Internal ballistics
A bullet in a case in a chamber initially has zero velocity. As powder burns and builds gas pressure in the cartridge, the bullet starts to move down the barrel, accelerating rapidly. The farther a bullet can travel in the restricted space of a barrel with continually expanding gas behind it, the faster it will go. Yes, powder selection has an important bearing on this, but let's just stick with the basics.
A short barrel gives a bullet less time to accelerate than a longer barrel. Short barrels produce less velocity than longer barrels, especially in handguns.
Heavier bullets either accelerate more slowly than lighter bullets, or if because of powder selection they do accelerate as fast as a lighter bullet, especially in a short distance, then there must be greater force applied to that movement. This translates to greater recoil.
While a slower moving heavier bullet may have as much energy as a faster moving lighter bullet, the resultant increased recoil, especially in a small, lightweight handgun, translates into reduced control with the gun, adversely affecting speed and accuracy of subsequent shots.
Often you can gain a slight tactical shooting advantage by using lighter bullets out of short barreled guns. Since the one shot stop from a small handgun is largely a myth, multiple fast well aimed shots have a much higher percentage of terminating the attack of a motivated individual.
IOW, more well aimed bullets delivered faster is an advantage.
I use the all copper Barnes TAC-XP bullets from various manufacturers (Including now Barnes ammo) as lighter weight bullets that perform like heavier weight bullets because of their monolithic design. They actually outperform other common, less expensive designs in all critical tests, but because copper costs 4x the cost of lead, LE does not use them based on cost.
Functional reliability, consistent upper end velocity with lighter bullets (copper is not as dense as lead), deep penetration with 150% expansion and 99-100% weight retention combines all the self defense attributes you could want, including barrier penetration. You don't have to feel like a grenade is exploding in your hand to launch this quality projectile.
Yes, I worked for Barnes but retired three years ago.
This 140 grn Barnes Military/Law Enforcement (M/LE) TAC-XP copper bullet is about as long as a lead 165 grn. The 10mm 155 grn bullet is almost too long to load successfully in a .40 S&W case, but some do it. The extra weight is not necessary for top performance.
M/LE TAC XP Pistol Bullets | Barnes Bullets