This Ammo Book is optimized for full-size rifle rounds (.30 caliber and up) and 12-gauge shotgun shells with .75" (3/4 of an inch) bullet slots built of suspender elastic. DETAILS ON FIT 1. Large Rifle Cartridges A .75" loop is standard for heavy-duty rifle rounds. The elastic provides tension to grip the "body" of the cartridge. Common calibers that fit this size include: .308 Winchester / 7.62x51mm .30-06 Springfield .300 Winchester Magnum .45-70 Government (This is a thicker straight-walled round that fills the loop well) 6.5 Creedmoor 2. Shotgun Shells A 12-gauge shotgun shell has an outer diameter of approximately .79" to .80". Since the loops are made of Suspender Elastic, they will easily stretch to hold: 12-Gauge Shells (Tight, secure fit) 20-Gauge Shells (Slightly looser, but typically held fine by the tension) 3. What won&';t fit well? Small Pistol Rounds: Calibers like 9mm (.355" diameter) or .45 ACP (.452" diameter) will be far too small. Small Rifle Rounds: .223 / 5.56 NATO is too thin. Note: this is a very limited run of 250 units. When it&';s gone, it&';s gone.