For inner barrels, the narrower the inner diameter, the stronger the fps would it gives, but at the same time, the shorter distance the BB would travel; on the other hand, the wider the inner diameter, the lower fps would it give, but the longer distance the BB can travel. So PDI creates this category of "DUAL" barrels as a balance between the fps and BB flying distance.
for Tokyo Marui hi-capa 5.1
inner diameter: 6.05mm+
outer diameter: 8.55mm
material: STKM-13C (ultra precise custom-made item)
error allowance: 0.007mm