Tunnels should not be too large. If an animal can turn around in the tunnel it may not be trapped cleanly. Tunnels for spring traps like the Fenn and Springer should be just wide enough to accommodate the set trap with its safety catch off, and just high enough for the jaws to close freely when the trap is sprung. In the case of the No 4 size; this is16cm wide by 13cm high. This gives the animal less ‘room for manoeuvre’, and reduces the risk of the trap not killing to a minimum.
It is also important to set the trap well back into the tunnel, and not in the entrance. In this way your trap is less likely to deter a shy or cautious predator. Also, once the animal is well into the tunnel, it is more likely to be committed to going right through, rather than trying to turn round.