What was the most powerful military round put into service?

[GONRA]

GONRA's pretty sure the Swedish 8x63 patrone M/32 cartridge was developed for a Swede light machine gun.
After that, Swedes developed the Gevar M/40 rifle - machine gunners could use the same cartridge in their rifles.

[M-Tecs]

Doesn't qualify but I like mini-guns

[JHeath]

The OP specified "shoulder fired rifle." I think he meant the most powerful ordinary rifle.

Anti-tank rifles are "rifled" but so were Parrot Rifles. They are fired from bipods. They mount to the shoulder, but so did twin 20mm antiaircraft guns on ships.

The M/36 cartidge was originally intended for machine guns. But was later chambered in regular 98s as a more or less ordinary rifle.

[Multigunner]

The 8X63 was sometimes called the 8mm Browning. It was intended to allow use of a very heavy boat tail 8mm bullet over a heavy charge to greatly increase the long range performance of the browning Machineguns in use by Sweden.
The Swedes chose large strong men has MG crews, because their tactics required hauling the weapons uphill and changing positions quickly.
As Gonra stated the rifles chambered for this cartridge were intended for MG crew men. the tactics of the day required that machinegun crews carry a rifle in the same caliber as their MG. In those countries that bought up many WW1 Vickers and Maxim guns in .303 after that war they tried when possible to arm the gun crews with .303 rifles. One country even commissioned a Mauser carbine modified to feed the rimmed .303 cartridge. only one prototype is know to exist. It used a Rigby style floor plate, deeper at the rear, developed for sporting rifles.