In August 1941, three LaGG-3s produced by GAZ-23 in Leningrad were retrofitted with a Boris Shpitalnyy's 37 mm Sh-37 gun. The planes were named K-37 and immediately sent to the front.
	This weapon was firing through the cylinder banks and the propeller's hub; its barrel protruded from the spinner for about 60% of the spinner length. The weapon had a loop ammo reserve with 20 rounds. Here we see a drawing of the Sh-37 mounted between the cylinder nanks pf a M-105P engine (left side cylinder bank is omitted). Image from Aviakollekcia 5/2005

The gross weight of the plane was increased to 3363 kg, and its speed decreased of 5-7 km/h ; the climb rate and ceiling were lower than the standard LaGG-3s of the same series. The handling qualities were preserved due to the installation of automatic slats, as those installed on later versions.
The tests demonstrated that the plane was a stable platform of fire, and the gun had a good kill probability against targets at a range of 300-400 m; the gun was intended both for use against enemy bombers and ground targets.

20 LaGG-3s K-37 were built in late 1941, and they were delivered to 42 IAP on the Bryansk front in early 1942. The first combats were successful, and three enemy bombers were destroyed. However, it was noted that 20 rounds for the gun were an insufficient supply. The pilots had to fire only after having aimed carefully, and only in short burst of 1-2 rounds, both to save ammo and because the recoil forces caused the plane to drop on its nose and to lose the alignment with the target, particularly if the fighter was flying at low speed. Incendiary-explosive shells were used against flying targets, while armor-piercing shells could be used against ground armore targets.

In September-October 1942, these planes were utilized during the Battle of Stalingrad by the 291 IAP with good success, shooting down 13 enemy bombers; 7 K-37 were lost in combat on that occasion. During their use, they were escorted by Yak fighters. A.Ootin, commander of the 220 Fighter Air Division (IAD) to which the 291 IAP was assigned, was very satisfied with these planes, and wrote that one or two hits in any part of an enemy plane were sufficient to shot it down.
 
 

	The new 11P37 (later renamed Nudelman-Soranov NS-37) 37 mm gun was developed by OKB-16 design bureau guided by A. Nudelman and A. Soranov; they continued the work of Y. Taubin and M. Baboorin that were shot for their previous failure.
	The barrel was less protruding than the Sh-37, about 40% of the spinner length. The performance of NS-37 was very high, despite its relatively low mass (150 kg). When armed with armour-piercing shells, it could pierce an armour of nearly 50 mm at a range of 200 m.  The tests of the weapon were completed in March 1942; this gun was lighter (about a half ) and more reliable than the Sh-37, and replaced it.
	This image shows the installation of UBS and NS-37 and relative ammunition. The UBS and its ammo boxes are clearly visible, while we see only the rounds of the NS-37 gun, apparently installed on a loop chain reserve as on Sh-37. The NS.37 gun is scarcely visible in this image; it is well below the UBS, coaxial to the prop. Image from Aviakollekcia 5/200
	This image of the installation of a NS-37 and an UBS on a Yak-9T, that had the same armament and the same engine, shows a somewhat different arrangement of the rounds reserve when compared to the LaGG3-37..

 

The first batch of LaGG-3s with NS-37 were delivered on December 1942, and were known as LaGG-3-37 (series 34, according to other sources); they looked as the well-known series 35, but were distinguishable for the long protruding barrel. It looks that many of them, but not all, had the water cooler air intake enlarged as on series 35. Some of them were delivered to the Normandie-Niemen Fighter Air Regiment.  Despite having obtained some success, particularly against bombers, the uparmed LaGG-3s had some problems due to the reduced performances, the shifted center of gravity and the strong recoil forces (5500 kg) that led to damages to the fuselage.  The Il-2 3M armed with two NS-37s in underwing gondolas was more suited to the antitank role.  The true successor of the LaGG-3-37 in the air-to-air gunship role was the Yak-9T, that was tested in February 1942 and became operative in the late spring. It was faster and more stable while firing than the LaGG-3-37, so no further batches of this type were produced.

 

K-37 Leningrad 3 one of the first three upgunned planes

Chkalow 04 a plane gifted by the farmers of Gorky region;  21 IAP, Kalininksiy front, May 1943

white 13 a plane seen on a demonstrative movie

Chkalow 14 a plane gifted by the farmers of Gorky region;  21 IAP, Kalininksiy front, May 1943

 

white 27 A standard LaGG-3-37 with NS-37 gun, probably in spring 1943.