The standard Boys AT rifle recoiled hard enough to discourage the average infantryman from using it on a regular basis ..... that and it weighing more than twice as much as a standard infantry rifle, so would only be used by the hardest-core sniper specialists.
The rare paratrooper version had a shorter barrel and brutal recoil.
A .55 caliber sniper rifle would have been most valuable in the Western Desert or Italian mountains .... most valuable when killing forward artillery spotters on the next mountain ridge.
Modern .50 calibre (firing Browning Heavy Machine gun ammunition) were popular between 2001 and 2010.
.50 caliber sniper rifles were most valuable at the long ranges across Afghan mountain valleys. .50 cals also proved effective when fired from the top of Iraqi 20 story buildings. In both cases .50 cal. sniper rifles helped push the enemy back beyond the effective range of most infantry weapons, giving western troops a breather.
The most recent generation of sniper rifles are smaller and lighter and fire very high velocity .338 Lapua ammunition.