The inventive ablation element comprises an elongated cannula having a proximal end and a distal end. In one form, each projectile assembly further includes an internal wedging surface, at or adjacent the trailing end of said cylindrical extension which accommodates a tapered nose part of a following projectile, for expanding said trailing end into enhanced sealing engagement with the barrel upon engagement of said wedging surface with said tapered nose part. Such advantages enable the formation of stable ultrafine patterns suitable for 64M, 256M, 1G, 4G, and 16G DRAM semiconductor devices and improve the production yields. The hub is connected to a motor. The porous material used for the guiding surface enables the line to glide on a fluid pad of less than 1 mm.