This is actually a Anti-Tank Gun - a truly poor and desperate attempt of an anti-tank weapon.
It wasn't that poor considering the opposing armoured vehicles at the time. The weapon was designed in 1939 to partially fill the desperate need of any anti-tank weaponry, since about the only AT weapons available were some odd less or more portable anti-tank cannons. And considering how much more portable it was than a full-fledged AT cannon, and how the shaped charges as anti-tank weapons would be developed by the Germans only later during the war as infantry AT weapons, it wasn't that bad of an effort.
After it stopped being useful against tanks, it was used as a long range heavy sniper rifle, against lightly armoured or non-armoured vehicles and against fortified positions - as well as an anti-aircraft weapon against slow ground attack planes (Sturmoviks).
Winter War
During the Winter War Finland lacked anti-tank weaponry. Only two 20 mm rifles and a few 13.2 mm machine guns made it to the front, where the 13.2 mm machine guns were found to be ineffective and unreliable while the larger 20 mm ones proved successful against Soviet armour. Because of this, Finland finally settled on the 20 mm design and started production. The gun was also widely used in the "Cold Charlie" technique, where the Finns would use a mandequin to pose as an officer sloppily covering himself, Soviet snipers would fire upon it, and the Finns would use the Lahti L-39 to fire at the Soviet sniper.
tches (especially with phosphorus ammunition). It was even able to damage tank turrets and pin them to stop traversal of the cannon.
Users noticed the L-39 was heavy and difficult to move in the battlefield. Even its magazine weighed almost two kilograms more than the Finnish Suomi M-31 SMG. The whole weapon weighed some 50 kilograms and it was usually towed by reindeer or horses. In the field, a two man team was assigned to the gun's use to move and fire it. Some of the rifles were simply abandoned in the heat of battle. They were easy to replace, however. By the end of the war over 1900 of the L39's, manufactured by VKT (Valtion Kivääritehdas, "State Rifle Factory"), had been produced and put in the field.
I have to admit, though... 50 kg rifle... Pretty much like NSV 12.7mm machine gun along with the tripod mount and belt box full of rounds.
Having been part of some wonderful exercises concentrationg on how to transport an NSV from a firing position to another (and another, and another...) I would definitely not want to haul this piece of metal around, especially if it wasn't possible to take apart fast to make carrying the parts easier...
And armor piercing is 
largely obsolete.
No, it isn't obsolete.
It is true in the sense that infantry troops do not use directly piercing ammunition against heavily armoured tanks but instead use different kinds of shaped charges, be it by mine, propelled grenade (recoilless rifles) or anti-tank missile.
Tanks, however, do use armour piercing ammunition against each other, in addition to HEAT (high explosive anti-tank) rounds. These sabot rounds are essentially winged spikes or darts made of depleted uranium, concealed in a shell that fits the tank gun barrel and detaches after exiting the barrel, leaving the heavy, dense spike of death fly freely towards the target at extreme accuracy. The armour piercing effect of the sabot rounds is devastating due to extremely high muzzle velocities and high density and hardness of the sabot material.
And it goes without saying that as body armour becomes more effective, infantry weapons will further specialize on armour penetration. Rifle caliber rounds are already incapable of penetrating modern body armour (although they'll definitely leave a mark).
Aand... to say something about the suits. They might offer better endurance and much better ballistic protection than traditional fibre/ceramic body armour, with better coverage, but even the weakest of AT weapons would be a damn big risk. Getting hit with even an old M72 LAW (or 66 KES 75/88) grenade would cause heavy damage, as that in itself penetrates ~30 cm of panzer steel.
Getting suitable protection even against 12.7 mm sniper rifles would be far-fetched. Hell, even a Dragunov could be a hard one to deal with without making a Michelin man out of kevlar... Which kinda means that to be truly useful for an infantryman, the exoskeleton would need to be:
-small enough to not increase target profile notably
-not hinder normal movement and concealment at all
-offer enough increase in the soldier's physical abilities to be of any use.
Strength doesn't really help much in a modern firefight in itself, as long as you don't get into hand-to-hand fight against the enemy... The main advantage of the suit would be better endurance, better weapon aim (increased stability) and perhaps the most advantageous feature (if possible) would be increased running speed.
