For this discussion we are going to focus primarily on Self Defense ammunition.
Practice ammo should be appropriate for your gun and ideally of similar weight to what you will use for self-defense. However, this is by no means a requirement! Almost any brand or "reloaded" ammo is acceptable.
The purpose of self-defense ammo. is to STOP a threat as quickly as possible. While any ammo can be used in self-defense there are important factors to consider for any you will bet your life on.
1. Reliability: Only use factory loaded ammunition designed for self-defense. Do not use hand-loaded or reloaded ammunition. While these will cost around twice as much as practice ammo. Additional QC practices are in place to make them more dependable and reliable when you need it most.
2. Effectiveness: Self Defense ammo is designed to do TWO things. First is to effectively transfer the energy of the bullet to the target. Second, it is designed to NOT pass through and cause harm to anything beyond it. A great deal of testing goes into designing a bullet that will safely pass-through clothing but not continue through the target and cause harm to an innocent person beyond it. This is usually accomplished by using a "hollow point" that is designed to expand when it hits a soft object.
3. Bullet Size and Weight: Bullets are measured by their diameter (referred to as Caliber) and Weight (measured in Grains abbrev. 'gr.'). A bullet with a larger caliber will typically weigh more but move slower than a smaller caliber. (Softball vs. baseball). The advantage of a larger caliber is that it produces a larger wound and can transfer more of it's energy to the target - this is known as "stopping power". The advantage of a smaller caliber is the weapon can usually carry a greater capacity and will have less recoil. The lower recoil usually means the shooter will translate this into better accuracy. Don't get too caught up in weight but a pistol with a small barrel may benefit from a slightly lighter grain bullet (115 or 124gr) compared to a full-size gun that will get the full power out of a heaver (147gr) one.
4. Legal Questions: What??? Why is this here? In the age of more gun control and lawyers what you carry may be held against you in the court of public opinion. There are many articles by industry experts such as Massad Ayoob about the legal ramifications of things like gun modifications and ammunition choices. If you avoid exotic named ammunition like "R.I.P." and stick to what law enforcement uses - especially in your area - you may avoid being labelled a blood-thirsty killer.
Caliber Recommendations
Semi Auto Calibers
• 9mm (Parabellum and Luger are the same!)
• .40 S&W
• .357 Sig
• .45 Auto
• 10mm
• .380 Auto
Revolver Calibers
• 38 Special
• 357 Magnum
• 44 Magnum
TYPES
SELF DEFENSE: Jacketed Hollow Point (JHP)
PRACTICE: Full Metal Jacket (FMJ), Ball, Wadcutter, Frangible (esp. if using steel targets)
RECOMMENDED BRANDS:
• Federal Premium HST
• Sig Sauer Elite V-Crown
• Hornady Critical Defense
• Speer Gold Dot
• Remington Golden Saber
AVOID:
• G2 Research R.I.P.
• Any ammo with a scary name
MATERIAL:
• USE: Brass or Nickel Casing
• AVOID: Steel Casing, Steel Core, Overpressure (+P,+P+)