There have been two main competing types of military ammunition, the slug and the missile. The slug comes in various sizes from bullets, to artillery and naval gun and mortar shells, to dumb bombs dropped from airplanes. The alternative is a missile. The prototypical missile is self-propelled, but increasingly, the self-propelled v. hurled distinction is giving way to the distinction between guided and unguided weapons.
The trend is to favor smaller, guided weapons, over larger unguided weapons. The latest artillery shells and airplane dropped bombs have GPS guidance systems that make them look a lot like missiles. This is due largely to a desire to avoid collateral damage caused either by an inaccurate strike, or a strike larger than needed to hit the target. The true target, apparently, is often small, but historically, with inaccurate ammunition, a large explosion has been necessary to make "close" good enough. The trend may also reflect a shift in U.S. military operations from planning for Cold War mass invasion scenarios to a focus on dealing with counterinsurgency actions.
The Navy no longer has any ships with a naval gun larger than the 5" gun on its destroyers and cruisers. The 8" guns and 16" guns on older models of ships have been removed from service, largely in favor of cruise missiles, which deliver similar sized packets of high explosives, more accurately, for a longer range, at a cost of about half a million dollars per missile.
The Army has likewise retired its 203mm (8") howitzer (except for a few remaining national guard units). The largest slug thrower rounds now in use by the U.S. military is the 155mm howitzer round.
Even old school weapons, the Army's 155mm howitzer and the Navy's 155mm advanced gun system on the DD(X), proposed to mute the concern about shrinking naval gun size, are increasingly seen as tools for delivering GPS guided advanced rounds, rather than mere dumb slugs. Some rounds for the Army's new M777 light towed howitzer are even "rocket assisted", which starts to sound a lot like a missile. Proposed new mortar rounds also come in rocket assisted versions. When does a howitzer or naval gun become a small multiple rocket launcher?
There is increasing pressure to convert all 5" naval guns and 155mm howitzers to guided munitions, which would leave the 120mm round as the U.S. military's largest unguided munition. And, the M1 Abrams tank, with its 120mm direct fire main gun, is facing pressure for multiple reasons, such as the limits that its large weight places on its ability to be rapidly deployed, and the effectiveness of other platforms in anti-tank warfare. In the Iraq War, the TOW missile armed Bradley fighting vehicle destroyed more Iraqi tanks than the M1 tanks did with their 120mm rounds, and attack helicopters with Hellfire missiles, and A-10 attack planes with Maverick missiles also destroyed a large share of Iraqi tanks from above. This gets noticed when the M1 was originally designed to engage in tank on tank battles.
The line between the 250 pound "small diameter bomb", which has an integrated guidance system but not propellant of its own, and the tactical missiles carried by fighter aircraft is also growing thinner and thinner.
This doesn't come cheap. The Army's new guided artillery rounds will cost $80,000 each (half of which is R&D). This is twenty times or more what an unguided round would cost, and is similar to the cost of the Army's Hellfire missiles, which it mounts on helicopters as an anti-tank weapon. Both are about 100 pounds in weight as well.