The U.S. Army ERCA (Extended Range Cannon Artillery) has overcome reliability problems with the new cannon artillery itself and recently successfully fired the new BAE SCALRP (Sub-Caliber Artillery Long-Range Projectile) guided 155mm shell at a target 110 kilometers away. The current Excalibur guided shell has a max range of 38 kilometers and unguided shells have a range of 25 kilometers.
SCALRP uses a sabot design, which means that when the 155mm SCALRP shell is fired the actual shell separates from the 155mm case and the smaller caliber shell is able to reach a longer range than a regular 155mm shell. SCALRP also contains sensors as well as a GPS guidance system and is able to hit stationary or moving targets. SCALRP is still in development and more test firings and some tweaking to the shell are required before it is ready for service in the ERCA for as well as, at shorter ranges, in other 155mm guns and howitzers. The cost per shell is about the same as the Excalibur. SCALRP contains less explosives than an unguided 155mm shell, but that makes little difference because the guided shell depends on accuracy, not explosive effect, to destroy or damage a target.
Back in 2021 the ERCA prototype 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 ranges 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 propellant that could provide the power to push the ERCA Excalibur out to 70 kilometers. This is what the SCALRP sabot-type shell did and now there is artillery ammunition completive with the 227mm GMLRS guided rockets that have a max range of about 80 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 has 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 initial 2020 tests and it worked well in late 2020 tests that sent a GPS shell 70 kilometers.
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 goes, the more its 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 were conducted in 2021 to verify that the 70 kilometer Excalibur is reliable and ready for production. The longer barrel M109A7 was to enter service with the new Excalibur shell by 2023. Then it was discovered that the longer barrel (including the shell loading mechanism) had reliability problems. This has delayed final testing and approval indefinitely until the reliability problem can be fixed.
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 is no big thing. Making all the modifications to the recoil system, breech, and interior of the M109 did not require any new technology. Now it turns out that some new tech is required and if that cannot be obtained soon and at an affordable cost, the 70 kilometer ERCM is dead. A shorter barrel, providing less range, might be acceptable if it can be developed in time and without costing so much that the ERCM Paladin becomes another new weapon that was canceled because it was too expensive to procure in useful numbers.
The longer barrel and 70-kilometer range of the Excalibur shell was a big deal for the Paladin, which was 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 updated Paladin versions of the M-109 around until 2050. The army planned to acquire as many as 500 M109A7 Paladins by 2027, reflecting the impact of the GPS guided shells, and the number of older M109s that are still fit for service.
The primary competition for ERCA equipped Paladins is the HIMARS (High Mobility Artillery Rocket System) vehicles that carry six GMLRS (GPS/INS guided MLRS). Ukraine has had much success with the HIMARS vehicle and the capabilities of the GMLRS rockets. The latest version of the rocket can hit targets up to 85 kilometers away using INS/GPS (Inertial Navigation System/Global Positioning System) guidance. The INS component is important because it takes over if the GPS signal is lost due to jamming or when mountainous terrain blocks GPS as the rocket approaches the target. Over the last two decades INS has become more accurate. Russia has been a major source of GPS jammers since the 1990s.
Two East Europe NATO members (Romania and Poland) use HIMARS. Several other NATO countries use HIMARS or the larger tracked MLRS vehicle (M270) that carries two pods. HIMARS is gradually replacing these because the tracked M270 requires significantly more maintenance. The HIMARS truck-mounted GMLRS system is ideal for Ukrainian artillery tactics, which currently use individual self-propelled artillery to fire unguided shells at targets up to 40 kilometers away. This avoids return fire from the Russians, who are more vulnerable to Ukrainian tactics because Russia still employs mass artillery fire from batteries (six guns) or battalions (three batteries).
Each HIMARS system is carried in a 12 to 16-ton 6x6 truck with the heavier version having armored (against small arms fire and shell fragments) crew cab. The vehicle carries one MLRS (Multiple Launch Rocket System) six rocket pod instead of two in the original larger, tracked, MLRS vehicle. Initially, a major attraction was that a HIMARS truck could fit into a C-130 transport (unlike the 22-ton tracked MLRS) and was much cheaper to operate. The HIMARS vehicle can move at up to 85 kilometers an hour on paved roads and travel 480 kilometers on internal fuel. In 2016 it was revealed that HIMARS vehicles in American service had reached a million operational hours with a 99 percent readiness rate.