Numerical control (CNC) boring and routing machines use curtains made from resistant, but flexible materials to protect end-users from the projection of tool pieces. These curtains allow the work piece to gently pass through, but have to firmly stop every small sharp piece or fragment ejected by fast drilling tools at the highest speed. Nowadays curtains are made in flexible thermoplastic materials: polyamide, polyurethane, polyvinyl chloride or similar materials. Safety issues to be addressed related to the risk of projection of parts during processing are defined by EN 847-1 and EN 847-2 standards, both collecting practical experiences from manufactures and users. However, the effectiveness of these barriers has been compromised by several experts and this state of uncertainty has led to new technical observations, experiments and numerical simulations, but no conclusive results are available at the moment. This independent research, realized by ballistic impact simulation, aims at verifying the effectiveness of protection provided by barriers installed on CNC boring and routing machines. These barriers consist of curtains realized on the basis of the standard UNI EN 848-3. As additional goal, the study optimizes the barrier parameters (e.g. materials, shape, depth, mass, cost,..) in relation to the projectile parameters (shape, mass, speed, direction...) identifying their mutual influence. Additional outcomes lead to the formulation of a practical methodology for design and evaluation of protection efficacy.