So just to be sure i well understand your problem. You said the CO2 spartlet get puncture when you screw the bottom cap of the aluminium cylinder ?
If it's right, do you place the rubber ring correctly ?
When you assembled the grenade head do you:
1- Pull the hammer and secure it with the safety pin.
2- Insert the clear plastic spacer in the black threaded cylinder.
3- Put the rubber part with the perforation pin over the clear spacer
4- Then screw the black threaded cylinder in the grenade head
5- Crew the aluminium cylinder in the grenade head (not to tight)
6- Put the rubber ring (thin side down)
7- Add the CO2 spartlet
8- Screw the bottom cap (not to tight)
You may also watch my video
YouTube- CO2 Grenade Airsoft \ Paintball