Airsoft FPS Calculator

Spring (M-rating)

Compression spring inside the gearbox that drives the piston. Higher M-number = stiffer spring = more FPS, but more wear on gears, piston, and air-nozzle seal.

Bore size

Inner barrel diameter. Tighter bore (6.01–6.03mm) improves air-seal efficiency and consistency; wider stock bores (6.05–6.08mm) are more forgiving of imperfect BBs.

Gear ratio

Motor revolutions needed per piston stroke. Higher ratio (24:1+) = more torque per stroke = better cold-weather/heavy-spring performance. Lower ratio (13:1) = faster cycling = higher ROF.

DSG (Dual Sector Gear)

A sector gear that fires two piston strokes per full gear revolution, roughly doubling effective ROF for a given motor/ratio. Requires a DSG-specific gearbox shell.

MOSFET

An electronic switch that routes high current away from the mechanical trigger contacts. Effectively mandatory above 9.6V to prevent arcing damage.

NiMH vs LiPo

LiPo holds voltage steady as it discharges (consistent ROF shot to shot); NiMH sags under load, so ROF slows as the pack drains.

Cylinder volume

How much compressed air the piston pushes per stroke, shown as a % of a stock "full-stroke" cylinder (100%). Needs to roughly match the barrel's internal air volume: too low starves long barrels, too high wastes stroke on short ones.

"M-rating" is informal industry shorthand. Most brands roughly track it, but numbering is set by each manufacturer. Treat this as a starting point and always chrono after installing.

Guarder note: Guarder's retailer-published chrono data shows SP-series springs shooting noticeably hotter than their label implies, often a full M-band higher at 0.20g. So the table places Guarder springs by tested FPS: SP130 sits in the M150 row, not M130.

FPS figures are for 0.20g in an otherwise stock gearbox. "Tested" figures come from a UK retailer's own chrono testing of stock springs, not marketing claims. *M160/SP160 extrapolated beyond commonly published data. Blanks mean no reliable published reference was found, not that the part doesn't exist.

Green gas

~90% propane / 8–10% butane, usually pre-lubricated. ~115 PSI at 20°C for a generic can. The all-round standard gas.

Puff Dino (9kg / 12kg / 14kg)

A Taiwan-made green gas brand very common in PH shops. The kg names are pressure tiers, not literal PSI. 12kg and 14kg run meaningfully hotter than generic green gas (14kg ~10% stronger than 12kg); 9kg is a lighter fill for Japan-spec and NBB guns, or to tone down a gun that runs hot.

Red / black gas

Higher-pressure propane blend for more power and better cold-weather performance. Check your valve and seals are rated for it.

CO2

~850–900 PSI raw, regulated down at the valve. Barely affected by cold, very consistent, but hits noticeably harder. Many fields restrict CO2 on full-auto GBBRs.

NPAS

Negative Pressure Air System / nozzle air volume adjuster. Tunes how much gas reaches the striker per shot.

Blowback

The slide or bolt cycling on each shot. Diverts some gas energy from the BB, so locking it sends more energy downrange.

The HT-X3005 and HT-X3006 are the budget chronographs most of us actually own: cheap, pocket-sized, everywhere in PH shops. They're genuinely useful for tuning. The problem is what happens when you take that number to a game.

Both our own bench testing and the published comparisons point the same way: the HT reads low against reference-grade units, by roughly 3%, around 10–15 FPS at typical AEG velocities. Your gun is hotter than your chrono is telling you.

That direction is the whole point. Over-reporting would just leave performance on the table. Under-reporting means you can walk up to a field chrono believing you're legal and fail, or pass a lax check and spend the day shooting people harder than the site allows.

Where it bites: on a 420 FPS PH CQB cap, a bench reading of 419 looks like one FPS under. You're realistically around 428–439, over the cap across the entire range. For even the worst case to stay under 420, your HT needs to read 401 or lower.

It's worse in joules. Energy scales with the square of velocity, so a 3% velocity error becomes a 6–8% energy error. Real example from the unit pictured above: it displayed 321.8 FPS / 0.961 J on a 0.20g BB, apparently inside Japan's 0.98 J ASGK limit. Corrected, that shot is more like 328–337 FPS / 1.00–1.05 J, over the legal limit across the whole range.

The honest caveat: this shows a range, not one number, on purpose. Published HT-vs-reference comparisons don't all agree; some units are nearly spot-on, others far worse than 3%. Unit-to-unit variation is a bigger source of error than the average bias, so no single percentage is right for every device. If you can get one reading from a field Acetech or Xcortech on the same gun, use Calibrate my unit.

This tool exists so the gap doesn't surprise you at the safety table. It does not replace the field's chrono.

Our own testing: 20-shot strings on two deliberately different platforms, a WE M4A1 GBBR (gas, higher shot-to-shot variance) and a King Arms P90 AEG (electric, tighter grouping), each chronographed on the HT and on reference-grade equipment.

The independent test we lean on most: AirsoftLab mounted an HT-X3005 directly in front of an Acetech AC6000 and fired three 10-shot strings. The HT read lower on every shot, averaging about 11 FPS low. Two Acetech units tested against each other differed by only ±2 FPS (±0.5%) at ~405 FPS. They also noted the difference was "not really stable, so it's not just an offset": once the average bias was removed, residual scatter was around ±3 FPS.

Why a percentage, not a fixed FPS. An optical chrono works out speed as distance ÷ time between two light gates. If the gate spacing or clock is slightly off, every reading is scaled by the same ratio, so the error grows with velocity. A flat "+11 FPS at every velocity" is the one shape no plausible mechanism predicts, so this applies ÷ 0.97 (about 3.1%) with a band from 2.0% to 4.7%.

The energy gap is roughly double the FPS gap because energy scales with velocity squared. Where sites regulate in joules (much of the EU, Japan's 0.98 J), the under-read hurts about twice as much.

The HT's own spec sheet claims ≤0.5% accuracy, which independent testing plainly doesn't support. A generic correction narrows the gap; it doesn't close it.

Sources: AirsoftLab's HT-X3005 chrono and AC6000 chrono accuracy bench comparisons, plus owner reports across airsoft and airgun forums.

Derived from field testing (20-shot strings, WE M4A1 GBBR + King Arms P90 AEG) plus cross-referenced owner reports on the HT-X3005/X3006, not a factory calibration certificate. Treat outputs as a practical estimate, not legal or competition-grade certification.

Individual units vary. Always re-verify against the field's chronograph before relying on any number for an FPS cap.

Assumes the device is set to 6mm and the correct BB weight. Wrong setup gives wrong output regardless of the maths.