5c vs ER-40 collet system for workholding of slugs

[JCDFCM]

    Is there any quality ER Collet manufacturer that offers relieved collets from the back? We are machining slugs on a vertical mill and that requires a longer slug to overshoot the length of the collet clamping surface to prevent damaging the collet. The 5c is relieved and doesn't have this issue and additionally allows for a dead stop and can handle different shapes...flexible. One can argue accuracy but if you are machining a finished part on a 5-axis from a slug, it doesn't have to be perfect. If ER Collets can be ordered relieved from the back, that would solve the long slug issue. With everything considered (accuracy, flexibility, etc.)    5c vs ER-40...which is better? Light high-speed milling on small workpieces will be performed on bar stock cut into slugs or ordered as slugs. There will also be C-axis milling performed as well.