Despite the popularity of the American M-109 155mm Paladin self-propelled howitzer the U.S. (where the M-109 originated in the 1960s) has been seeking a replacement for over a decade without much success. For example, in 2009 the U.S. Army cancelled its second attempt (the XM1203 NLOS-C) to develop an M-109 replacement. The third attempt was the PIM (Paladin Integrated Management program) and, fortunately, it worked.
PIM was not a new design but more of a very thorough refurbishment. That army is going to rebuild many of its existing 900 M-109s, rather than trying to come up with another new design. But PIM will be a very extensive upgrade. It will use the same chassis as the M-2 Bradley infantry vehicle, and a new engine control system. Added to that will be the cab and gun mounts from the Paladin. From the XM1203, the automatic rammer, but not the automatic loader, will be used. So troops will still have to manually load the propellant and 41 kg (90 pound) shell, but the semi-automatic rammer will then push the propellant and shell into the firing chamber and close the breech. Automating this part of the process improves accuracy somewhat, because when troops manually shoved (rammed) in the propellant and shell, they often applied too much, or too little, pressure and left the shell out of position by a tiny bit, just enough to hurt accuracy. PIM will get new electronics, and numerous small improvements, many based on user suggestions. This will make the M-109 PIM competitive with some new European self-propelled 155mm howitzers. The PIM is also heavier, at 35 tons and the new chassis can support up to 50 tons. Even at that weight PIM is as fast as the existing M-109 and more maneuverable.
The NLOS-C was to have entered service in 2009 as part of the FCS (Future Combat System) program. But the FCS project lost its focus, budget discipline and Congressional support. It was cancelled in 2009. It was not for want of effort. In 2002 the prototype NLOS-C was cobbled together in six months, after the new (and very high tech and high priced) Crusader SP artillery system was cancelled. The 45 ton Crusader used an autoloader and an engine similar to the one used in the M-1 tank. It was deemed too heavy, too expensive and too much of a Cold War era weapon. Thus died the first attempt to replace the M-109.
Although the M-109 has been frequently updated, the NLOS-C incorporated many more new technologies. This included an auto-loader (from the Crusader) and a more modern 155mm gun and an APC chassis with a hybrid-electric engine (to reduce fuel consumption.) Some of the electronics developed for Crusader were also used in NLOS-C. This all weighed 23 tons, about the same as the M-109. But the NLOS only had a two man crew, compared to five in the M-109.
The final version of the NLOS-C was to be heavier (about 27 tons), because more defensive systems were added, to reflect experience in Iraq. The vehicle carried 24 rounds of 155mm ammo. Congress originally demanded that NLOS-C be in service by 2008, but development needed a few more years. Field testing (operating as one would in combat), began in 2007 using six prototypes.
One problem the brass were concerned about was the ability of the two man NLOC-C crew to hold up during 24/7 operations. The M-109, with a five man crew, has enough people to take care of maintenance, standing guard and, basically, always having one or two people rested and alert. This is not so easy when you only have two guys. One solution was to have two or more crews per vehicle, as combat aircraft (and some warships) have done for years. The off-duty crews would be back with the support troops. PIM will have a crew of four, compared to five in the Paladin M-109 and that is expected to be enough.
One of the things that helped kill the NLOS-C was the new GPS guided Excalibur shell. This smart shell entered service in 2008 and changed everything. Excalibur has worked very well in combat, and this is radically changing the way artillery operates. Excalibur means 80-90 percent less ammo has to be fired, meaning less wear and tear on SP artillery, less time needed for maintenance, and less time spent replenishing ammo supplies, and more time being ready for action.
During the fighting in Iraq and Afghanistan even the M-109 was not used much. The lighter, towed, M777 has proved more useful, especially when using the Excalibur shell. Currently, the army plans to keep PIM versions of the M-109 around until 2050. The army plans to acquire at least 551 PIMs by 2027, reflecting the impact of the Excalibur shell, and the number of older M-109s that are still fit for service. The M-109 was a solid design, which is pretty clear from how difficult it's been to come up with a replacement. So, in the end, the army replaced the M-109 with another M-109 upgrade.