Mag shims can greatly improve the seal between gas magazines and GBB's. The following is a magazine from a KWA ATP, which is notorious for poor seals and performance. Better seals between the mag and cylinder as well as a good piston head seal make for far higher efficiency and performance. However, when installing the shims techs often forget to modify the seal to actually allow it to sit higher and may be best to remove material from the top edges of the seats. I found that a Dremel and cut-off wheel works well for "grinding" rubber edges and seats. This means the seal can sit higher without compressing which can block the effects from the shims.