Decisive Darkness: What if Japan hadn't surrendered in 1945? by @The_Red
The M1B Garand under ownership of an unknow men
While the devastation of the Typhoon Louise in Okinawa forced the US armed forces in said area to be armed with some old firearms (the oldest been the M1917 Lewis and M1917 Enfield - one Marine Defense Battalion was armed with the Mendoza RM-2*), all the Americans Armed Forces (and the foreign forces that also send ground forces - for logistical purposes) were all armed with the most modern equipment at that time.
The handguns were still the same as during the entirety of the war
The main submachines guns became either M3 and M3A1 Grease Guns or Owen** guns, with the rare M1 Thompson been very rare (even rarer was the Lanchester smgs, but it would only be found inside of the British Pacific Fleet ships);
The rifles were almost the same situation as with the handguns, but with several variants, and outside of a M1903A4 Springfield, the Garand was off course the most common Allied firearm, but for late 45, the original M1 became itself been replaced by their new "brothers", been:
- the M1B (a 20 magazine selective-fire version - the mass production version of T20E2),
- the M1C/D (sniper rifle variant of the base M1) and
- the M1T1/2 (mass production of the T26 (T1) and M1E5 (T2) - while nicknamed "Tanker" (and indeed) some tank crew did use such rifle, their were mostly used by the paratroopers and replacing the M1 Carbines in certains formations);
meanwhile, the M1 (and M1A1), while still common, were still been replaced by the earlier mentioned M1T, or the automatic fire M2 or the M3, with a infrared sight attached.
The M1919 still was the main Medium MG (the M1917A1 was still used in some units in the Marines and the M2 was still the main HMG, but kept in the fixed positions in the last defenses of Kyushu), with the A4, A6 and the new A7 been introduced (based on the AN/M2 "Stinger" customization); the M1918A2 BAR was actually been replaced with the Winchester Automatic Rifle (ironically shorten for 'WAR') in the US Army, while the Marines (and some foreign units ) were replacing it with the M1945 Johnson (a modernized M1941)
The M9/M9A1 Bazooka (and the M18 - an M9A1 made off alluminion alloy) was still the main "RPG" of 1945, but was been supplement by the Recoiless Rifles (the M18, M20 and M27) by the paratroopers (alongeside some Panzerfausts***), plus the new M20 Rocket Launcher - but unlike in Europe, they were mostly deployed against bunkers and other hardened defences, as enemy armored vehicles were almost non-existing by that point.
Both shotguns and flametrowers were still equal prior to the invasion.
This is off course (outside of video games) a pure contrast to the Japanese, as outside the Type 4 (a near Japanese Garand) and the convertion of Type 97 aircrafts machine gun to ground use, there were units that could barely arms a single company with rifles and ammunitions, let alone whole divisions.
*The US, needing to replace the Typhoon Louise losses, ordered an Mexican company 5,000 Mendoza RM-2 in .30-'06 Sprg., and been proven a success, send them to overseas, and outside of the single Marine Defense Battalion in Okinawa, been used by the occupation forces in Bavaria and Austria (including the Brazilian Occupation Force)
**Been considered the best submachine gun in the Pacific War, MaCarthur (the man that pretty much putts the Americans front and center of every situation) requested that it own forces receive 45,000 Owens from the Australians, and the Aussies had to built several more workshops for the need of almost the entire world; and in a tit-for-tat, received in return some P-51M Mustangs and A-26 Invaders.
***The 82nd Airborne brought a few hundreds from Germany, and they also proven a success, with the caviat that the shorten range meants higher chance of injury or death by the user.
Type 3 Ho-Ni III from wardrawings website
Both the Type 3 Chi-Nu and Type 4 Chi-To were the most common tank of the Japanese in their Home Islands (according to video games and certain documentaries), the reality was much different - other tanks were still the backbone of the IJA, mostly the light tanks Type 95 Ha-Go (and Type 4 Ke-Nu) and Type 98 Ke-Ni (and even improven Tye 2 Ke-To) and the medium tanks Type 97 Chi-Ha (mostly the improved ShinHoto) and Type 1 Chi-He. But while a few hundreds Type 3 and Type 4 were built (the Type 5 Chi-Ri was cancelled, but the single prototype was used in a single battle, more a little latter), lack of fuel and the complexity (and lack of spares to made the situation worse) made many been used more as either mobile or fixed artillery or AT guns wit a turret. With this in mind, many factories began instead focusing on the Anti-Tank variants, as they lack of turret and lighter weight made them much more "economic" in a articional war against the world. In fact, more Type 3 Ho-Ni III and Type 5 Ho-Ru (an open top "Hetzer" based on the Ha-Go with a 47mm gun) were reported than any other IFV.
Most of the listed tanks were present whiting the 1st Armored Division (wit a few add-hoc Independent Armored Brigades in the mix also having one or another) on the Battle of Isesaki, in mid-April, that became the biggest "tank" battle of the Pacific War, were the Division, in a defensive position, was utterly destroyed by just half of the American 13th Armored Division. From the common Type 3 Ho-Ni III's, to the single Type 89 I-Go, Type 5 Chi-Ri prototype and even a single M3 Stuart, the Japanese destroyed just a American company worth of tanks (the most "destroyed" company only lost a platoon worth of tanks), and off course, this doesn't count the tanks that could be repaired and re-used, and off course, half of those destroyed were done so mostly by suicide soldiers with explosive that with direct tank vs tank engagements.