The Lee-Enfield rifle has been in active service with at least one military since 1895 (currently still used by the Canadian Rangers).
The modern assault rifle SA80 used by British forces still allows for fixed bayonets, which is rare in modern rifles. Orders commanding soldiers to fix bayonets, in heavy fighting, has happened multiple times in the modern wars in Iraq and Afghanistan.
The US Military has a laser rifle called the PHASR rifle, which disorients and temporarily blinds the target. It also looks like Storm Rifle in Halo 4.
The Soviets produced an assault rifle, the TKB-517 that was more reliable and accurate, and easier to produce and maintain than the AK-47. It was only rejected because of a greater proficiency with the AK-47 among the Russian military.
The FP-45 Liberator was an extremely cheap single shot pistol, which was developed by the US military to be airdropped in occupied territories during World War 2. It was intended for use by resistance forces fighting axis powers.
In 1967, a then-classified Army report showed that out of 1,585 troops questioned, 80% experienced a stoppage while firing the M16 rifle. This occurred while the Army insisted to the public that it was the best rifle available for fighting in Vietnam.
One of the weapons U.S.A. would have used in the planned invasion of Japan was Little David, a 36 inch (914mm) mortar used to test aerial bombs. Converted into a mobile siege gun, it fired 3650 lb (1600 kg) shells at a range of 6 miles and would have been the largest caliber gun ever fielded.
8T13 Beano Grenade
The US Army during World War 2 developed a grenade (T13 Beano Grenade) the same size and shape as a baseball, to make it more effective for the soldiers who had grown up playing baseball.
9Mk 3 Centurion 169041
The Australian Mk 3 Centurion 169041 was nicknamed "The Atomic Tank" due to it surviving a 9.1kt blast from 460 meters away. The tank was simply driven off after the test and went on to spend another 23 years in service, including time in the Vietnam war.
The US military at one time invented the M65, the recoilless nuclear rifle. It fired nuclear rockets.