1-K was a Soviet anti-tank gun initially developed by the German company Rheinmetall. The gun was closely related to the German PaK 35/36. It lacked some improvements eventually introduced in PaK 35/36, but it was basically the same design; each gun could use ammunition of the other. 1-K had split trail carriage with unsprung wooden wheels (while PaK 35/36 received suspension and new wheels). It was equipped with horizontal sliding block breechblock, hydraulic recoil buffer and spring recuperator.
1-K was the first dedicated anti-tank gun of the Red Army (RKKA) and as such was actively used for training of anti-tank units. On 1 January 1936 RKKA possessed 506 guns of the type, of them 422 operational, 53 waiting for repair, 28 used for training, 3 unfit. When RKKA received large numbers of more powerful 45 mm guns, many 1-Ks were apparently relegated to training facilities and depots. The exact number of 1-Ks in service in June 1941 has not been determined. It is known that the gun was present in some units, e.g. 8th Mechanized Corps and it is likely that pieces stored in army depots were also rushed into active service. However there are no certain reports of their combat use. Apparently most of the guns were lost in combat at the early stage of the German-Soviet War.
The significance of the 1-K lies in the fact that it was the first Soviet anti-tank gun. As such, it gave some valuable experience. It also became the base for a series of Soviet 45 mm anti-tank guns.
It was a light and compact gun which could be easily moved by its crew. The drawbacks were a lack of suspension, weak fragmentation shell (because of small caliber) and poor manufacturing quality. RKKA wanted a larger-caliber gun that could be used as a battalion gun as well as in an anti-tank role, so the 1-K was quickly replaced in production by its 45 mm descendants.
By 1941 the gun was adequate only against lightly armoured vehicles. Modern tanks could only be penetrated from their side and only at short (less than 300 metres) range. The situation was aggravated by low ammunition quality, which explains smaller penetration figures compared to the PaK 35/36. As noted above, the 1-K could fire German shells, improving its anti-armor performance roughly to the level of early Soviet 45 mm guns, as those also suffered from problems with ammunition quality.
This was due primarily to the use of gunpowder with a nitrocelluose primer as propellant in all Soviet small arms, artillery and anti-tank artillery up until well into 1942, when British convoy shipments of Cordite and more advanced propellant technology became available. Also the continued use of obsolete APHE technology ammunition, which most nations had long abandoned for considerably improved penetration performance of solid shot AP, APC and APCBC ammunition technology.
The APHE shell itself was of the 1890s Hotchkiss naval type consisting of a hard-nosed forged-steel projectile with a base-mounted inertial deceleration shock-delay fuse with a stable explosive filler, most likely Picric Acid. APHE shells are more effective against battlefield sandbag, earthwork or log improvised fortifications and domestic buildings than equivalent-calibre impact detonating HE or fragmentation shells. APHE can be regarded as a useful dual-purpose round in many respects.
German Rheinmetall-Borstig evaluation tests on a captured 1-K, during 1941, gave a maximum penetration of up to 42 mm of perpendicular rolled homogenous armour plate at 100 metres with APHE and up to 61 mm of perpendicular hardened carbon steel armour plate at 100 metres with APHE. (Note that all tank-building nations had abandoned the use of carburized hardened carbon steel in favour of the increased protection offered by rolled nickel-chromium homogeneous steel armour plate, cast nickel-chromium steel and cast ferro-nickel based armoured alloys by the mid-1920s onwards).
By comparison the German 37 mm PaK 35/36 could penetrate up to 44 mm of perpendicular rolled homogenous armour plate at 100 metres with PzGr.18. APHE, up to 64 mm of perpendicular hardened carbon steel at 100 metres with PzGr.18. APHE, up to 65 mm of perpendicular rolled homogenous armour plate at 100 metres with PzGr.39. APCBC and up to 79 mm of perpendicular rolled homogenous armour plate at 100 metres with PzGr.40. APCR.
The PaK 35/36 used Binatol as a propellant.