Kudos to the poster who defined what HE thinks a combat knife is. The definition can be pretty much in the air. People that carry "combat folders" consider them to be combat knives for a new generation. The description that Spookey gave is a more "traditional" definition. If you compare some of the more well known (sucessful) combat knives and look at thier similarities they fit his description. KaBar, S-F commando, Gerber MKII, although different, are all fixed blade and are around 11"-12" overall in length (blade is long enough to penetrate internals).
I found a good article online in regards to this topic (keep in mind this is just one person's opinion):
Combat Knives By Allen Elishewitz
Throughout history combat has taken troops into different environments like desert, jungle and even urban warfare. There are certain characteristics a knife must possess for different warfare conditions. In my opinion, I consider a fixed blade to be the primary cutting tool of a basic soldier. The problem with any type of mechanical knife is that the chances of something going wrong are greater. My experiences as a team leader in a Recon unit gave me the insights of what a real combat knife should possess. The inherent properties that determine a combat knife are: blade,
guard, handle and sheath. In combat, a knife's main purpose is a tool, secondarily a weapon.
Blade
A combat knife's blade should not be so small that it cannot be used as a tool. But on the other hand, it should not be so long that it gets in the way. A good length for a blade is from 5½ to 7½ inches. The best thickness for the blade is 3/16" but if the individual does not mind the extra
weight 1/4" is also good. The finish is very important it must not reflect light in a combat situation. Bead blasting or a dark satin finish is the best way to subdue a blade. However, you can use PVD (Physical Vapor Deposition), bluing, Parkerizing, or any darkening method to cover the glare. It is very important to have a single edged blade for combat. You loose strength and the ability to use it as a tool if the blade is double edged. Serrations are an excellent addition on a combat knife,
however saw blades will lessen the necessary characteristics for use as a tool. The weight of the blade can make some work easier, but proper cutting technique can achieve the same amount of work with a thinner, smaller knife. For example instead of hacking at a piece of wood with a big
knife, one can achieve the same task with a smaller knife using a chisel cutting technique. When patrolling, you might consider carrying a sterile knife, this means the knife has no markings. If the knife is lost in combat, the enemy cannot tell who or what unit is in the area.
Guard
The best material for combat guards is stainless steel because it is strong. The guard can also be incorporated as an intrical part of the handle material. Like the blade, the guard must not reflect any light. The guard even more so because the majority of the time the knife will be
sheathed and the sheath will not cover the guard. Bead blasting or some kind of coating on the guard will eliminate that problem. Whether you have a bolster or a slip over guard they both must be soldered or epoxied. Both methods not only secure the guard but most importantly protect the
tang from moisture. Inadequate protection can lead to rusting in joints, which will weaken the knife at its most critical location. A full double guard is impractical because it will get in the way and add extra weight to the knife. A more sensible guard style would be single or one and a half.
A single guard allows you to draw the knife from the sheath with limited possibility of snagging. If you find it necessary to have a top guard, go with half a guard. The top guard is half the length of the bottom guard. This gives you added protection and leverage when using the knife.
Handle
The handle material must not absorb any kind of liquid nor crack or shrink. The handle should be able to absorb continuous vibrations and shocks from chopping or cutting. The ideal handle materials for combat knives are G-10 or linen Micarta. A good dark color and nonreflective surface is the best finish for a combat knife. Make sure the knife fits snugly in your hand
and does not slip when wet. If the handle is too short, your hand will feel cramped and you could not use the pommel as a hammer. The handle should be long enough that you can grip it in several different positions when necessary. On a full tang knife, make sure the maker uses
screws and bolts or no less than five pins to attach the handle onto the tang. Personally I prefer a hidden tang knife for a combat situation because the tang is completely protected within the handle and the handle absorbs more shocks than a full tang knife. A thong hole is another good idea, it allows you to attach a cord and tie it to your belt. This will ensure against losing it in the brush. Round off all the corners on the handle to prevent the knife from feeling uncomfortable after a long period of use.
Sheath
The sheath should be attached to your body in a way that it does not produce any sound during movement. At the same time, it should not obstruct your physical movement. The sheath must not absorb moisture, crack, dry rot or tear. It must not reflect light or make a sound when
drawing the knife. The two best materials for combat sheaths are Kydex and nylon. Both are extremely durable and will not rust the blade. The ideal combination is a nylon sheath with a Kydex insert, this blends the best of both worlds. Also, adding two securing devices will allow you to be airborne and water qualified. Do not forget to add a drain hole in your sheath, which will help keep the inside clean and dry.
