Cap is the power source of  model gun.  When model gun was first developed, paper Cap is the only power scource.  Paper Cap is dangerous to handle and it does not have protective shield over the fire powder.  Depends on different model gun, each cartridge will need a couple of paper caps in order to function.  Usually 2 to 5 paper caps per cartridge for semi-autos and 4 to 7 per cartridge for full-autos.

A packet of paper cap and it was sold for 100yen for 200 pieces

From the early 80s, the invention of Plastic Cap saves many model gun enthusiasts' effort.  Containing 1g of fire powder in a plastic shield, Plastic Cap is safer to handle and easier to load.  The design of cartridge was also changed to suit plastic cap, making model gun safer and easier to handle.  7mm and 5mm Cap is developed for normal size and compact size cartridge.

7mm Cap and 5mm Cap, length is representing the diameter of plastic shield

As most model guns are using 7mm Cap, variances of 7mm are developed to suit different needs.

Top to bottm: 7mm R Cap for "Shooter One", 7mm B Cap for blowback, 7mm S Cap for Revolvers and 5mm B Cap

7mm R Cap
It is used for infrared target sensor.  For sport shooting, targets with infrared sensor is used to detect wether the shooter has hit the target or not.  R Cap can produce extra infrared light for such target.  400yen for 100 rounds.

7mm B Cap
It is used for model gun with blowback. Moderate sound and spark.  400yen for 100 rounds.

7mm S Cap
Formulated for revolers.  Great sound and spark.  300yen for 100 rounds.

5mm B Cap
Only this one to chose from if you are having a 5mm model gun.  400yen for 100 rounds.

!!
Remember cap is fire powder and it is dangerous if you are not handling it with care.  Don't use Cap for purpose other than model gun.  Please bear in mind that Cap is illegal to hold in Hong Kong.