"Knockdown power" is subjective, and truthfully, isn't really a correct term. However, .38s and 9mms have different efficiency depending on the specificication of the particular cartridge and the barrel length of the gun from which it's fired. It's difficult to compare the two with any real accuracy because there are too many variables. If you don't have access to any ballistics charts, try an internet search for handgun ballistics and compare them for yourself.
IMPROVING ANSWER: If you believe that a lighter but faster bullet is the one that dumps the most kinetic energy into the target, the 9mm will be measurably better at "knockdown" because although the bullet is slightly lower in mass it will hit with maybe 30% faster velocity (compare 800 f.p.s. for the .38 with maybe 1,200 feet-per-second for the 9mm).
But the .38 special will use longer and heavier bullets, with 158 grain being common and even 200-grains sometimes.
As a matter of the science of physics, the energy formula for a moving object depends on velocity squared, or velocity x velocity. So that means that velocity is more important than mass for achieving a high level of energy.
But in a practical sense, it's not "kinetic energy" that knocks down a bad guy or a big game animal. The most kinetic energy a bullet can deliver is equal is to the kinetic energy that the gun produces on the shooter's end-- recoil.
The caliber for one.
Depends on which cartridges- there are dozens of different .32s. If you mean a .32 S&W, the .380 has more energy- but the term "knockdown power" is pretty misleading. You should not expect any handgun to "knock" a person down. Works in the movies, not in real life.
I think you have to identify your definition of power. Do you mean speed or knockdown power? A .177 caliber pellet will travel faster than a .22 caliber pellet but a .22 pellet will have more knockdown power. With 200 GDP ($324) you should be able to find what you are looking for. I suggest you look at the web site below and read the owner reviews. Then you will have more information about your selection.( See the related link below)If this rifle is avalable in the UK then this one has the most power I've seen in an air rifle that is not a PCP class rifle. Its the "Benjamin Trail NP XL 725" its a 25 caliber Nitro piston air rifle.
Size and relative power. A .40 caliber bullet is .40 of an inch and the entire cartridge is 28.8 millimeters long. A .50 caliber (handgun) bullet is .50 of an inch and the entire cartridge is 40.99 millimeters long. The .50 caliber has more powder, more velocity, and more muzzle energy.
What the difference between process piping and power piping?
The knock down power of a 12 gauge shotgun depends on the ammo used. Using bird shot it had low knockdown power, however using double or triple ought buck packs quite a lot more. A slug would have even more knockdown power.
help!
The .22 is a larger slug and will have more knockdown power than the .177.
There really is no difference.
This all depends on your needs. FXAIRGUNS makes 10 different models in both .177 and .22 caliber. The .177 will be faster but the .22 will have more knockdown power. So it's really up to you, your needs and the cost. Remember, there is an additional cost of the hand pump or C02 tank.
Many people misunderstand power verses speed. Lightweight pellets can obtain incredible speed well over 1200 FPS, but that doesn't equate to power. The larger the pellet the slower the speed, but it will have more knockdown impact and will fly straighter than a lightweight pellet that is more effected by crosswind. At present the most powerful air rifle is a 50 caliber PCP like the SAM YANG Dragon Claw or any 50 caliber PCP. See the link below. It is more common to find a .25 Caliber like the Marauder or Condor with a .25 cal barrel. Benjamin even makes a new 357 caliber for hunting.
The difference is in the output frequency.