• Available for most popular primers
• Speed up reloading
• Protect primers
• Extremely safe and reliable

It’s conclusive that large primers give more velocity with all powders tested especially the slower ones such as Herco. That is because the large pistol primer has a stronger ignition on the powder charge. Felt recoil was different with some of the loads that read 100 FPS difference or so.
Some cartridges require a large pistol primer (45acp, 44 special, 44 magnum, …) while others require a small pistol primer (9mm, 38 special, 357 magnum, …). Depending on what cartridge you are reloading, you need to buy the proper size primer for that application.
There are two sizes and four types of primers. Pistol primers have thinner and somewhat softer primer cups than their rifle counterparts. Small pistol and rifle primers utilize a cup 0.175″ in diameter, while large pistol and rifle primers measure 0.210″ across.