At the end of 2020 the U.S. Army ERCA (Extended Range Cannon Artillery) finally achieved a decade old goal of using a longer barrel cannon firing an extended range Excalibur GPS guided shell to accurately hit a target 70 kilometers distant. Over a decade ago it was realized that this required three new technologies. The primary need was a version of a reliable GPS guided artillery shell, like Excalibur, that could handle the more difficult task of keeping the shell on course over such a long range. The first version of Excalibur only worked at ranged between 20 and 25 kilometers. Second, there had to be a longer barrel 155mm artillery system that could fire the longer-range Excalibur shell. Finally, there had to be a new propellent that could provide the power to push the ERCA Excalibur out to 70 kilometers.
A longer-range Excalibur shell began development in 2010 and by 2011 Version 1A-2 of Excalibur was approved for combat use. During tests Excalibur 1A-2 accurately hit targets 40 kilometers distant. During its first combat use in 2012, a U.S. Marine Corps M777 155mm lightweight towed gun hit a target 36 kilometers away using Excalibur 1A-2. During development of Excalibur 1A-2 it was noted that, with a longer barrel gun and more powerful propellant, Excalibur could hit targets at least 60 kilometers and theoretically 70 kilometers away. Even with the short range of the original Excalibur, the GPS guided shell was quickly accepted as an essential weapon.
A more difficult problem was obtaining a new 155mm gun with a longer barrel. The relatively new and popular 4.2-ton M777 lightweight howitzer had a 155mm/32 barrel. That means the barrel length is 32 times 155mm or 5.1 meters (16.7 feet) long. The first long barrel experiments resulted in the longer XM907 barrel for the M777. This was a 155mm/52 barrel which was 8.1 meters (26.4 feet) long. XM907 worked, but was not practical for the towed M777 howitzer.
Another long barrel, the M1299 was built. This was a 155mm/58 barrel which was 9 meters (29.3 feet) long, and designed to be used in the latest version of the Paladin 155mm self-propelled howitzer. Normally the Paladin carries a 155mm/39 barrel that is 6.05 meters (19.3 feet) long, but similar vehicles have had longer barrels and been successful. An M1299 barrel on a M109A7 Paladin was used for the 2020 tests and it worked well in late 2020 tests.
A new XM1113 propellant system was first used with the long-barrel Paladin in early 2020 and was able to send a shell 65 kilometers. The XM1113 uses the decades old RAP (Rocket Assisted Projectile) technology that has been updated to provide unprecedented ranges of up to 70 kilometers. Such extreme ranges were not practical before GPS guided shells became available as the longer range, the unguided shell accuracy gets worse. The late 2020 test showed that Excalibur could put shells on targets 70 kilometers away. Only one of the three XM1113/Excalibur shells fired hit the target. The other two missed because the Excalibur guidance system was not able to handle some weather conditions, like exceptionally heavy winds. Excalibur can be tweaked to deal with that and more tests will be conducted in 2021 which should verify that the 70 kilometers Excalibur is reliable and ready for production. If that happens the longer barrel M109A7 will be in service with the new Excalibur shell by 2023.
There are already vehicles similar to the M109 with longer barrels, although not quite that long. From past experience the longer barrel on a self-propelled howitzer chassis no big thing. Making all the modifications to the recoil system, breech, and interior of the M109 did not require any new technology.
The longer barrel and 70-kilometer range of the Excalibur shell is a big deal for the Paladin, which is seen as a system eventually being replaced by cheaper, more effective artillery. During the fighting in Iraq and Afghanistan, the M109 was not used much. The lighter, towed, M777 has proved more useful, especially when using GPS guided shells. The army plans to keep PIM/Paladin versions of the M-109 around until 2050. The army plans to acquire as many as 500 M109A7 by 2027, reflecting the impact of the GPS guided shells, and the number of older M109s that are still fit for service.
The M109 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 M109 with another M109 upgrade. Along with the new M109A7 will come a hundred or so of the upgraded M992A3 CAT ammo resupply vehicle which is basically an M109 without the turret and space to carry nearly a hundred rounds of 155mm ammo as well as automated systems. These vehicles can quickly transfer ammo to a M109, which can only carry 36 15mm shells and propellant. Fewer M992s are needed because M109s fire fewer of the guided shells. The GPS shells have also been improved with Excalibur complemented, and often replaced by the new M1156 PGK guided shell. The advantage PGK has is that the GPS guidance is not built into a shell but instead it is a slightly heavier (about 1.4 kg/3 pounds) and larger fuze that screws into the front of a 155mm shell. This PGK fuze contains a GPS and small fins to guide the shell to a precision hit. Normally the fuze just controls how the shell will explode or when by using a timer or small radar. No one had been able to put GPS guidance in such a small package before but many kept trying for several decades. A PGK version for longer (70 kilometer) range has to be developed and, if the demand is there, it will be.
The original (2009) version of PGK was much less precise than Excalibur and could only ensure that the shell landed within 50 meters (160 feet) of the target. If it did not hit within 150 meters, PGK deactivated the shell so it did not explode. The original version of PGK was subsequently tweaked so that by 2012 it landed within 32 meters (100 feet). PGK was soon further improved to put a guided shell within 19 meters (60 feet). By 2015 accuracy was improved to 10 meters or less and the reliability problems largely eliminated as the PGK was now over 90 percent reliable. At that point, Excalibur became a premium item, which was more accurate than PGK but nearly twice as expensive. PGK proved itself in Syria where small numbers of American towed howitzers supported the Syrian Kurd forces that took the ISIL (Islamic State in Iraq and the Levant) capital Raqqa in 2017. ISIL thought they could defeat the attackers by defending many hidden positions inside the city. PGK made that ineffective. Most of the civilians had fled Raqqa and once a group of ISIL fighters was encountered one or two PHK shells would kill them. This broke the morale of many ISIL fighters, who fled Raqqa rather than fight to the death without much chance of hurting any of the attackers.
One of the primary advantages of longer range 155mm guns is that more of them can be quickly shifted to targets that were earlier out of range. The longer-range shell makes it easier to concentrate a lot of fire for an emergency situation. This longer range is monopolized by the M109A7 because the longer barrel does not work with the M777 or towed 155mm artillery in general. The U.S. got rid of most towed artillery decades ago and ultimately kept one “lightweight” 155mm model because it could be easily moved by helicopter.