Short answer: a fitted ammo case holds about 64 rounds of 9mm, 5.56 or .380 per 100 x 100 mm (4 x 4 in) of floor, 36 rounds of .45 ACP or .308, 100 rounds of .22 LR and 9 shells of 12 gauge. Height is the cartridge length plus about 8 mm for the floor, headroom and lid.
How is a round pocket sized?
Each round stands upright in its own pocket. The pocket is the cartridge rim diameter plus 0.6 mm of clearance, and pockets sit 1.6 mm apart, so the spacing (pitch) is rim + 2.2 mm. Rounds per row in a case = (inside width - 1.6 mm) / pitch, rounded down.
Pocket size and rounds per 100 mm by caliber
| Caliber | Rim (mm) | Pocket (mm) | Pitch (mm) | Per 100 mm row | Per 100 x 100 mm | Case height (mm) |
|---|---|---|---|---|---|---|
| .22 LR | 6.9 | 7.5 | 9.1 | 10 | 100 | 33 |
| .380 ACP | 9.5 | 10.1 | 11.7 | 8 | 64 | 33 |
| 9mm Luger | 10.0 | 10.6 | 12.2 | 8 | 64 | 38 |
| .40 S&W | 10.8 | 11.4 | 13.0 | 7 | 49 | 37 |
| 10mm Auto | 10.8 | 11.4 | 13.0 | 7 | 49 | 40 |
| .45 ACP | 12.2 | 12.8 | 14.4 | 6 | 36 | 40 |
| .38 Special | 11.2 | 11.8 | 13.4 | 7 | 49 | 47 |
| .357 Magnum | 11.2 | 11.8 | 13.4 | 7 | 49 | 48 |
| .44 Magnum | 13.1 | 13.7 | 15.3 | 6 | 36 | 49 |
| .223 Rem / 5.56 | 9.6 | 10.2 | 11.8 | 8 | 64 | 65 |
| .300 Blackout | 9.6 | 10.2 | 11.8 | 8 | 64 | 65 |
| 7.62x39 | 11.3 | 11.9 | 13.5 | 7 | 49 | 64 |
| .308 Win / 7.62x51 | 12.0 | 12.6 | 14.2 | 6 | 36 | 79 |
| 6.5 Creedmoor | 12.0 | 12.6 | 14.2 | 6 | 36 | 80 |
| 12 ga 2.75" | 22.8 | 23.4 | 25.0 | 3 | 9 | 78 |
| 12 ga 3" | 22.8 | 23.4 | 25.0 | 3 | 9 | 84 |
| 20 ga 2.75" | 20.6 | 21.2 | 22.8 | 4 | 16 | 78 |
Case height assumes a hinged lid: cartridge length + 2 mm headroom + 3 mm floor + 3 mm lid. A slide-off lid adds about 2 mm. Mixed calibers share one case - shorter rounds sit on a raised floor so every round stands at the same height and is easy to pull.
Real case sizes for common loadouts
These are the outer sizes our case builder produces (width x length x height). Hinged cases include the hinge and latch; slide-off lids add rails instead.
| Holds | Hinged lid (mm) | Slide-off lid (mm) | Printed as |
|---|---|---|---|
| 50 x 9mm | 69 x 140 x 38 | 71 x 130 x 39 | one plate |
| 100 x 9mm | 106 x 177 x 38 | 108 x 167 x 39 | one plate |
| 100 x .380 | 102 x 171 x 33 | 104 x 161 x 35 | one plate |
| 100 x .45 ACP | 109 x 235 x 40 | 111 x 225 x 42 | one plate |
| 50 x .357 | 76 x 153 x 48 | 78 x 143 x 50 | one plate |
| 100 x 5.56 | 103 x 172 x 65 | 105 x 162 x 67 | one plate |
| 40 x 7.62x39 | 76 x 127 x 64 | 78 x 117 x 66 | one plate |
| 50 x .308 | 80 x 161 x 79 | 82 x 151 x 81 | one plate |
| 500 x .22 LR | 236 x 201 x 33 | 236 x 193 x 35 | two parts |
| 25 x 12 ga | 109 x 194 x 78 | 111 x 184 x 80 | one plate |
| 25 x 20 ga | 100 x 178 x 78 | 102 x 168 x 80 | one plate |
"Two parts" means the base and lid are printed separately because together they do not fit one print plate - the case is the same, it just takes two prints.
Why not just use a bigger box?
Loose rounds in an oversized box rattle, dent and are slow to count. A fitted pocket for every round keeps them protected and makes the count obvious - an empty pocket shows exactly what is missing.
Frequently asked questions
How many 9mm rounds fit in a 4 x 4 inch area?
About 64. A 9mm pocket is 10.6 mm wide with 12.2 mm spacing, so 8 rounds fit across 100 mm in each direction.
How big is a 100-round 9mm ammo case?
Our builder makes it 106 x 177 x 38 mm (4.2 x 7.0 x 1.5 in) with a hinged lid.
How tall does a 5.56 ammo case need to be?
About 65 mm (2.6 in) outside: the 57.4 mm cartridge plus floor, headroom and lid.
Can different calibers share one case?
Yes. Each caliber gets its own block of pockets, and shorter rounds sit on a raised floor so they are as easy to grab as the long ones.
How many 12 gauge shells fit in a case?
About 9 per 100 x 100 mm of floor. A 25-shell case is 109 x 194 x 78 mm (4.3 x 7.6 x 3.1 in).