Armour forces in the Finnish army of World War Two were an unbelievable pick and mix of types and models. A large part of them were stolen from the Soviets after successful battles. Through desperation they continued in service far beyond the point when other nations had declared the model obsolete. For the most part they met with success, but not always.

One of the tanks the Finns captured a large number of during the Winter War and start of the Continuation War were BT-7's. After a brief period of use in the front line, where the tank was judged to be a failure, about 53 vehicles were in Finnish hands. Of those the 18 best chassis were rebuilt with the best components that could be salvaged. Instead of the BT-7's original turret, a new home built one was fitted, and armed with a 114 Psv.H/18 howitzer. These were World War One era 4.5" guns given to the Finns during the winter war by the British. The parts for the BT-42 had been cobbled together from what ever the Finns could find. The gun sight was from a K/02 field gun, and the mount was from a Soviet 45mm anti-tank gun.
These tanks were first deployed in 1943 at the fighting at the Svir River. Some success was met, when the large calibre round was used against bunkers or other soft targets, however the effect of the HE rounds against armoured targets was nil.

During the latter half of 1943, and the first half of 1944 the Finnish armoured forces started undergoing re-organisation. Supplies of German Stug IIIG's started to arrive. The BT-42's were split off from the main armoured force into the Separate Armour Company, or Erillinen Panssarikomppania, abbreviated to Er.Ps.K. The crews for the 12 BT-42's (some accounts say 11 BT-42's) within this formation were drawn from two branches. Drivers were taken from the armoured division, while the rest of the crews came from the artillery branch.
In the same time period a new shell was developed to help the BT-42's deal with armoured targets. Like the rest of the BT-42 it was cobbled together out of spare parts. The British 4.5" howitzer used a two piece projectile, so all that was needed was a new warhead. The anti-tank shell was a 105mm HEAT round developed by the Germans for their 105mm artillery. Obviously the 105mm round was too small to be fired down the 114mm barrel, so the base of the shell was enlarged to 114mm.

The new HEAT shell

In the summer of 1944 the soviets launched a massive offensive with the aim of knocking the Finns out of the war. The first Finnish defence line was the VKT line, with its Western most point at Vyborg (also spelt "Viipuri"). Finnish forces holding Vyborg were in a bad state. The main part of the defence was the 20th infantry brigade. This had been formed out of the remains of the 22 infantry regiment in 1944, and spent its time on labour duties. So while the individual soldiers were experienced combat veterans the brigade hadn't trained and lacked cohesion. To add to the problem modern weapons in the form of Panzerfausts had been supplied, but no training had been given and no one in the Brigade knew how to operate them. So they weren't issued to the troops. A smattering of Panzershrecks were available to the troops.
The defence also had support of Heavy Artillery Battalion 40, equipped with twelve 152mm M1937 howitzers, and a light artillery battalion with 105mm guns. However both were low on ammunition. For example the Heavy Artillery Battalion only had 400 rounds.
There was a supply dump, however the supply officer refused to release the ammunition. Even when the headquarters gave verbal orders. Without the right paperwork the supply officer still wouldn't hand over the shells. Eventually in desperation a party of armed infantry were dispatched and they held up their own supply dump. Both artillery battalions were resupplied with ammunition. Much to the horror of Battalion 40, the shells in the dump were for M1938 guns, and wouldn't fit their guns.
Although the light battalion did join in the battle the delay had cost several days of what turned out to be a 5 day battle.

Into this mess the Er.Ps.K was deployed. The company commander, Lt Stig Sippel, requested that his company be used a self propelled guns, a role they were entirely suited for. The 20th brigade commander, who by that point hadn't slept for four days refused, and ordered the Guns to the front line to provide anti-tank cover. So over the night of the 19th/20th of June the Er.Ps.K deployed. The tanks were deployed in pairs along the line, and with the help of the local infantry they dug in.
Number three platoon was the first in action, after finishing the digging in about 1130, the crews watched retreating Finnish forces fall back along the road past them. About 1430 the first enemy tanks were seen. One IS-2 and four T-34's were seen coming down the road. The commander of 3rd platoon let them close. Waiting until the last moment to open fire. Due to the excellent camouflage the Soviets didn't spot the BT-42's in ambush. When the Soviets reached a range of 70m, the BT-42's unleashed the devastating new HEAT warheads in a point blank volley.

Only the new shells were anything but devastating. The IS-2 took 18 hits, with no effect. Maybe the shells were unstable due to the modifications and weren't hitting right, or the jet was being dispersed due to wobble of the shell. Whatever the cause the shells had absolutely no effect. There are also accounts mentioning hit chains of 11 and 15 hits with the HEAT shells to no effect, although those might be misreported versions of this engagement.

After this prolonged salvo the Soviets identified where one of the BT-42's was dug in and started shooting at it. The commander threw the tank into reverse to get out of danger, he backed into cover and turned the tank. The chronic reliability problems that had caused the BT series to be withdrawn struck again. The tank threw its track so badly it couldn't be repaired. The crew abandoned the vehicle and joined the infantry defence. It is reported that the Crew managed to obtain a Panzershreck and proceeded to knock out the IS-2 and forced the T-34's to withdraw. About 2130 they bagged another four T-34's as the Soviets mounted a large scale attack.

Meanwhile the rest of the company had been in action. Due to unknown reasons, some say Soviet saboteurs, others say misunderstood orders, one battalion in the front line had started to retreat. The Soviets had forced the front line meaning the BT-42's had to withdraw while fighting.
About 1730, in the city, Lt Sippiel ordered a section of tanks to fall back over a bridge, which one of the BT-42's then demolished. Lt Sippel then ran into a T-34. He struck it twice, again with the new shells, which had no effect. The T-34 fired back once and hit the BT-42 in the upper turret. The round ripped the roof off and killed Lt Sippel. The other BT-42's fell back across a second bridge, and that too was blown up by a BT-42 shooting the stone support away. With no further way to advance the Soviet advance was temporally halted in that area.

Lt Sippel's BT-42, after being knocked out

Er.PS.K had lost eight BT-42's from its starting strength of 12 vehicles in only a few hours of combat. Their impact on the battle had been almost nil. So on the 7th of July, 1944 the company was disbanded as there were no replacements vehicles, and even if there were replacement BT-42's the performance of the tank was so dire there seemed to be little point.