This is a nice idea, and helps you save on costs, provided that you have some Systema PTW M4 magazines lying around that have already lived their full lives and bound for the trash bin. The Vanaras inner case assembly can be used with the external case and you can determine how many rounds it should carry depending on your gaming or training needs. It just costs US$9.99 at RSOV.com, and can prolong the lives of the expensive PTW magazines even more, cheaply.

"Vanaras M4 PTW magazine inner case assembly for 0.20g-0.30g BB with 30 / 60 / 90 and 120 rds selectable capacity.

Designed to work with the original Japanese 'Systema' M4 PTW magazine and clones.

Product Features:

• Manufacturered of semi-transparent plastic for easy BB feed trouble shooting.
• Can fit on original Japanese 'Systema' M4 PTW metal housing and clones.
• Equiped with stiff spring to deal with 0.20g to 0.3g BB.
• Specially designed magazine follower without the need of additional 'Systema ' eye-drop style follower.
• Easy spring replacement system for spring cleaning, lubrication and even replacement.
• Designed with stoppers for 30 / 60 / 90 and 120 rds BB capacity to feed your combat needs.
• Labelling area on top of magazine for your personal note.
• Comes with pastic film color labels as shown in the pic."