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Replacements for the M4 have mostly focused on two factors: improving its reliability, and its penetration.[43] The first attempt to find a replacement for the M4 came in 1986, with the Advanced Combat Rifle program, in which the caseless Heckler & Koch G11 and various flechette rifles were tested, but this was quickly dropped as these designs were mostly prototypes, which demonstrated a lack of reliability.[43] In the 1990s, the Objective Individual Combat Weapon competition was put forth to find a replacement for the M4. Two designs were produced, both by Heckler & Koch: the XM29 OICW, which incorporated a smart grenade launcher, but was canceled in 2004 as it was too heavy, and the XM8, which was canceled in 2005 as it did not offer significant enough improvements over the M4.[43]
The Heckler & Koch HK416 was introduced in 2005, incorporating the same lower receiver as the M4A1, but replacing its direct impingement system with a gas-operated rotating bolt, more comparable to that of the G36.[44] The HK416 was adopted by the Navy SEALs, Delta Force, and other special forces.[43] In 2010, it was adopted by the Marines as the M27 Infantry Automatic Rifle.[45] The same year, the Rangers and Navy SEALs adopted the FN SCAR, but later withdrew their purchase, as it was not a significant enough improvement over the M4A1.[46]
SIG Sauer XM7, caliber 6.8×51mm (.277 in)
After the failure of the Individual Carbine program, the Next Generation Squad Weapon (NGSW) was started in 2017. The program aimed to replace the M4 Carbine and the M249 SAW with weapons that would compensate for their perceived deficiencies when fighting at longer ranges, as well as addressing concerns about the effectiveness of traditional 5.56x45mm ammunition against troops wearing body armor in a future peer or near-peer conflict. In order to achieve these goals, all weapon submissions were to be chambered in a new 6.8x51 mm caliber.
SIG Sauer, Textron Systems, FN Herstal, True Velocity (previously Lonestar Future Weapons and General Dynamics), and PCP Tactical took part in the program. Textron submitted a cased-telescoped (CT) ammunition-firing rifle for the program; FN Herstal submitted their HAMR IAR re-chambered in 6.8mm caliber; PCP Tactical submitted a modified Desert Tech MDRx; SIG Sauer submitted a redesigned MCX variant known as the MCX-SPEAR. In early 2022, the program concluded, with SIG Sauer being declared the winner. Their rifle entry was designated the XM5 (later changed to XM7), and their automatic rifle the XM250.[47] Operational testing and fielding are scheduled for 2024.
