Chrono explained: how your replica gets measured and how to pass

Chrono explained: how your replica gets measured and how to pass

Last updated: 18 July 2026

an hour and a half of driving and then being told at the chrono that you can't play. you want to experience that once and then never again.

this post explains what happens at the chrono, why you see a different number at home than at the field, and how to make sure beforehand that you just get through it.

what a chrono is

a chronograph measures the speed of your BB, usually with two optical sensors a known distance apart. from that speed and the weight of the BB it works out the energy in joules.

there are two types:

• shoot-through chronos with a little tunnel you fire through. what you'll see at most fields.
• muzzle chronos that clip onto your barrel, like the Xcortech models. handy for home.

what gets tested

here's the part that trips people up when they read airsoft advice from another country: there is no single universal limit. FPS and joule limits vary by country, and within a country they vary by site and by field. so the numbers below are the shape of the thing, not a rulebook you can carry between fields.

what's near-universal is the structure. fields group replicas by how they fire and set a maximum energy per group, roughly like this:

the principle underneath is simple: lower energy lets you engage closer, higher energy pushes your minimum engagement distance further out. that's the whole trade. a bolt-action allowed to hit harder has to keep more distance, so the energy on target stays sane.

so the one rule that travels: always chrono to your local site's limits. look them up before you build or tune anything, and don't assume the field down the road, let alone one in another country, runs the same numbers.

how it goes in practice

before your turn: battery in, mag with BBs in, replica on safe. the marshal doesn't want to take a replica that's set to auto.

at the chrono:

1. you say what kind of replica it is and which category you want to play in.
2. you usually fire five to ten shots through the chrono.
3. the marshal notes the values. some fields look at the highest reading, others at the average. ask if you're not sure.
4. you get a sticker, tape or a band on your replica as proof.

which BBs: this matters. some fields have their own chrono BBs, usually 0.20 g, and measure with those. others let you shoot your own BBs and enter the weight.

if you play with a different weight than you chrono'd with: say so. the energy stays the same but the field wants to know what you're using.

want to play in two categories

if you've got a replica with a fire selector that can do both semi and auto, you play on the auto limit, even if you only plan to shoot semi. the higher semi/DMR limit is generally reserved for replicas that are physically semi-only, mechanically or electronically incapable of full-auto. just promising you won't is not enough. how fields verify this varies, so check with the organiser beforehand if you're doing a DMR build.

why you get a different number at home

this confuses people every season, so let me explain it properly.

1. different BB weight

the biggest one. the same replica, at one fixed energy, gives different FPS depending on BB weight. as a worked example, at 1.20 J (used here purely to show the physics, not as a limit):

• 359 fps with 0.20 g
• 321 fps with 0.25 g
• 304 fps with 0.28 g

same energy, three different FPS numbers. if you measure at home with 0.25 g and the field measures with 0.20 g, you suddenly see a much higher FPS number and panic. while nothing has changed. i've got full tables in my post on converting joules to FPS.

2. temperature

on gas replicas this is huge. green gas has higher pressure at higher temperature, so more energy. a GBBR that sat at 1.10 J in a cold month can climb over the limit on a hot day.

if you play gas: chrono your replica in the sun at the temperature you'll be playing at, not in your cold shed.

on AEGs temperature matters much less, but a fully charged LiPo gives slightly more than a flat one.

3. differences between chronos

two chronographs rarely give exactly the same number. differences of five to ten fps are normal, and with poor light or a bad setup it can be more.

your home chrono isn't calibrated against the field's. so assume it isn't.

4. shot-to-shot spread

no replica fires every BB at the same speed. a healthy AEG might have five to ten fps of spread, a badly sealed gearbox much more.

if the field looks at the highest of ten shots and you look at the average of three at home, you're measuring different things.

5. bad BBs

uneven or deformed BBs give wild readings. chrono with the same quality BBs you play with.

how to make sure you pass

keep margin. aim comfortably under the limit, not right on it. leave enough buffer for spread, temperature and chrono differences. you won't notice the difference on the field.

buy your own chrono. a muzzle chrono costs somewhere between 40 and 90 euros and it pays for itself the first time you don't drive to a field for nothing. if you play in a team, buy one together.

measure after any change. new spring, different cylinder, different barrel, different hop-up rubber: all reasons to measure again.

break a new spring in. the first few thousand shots a new spring often gives slightly more energy. don't measure right after install and then trust that number.

bring a lighter spring if you're on the edge. an M90 or M100 in your bag, and tools. some fields give you the chance to adjust and re-measure.

on gas: let it come up to temperature and then measure.

if you fail

stay calm, it's not personal. the marshal is doing their job and those limits exist to stop someone getting hurt.

options:

• adjust and re-measure, if the field allows it and you've got tools with you.
• play in a higher category if your replica qualifies. if you're just over the full-auto limit and your replica is semi-only, you may be able to play as a DMR with a longer minimum engagement distance.
• rent something. a lot of fields have loan replicas.
• heavier BBs don't help. the chrono works in joules. this really is not a fix, it just gives a lower FPS reading at the same energy.

one more thing

chrono isn't harassment. it's the only practical way to make sure nobody gets onto the field with a replica that hits too hard. and because everyone goes through it, you also know the people shooting at you are within the limit. that's the deal.

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Also on Airsoft-FOB

• also available in dutch: Chrono: hoe wordt je replica gemeten en hoe zorg je dat je slaagt?