I suspect this one will require ongoing research. I know the basics but I continue to find odd antique caps at auction that I have been unable to figure out yet.

Percussion Cap Sizes

#10 Percussion Caps:

• Usage: Primarily used in black powder pistols and some smaller revolvers. They are smaller in both diameter and height compared to #11 caps.

• Characteristics: These caps have a smaller inside diameter, making them suitable for smaller nipples. They are often noted for not having as much "kick" as #11 caps, potentially requiring a slightly stronger hammer strike for reliable ignition.

• Examples: CCI #10, Remington #10.

#11 Percussion Caps:

• Usage: The most common size for muzzleloading rifles and many revolvers. They are larger than #10 caps in both diameter and height.

• Characteristics: Standard #11 caps provide reliable ignition for most black powder applications. They are the go-to size for many modern replicas and conversions from flintlock to percussion.

• Variations: Standard #11: Used for traditional black powder.

#11 Magnum: Designed for higher ignition temperatures, suitable for black powder substitutes like Pyrodex, which require hotter flames for ignition. They might have a slightly different composition or construction to achieve this.
Examples: CCI #11, Remington #11, RWS #1075 (which some describe as fitting between #10 and #11 in size).

Musket Caps:

• Usage: Designed for larger firearms like muskets or long rifles where a stronger ignition is needed due to the size of the charge or the length of the barrel.

• Characteristics: These are larger than both #10 and #11 caps, offering more explosive force to ensure ignition over longer distances or with larger powder charges. They are less common in modern muzzleloading but essential for certain historical replicas or larger caliber firearms.

• Examples: Specific musket caps are mentioned but less brand-specific, often just referred to as "musket caps" or by the firearm they're suited for.

In the Picture below I believe the order to be a #11, #10REM?, #10, #12. I may have to update this based on additional research:

General Notes:

Manufacturing Variations: Even within these sizes, there can be slight variations in fit and performance due to differences in manufacturing processes or materials. Brands like CCI, Remington, and RWS might have subtle differences in how well their caps fit certain nipples.

Compatibility: While there's some interchangeability, using the correct size is crucial for reliable ignition and to prevent issues like caps falling off or not seating properly on the nipple.

Historical Context: Percussion caps were a major advancement from flintlocks, offering better reliability in various weather conditions. The sizes evolved to match the needs of different firearms, from pistols to muskets, based on the era's technological capabilities and the firearms' design.