P-40 and 180gr
origonaly posted by Chan Bates on the KTOG message board.
In order to keep the necessary OAL of the cartridge for proper chambering, a 180 grn bullet has to be set deeper into the case than shorter, lighter weight bullets. Remember, if the diameter stays the same, the most usual way to increase or decrease the weight of a bullet is to increase or decrease its length. 180's are generally loaded to the maximum OAL, which can be a challenge for feeding.
This increased bullet length, combined with the additional powder required to push the heavier bullet at an acceptable velocity, yields compressed powder loads. Compressed powder loads generally increase pressure. This is where things can get critical.
It takes very little error here, in either powder quantity or bullet seating, to cause large increases in pressure.
Because .40 S&W bullets are generally not crimped, even hitting the feed ramp on the way up into the chamber can cause additional bullet set back, and therefore increased pressure. Hand cycling rounds, loading and unloading, increases the potential for bullet setback.
The .40 S&W is already a relatively high pressure round (35,000 psi, the same as the vaunted .357 mag.). There is no +P rating for this round, as it is already in the realm of most +P rounds for pressure.
Other factors can increase pressure, such as a hot primer, or elevated temperature of the whole cartridge, such as from a hot car in the summer.
Some full-size .40 guns can handle increased pressures better than the small, lightweight KT P-40. There have been many reported KABOOMS with Glocks and 180 .40 rounds because the increased pressure naturally heads for the weakest point: the part of the cartridge case that is unsupported at six o'clock.
The .40 S&W is an attempt to emulate the powerful 10mm pistol cartridge, but make it more manageable. The 180 grn weight bullet worked fine for the 10MM, with its longer case and higher velocities. Trying to get similar performance out of the same grain weight bullet in a smaller case gets risky.
Remember, with a KT P-40 you have a barrel/slide combination originally designed for 9mm. Now a larger, higher pressure round is going in the same barrel with a larger chamber and tube. That means thinner walls to contain higher pressure. Add to that a polymer grip and an aluminum frame with increased recoil for the heavier bullet, and you will wear your gun out faster than shooting lighter bullets.
Lighter bullets are more comfortable to shoot and therefore may contribute to greater accuracy and perhaps faster follow up shots.
You can shoot 180's in a KT and not have problems. That's true also for other .40's, including Glocks. What is undeniable is that there is an increased risk of an over-pressure round blowing up the gun in your--or someone else's--hand, or maybe right when you need it the most.
To me, the risks far outweigh the benefits. Chan
Again more on both the P-11 and P-40 from CB3 on KTOG:
Because the P-11/40's are lightweight, short-barreled guns, I prefer to stay in the mid-range of bullet weights for each.
I feel the heaviest loads, such as 147 grn for 9MM and 180 grn for .40, suffer more velocity loss from the short KT barrels, rendering them (perhaps) less effective for expansion/penetration than mid-range bullets.
I think there is also some additional recoil from the heavier bullets, which is another negative.
Remember also that heavier bullets are heavier because they are longer. The diameter does not change, so bullet length must add the weight. The overall cartridge length must stay within prescribed limits, so a longer bullet typically sits deeper into the case. Therefore, a longer bullet can produce compressed powder charges (such as in the .40 180 grn loadings) which are exacerbated by bullet setback issues causing higher than normal pressures (and potential Kabooms).
I prefer 124 - 135 grn bullets in the P-11, and 155 - 165 grn bullets in the P-40. I think you will find that all the parameters of bullet performance from a short barrel are met by standard mid-range bullet loadings.
Lightweight bullets have higher velocity, but that also means they spin faster and therefore may disintegrate faster or be deflected more easily. Higher velocity lightweights are generally known for fragmenting and having relative shallower penetration, especially if they hit bone (e.g., a forearm) before getting into the real target.
As with so many things in life, there is a balance you should seek to achieve. Extremes of anything do not tend to produce balance. With KT's, you are already at some of the design/function extremes for handguns--small and lightweight. Adding another extreme in going to the ends of the available bullet weight spectrum is more likely to add to problems than solve them.