Finnish BT-42 Assault Gun/tank

The BT-42 assault gun was a unique Finnish armored vehicle developed during World War II, born out of necessity and ingenuity rather than a formal design program. Its creation stemmed from Finland’s urgent need to bolster its armored forces during the Continuation War, fought against the Soviet Union from 1941 to 1944. The Finnish Army had captured a number of Soviet BT-7 light tanks during earlier engagements and, lacking more modern armored vehicles and heavy firepower, decided to repurpose these captured tanks into assault guns.

Eighteen BT-7 chassis were converted into BT-42s starting in 1942, with conversions completed by the end of 1943. Most of these were based on the BT-7 Model 1937, with one example using an older Model 1935 chassis. The conversion process was carried out by Finnish industry, with different companies handling various parts of the build. Lokomo and Crichton-Vulcan were responsible for restoring and modifying the BT-7 hulls, while the new turret structures were manufactured and armored separately. Final assembly and turret installation were handled by the State Artillery Factory.

The most distinctive feature of the BT-42 was its large, boxy turret, which housed a British 114 mm howitzer. These howitzers had originally been supplied to Finland by the United Kingdom during the earlier Winter War. The gun was a high-explosive support weapon intended for use against fortified positions, not enemy armor. The turret, although visually imposing, was lightly armored and contributed to the vehicle's high profile and increased vulnerability on the battlefield. There was no coaxial or hull-mounted machine gun, though crews typically carried a Suomi submachine gun for close defense.

The BT-42 retained the Christie suspension and many of the mechanical elements of the original BT-7. Powered by a Mikulin M-17T V-12 engine generating 500 horsepower, the vehicle was capable of speeds up to 53 kilometers per hour on roads. It had a combat range of about 375 kilometers. Its armor was modest, with the thickest sections only around 16 mm, offering minimal protection against anti-tank weapons or larger caliber rounds.

Combat experience revealed serious shortcomings in the BT-42's design. While the 114 mm gun was effective against bunkers and fixed positions, it lacked the armor penetration required to deal with Soviet tanks. During battles on the Svir River and later during the Soviet summer offensive in 1944, the BT-42 performed poorly. On one occasion, a BT-42 reportedly struck a Soviet T-34 tank numerous times without inflicting significant damage. The combination of inadequate armor, high silhouette, and underwhelming anti-tank capability made the vehicle highly vulnerable.

Attempts were made to improve its performance by introducing HEAT (High-Explosive Anti-Tank) rounds adapted from German designs, but the modifications failed. The rounds often did not arm properly due to issues with fuse compatibility, and thus were ineffective. During the defense of Vyborg in 1944, nine BT-42s were deployed, and eight were lost either through destruction in combat or mechanical failure.

After these failures, the BT-42 was withdrawn from front-line service. Postwar, the remaining units were retired, and by 1951, all but one had been scrapped. The sole surviving BT-42 is preserved today at the Parola Tank Museum in Finland.