The backpack doubles your ammo-carrying capacity, letting you carry up to 400 bullets, 100 shotgun shells, 100 rockets, and 600 plasma cells at once. It contains 10 bullets, 4 shotgun shells, 1 rocket, and 20 cells when you pick it up (20, 4, 2 and 40 on I'm Too Young To Die).
Stats for IWADs
| Game | ITYTD and HNTR | HMP | UV and NM |
|---|---|---|---|
| Ultimate Doom | 28 | 28 | 28 |
| Doom II | 48 | 48 | 48 |
| TNT: Evilution | 31 | 32 | 32 |
| Plutonia | 33 | 33 | 33 |