Correia and K80Geoff, the FG.42, (which is much shorter than "Fallschrimjagergewehr 1942", has kind of an odd history, too. The following excerpts are from German Small Arms by AJR Cormack (Exeter Books, 1979)...
"Although the total number of FG.42 weapons produced was relatively small, the weapon has always been held up as an example of the ideal weapon and was an innovation in weapons design. There are a number of reasons why the ideal weapon cannot, by its varied requirements exist, but the FG.42 did represent an attempt to provide paratroop forces with a fast-firing automatic rifle and an effective light machine gun. The Luftwaffe decided that the short bolt action carbine was inadequate in combat, as was the short range submachine gun. Availible machine guns were bulky and too heavy for comfortable air drop. Therefore, in 1940 the Luftwaffe put the design parameters of a new weapon to a number of firms, of which only two Rheinmetall and Krieghoff took up the project. This small acceptance becomes clearer when it is remembered that both Walther and Mauser were involved in Sturmgewehr projects and thus felt that another new weapon might confict with their army commitment. There is little doubt that service rivalry also played a part in the desire for a seperate Fallschirmjager weapon.
The Krieghoff weapon designed by Henry Krieghoff was not accepted. Some sources state that it was a gas operated rifle with a vertical sliding block type of locking system.
The design of Louis Stange of Rheinmetall was accepted and by early 1942 a prototype was availible for test. The weapon wass sufficiently successful for a request for a production batch of 3,000 was made. It is possible that the use of the full-power 7.92mm cartridge could be questioned as, when the FG.42 was used as a light machine gun, it fell short of expectations, being somewhat unstable. Permision which was required for all expenditure of labor and material, was granted. The design called for high quality alloy steel and although a lower grade manganese type was eventually used, this also was soon in short supply. Consequently, approximately only two-thirds of the desired quantity was produced. During the production run the trigger guard was modified to a stamping.
A major redesign was carried out. The new weapon was longer and heavier, the magazine was changed, the stock was manufactured from wood instead of steel, the pistol grip was made nearer to vertical and the trigger group was removeable. A four-position gas regulator was fitted, the bolt spring and the recoil spring were changed to a wound-wire type, a case deflector was fitted and the bayonet mount and muzzle brake were changed. Some of the early type weapons had a scope mount fitted as an extra, but all later types had the scope mount intergral with the weapon.
Although the changes were so extensive the the new weapons had only an external family resemblance, the method of operation was not changed. Major production was carried out by Kreighoff as the Rheinmetall plant lacked production facilities. All weapons had 'fzs' code stamp for Kreighoff, but none have Waffenamt numbers as they were supplied for a private Luftwaffe contract."
In small arms I would rate the Stg.44 as the most interesting, as it lead to all modern combat rifles, even if it was rather heavy. If you want to include the "big boys", I would think the Hochdruckpumpe (HDP) "supergun" would fit the bill. It was a 15-cm gun made up of 40 sections with 28 powder charges that detonated sequentially to boost the outgoing round. It fired projectiles that were eight-foot long, 150 lb. "arrows" at a muzzle velocity of 4500 fps with a range of 80 miles. According to Major General Leslie E. Simon, USA, "Its tests were regarded as successful although the barrel expolded about once in every three shots." I don't think I'd call that "successful"!
To unload after firing you had to shove a wooden dowell down the muzzle to extract the fired shell casing!