Good news first: the barrel is the most grateful part to measure. You only need two numbers – length and bore (inner diameter) – and both take a minute to read off. The catch: a surprising amount rides on them, so it pays not to eyeball it.
- Barrel length – measure from the muzzle to the rear end of the barrel, including the hop-up window section. Use a tape measure. Enter in millimetres (e.g. 363).
- Bore (inner diameter) – usually engraved on the barrel or listed by the manufacturer (6.01–6.08 mm). If unknown, leave the default 6.05 mm – most stock barrels are 6.05 or 6.08. Aftermarket (precision) barrels are usually 6.01 or 6.03, and this is normally printed on the barrel itself.
- Hop-up window – the length of the window at the rear end of the barrel, typically 5 mm. It belongs to the advanced hop-up settings; for a start just keep the default of 5 mm – it will usually be correct.
Tip: if the bore is printed on the barrel, trust that number! The inner diameter is hard to measure accurately with consumer calipers, especially to 0.01 mm precision.
Tip for complete idiots: don’t measure the outer barrel of the replica – only the brass, stainless or aluminium inner barrel that the BB actually travels through.
Rear end of the inner barrel, with an r-hop patch and a flatted hop-up bucking (turned inside out for your pleasure).
Where to enter it
In the simulator: Barrel → length / inner diameter. Length to the nearest millimetre is enough; the inner diameter is entered to a hundredth of a millimetre.
And remember the rule that holds for the whole simulator: the answer is only as good as the numbers you measured and typed in.