In my post on the 105mm howitzers, I was taken to task for calling the French 75 a howitzer, instead of a gun. If you go to Craig’s excellent blog on the American Civil War, he’s got quite a bit on the technical and doctrinal history of the artillery of the time.
Historically, artillery was divided into three categories: Guns, howitzers, and mortars. The primary distinction between the three types was barrel length, as expressed in calibers (that is the length of the barrel measured as multiple of the bore diameter).
Gun had the longest barrels, and thus tended to have the highest velocity, and the flattest trajectory. Howitzers were the middle ground, and a correspondingly higher trajectory. Mortars had very short barrels, and were used to loft their projectiles to the target.
Over the years, things have changed a bit. Since the introduction of the Stokes mortar, or trench mortar in World War I, the traditional mortar has almost disappeared from the battlefield, and mortars are now almost universally seen as infantry weapons organic to infantry units, as opposed to true artillery weapons. The are usually a smoothbore tube fired by simply dropping the round down the muzzle. They range from small man portable systems such as 60mm mortars, up through the (barely man portable) 81mm weapons, to the large 120mm mortars that pretty much require a prime mover for transport on the battlefield.
The gun/howitzer distinction was a little more blurred. The US Army did maintain a distinction between guns and howitzers for many a year, well through World War II. Craig has promised to get around to discussing some of the prime examples of US artillery in WWII, primarily the family of 155mm systems. But suffice to say for now that as the need for increased range became pressing, the difference in weapons started to fade. To increase the range for a given caliber is almost by definition to lengthen the tube. The longer tubes had traditionally added so much weight that maintaining the high elevation that howitzers had (and the plunging fire they gave, plus the ability to fire over high obstacles such as mountains) was too costly in terms of weight and size of a mount.
The drive for ever increasing range for artillery systems was a function of two technical developments. First, the ever increasing motorization and mechanization of the Army meant that troop units would be able to move that much faster, and consequently, be spread over larger areas. Second, the threat of atomic weapons on the battlefield meant that maneuver commanders wanted to keep their units dispersed as much as possible. Dispersed units were far less attractive targets for atomic weapons. Again, this dispersal meant the area that supporting artillery had to cover grew at an alarming rate. While artillery was faster to move than it had ever been, there was no way to achieve coverage of it supported units and still keep fires massed- unless the range of the guns was greatly improved. Adding air mobility to the mix in the 1960s accelerated this trend.
Thus, when Craig gets around to talking about post war Army 155mm artillery, you’ll see the original M109 had a rather stumpy little tube- a howitzer. But later versions of the venerable M109 had progressively longer tubes.
But high elevation plunging fire was still very desirable. Why? Imagine a young artillery battery commander today in Afghanistan. His battery is positioned in a valley, surrounded by high ridges on virtually all sides. Any Taliban that shows himself is liable to catch a 155mm round in the face. But if the Taliban merely pops over the other side of the ridge, he’s pretty much using the ridge for cover, much like an infantryman would use a wall to duck behind for protection. The flat trajectory of a high velocity round means our shells would simply pass overhead –and probably land on some innocent farmer’s goat! If our young artillery battery commander can fire at high elevations though, he can lob the rounds just over the ridgeline. To do so, his guns must have the ability to elevate at high angle. Again, this drives up the weight of the mount, but with the increased power of prime movers, and improvements in materials used (such as the widespread use of titanium in the newest M777 gun) the Army finally has been able to achieve gun like range with howitzer like elevations.