28g when you're actually feeding 0.32g — is a fast ban. Sites have BBs in the shop they can test you with.

What this means for you

• Buy a chronograph for £40–60. The Acetech AC6000 is fine for home use. Pre-test your gun before a site day.
• When you upgrade hop, barrel, or BBs, re-chrono.
• Note your gun's joule reading per BB weight. Keep it written on the spec card with the gun.
• If you're building a DMR, plan for the MED in your loadout — pistol is mandatory.

Sources & next steps

• BB weights explained — joule creep starts with which BB you use
• How to read an airsoft gun spec sheet — manufacturer FPS claims, decoded
• Air Weapons (Specified Maximum Kinetic Energy) (Scotland) Regulations 2016
• Browse chronographs in the catalogue →