They will have bolt bounce problems like authentic machineguns do. There can also be issues with timing if the "M16" bolt copyright or other sections are somewhat from spec. All complications that come up on machineguns but are invisible to semi car shooters. Mine closes with no concerns. Looks like https://frt-trigger24567.thezenweb.com/getting-my-rarebreed-triggers-to-work-74415577