When Mirbat in Oman was attacked by Adoo guerillas in 1972, the enemy were at one point so close to the SAS compound that one of the SAS defenders held the red hot barrel against chest and the base of the mortar between his legs. He fired several rounds at the attackers, so it must have worked.
To fire a small mortar (
http://www.army.mod.uk/equipment/pw/pw_lm.htm), you put the base on the floor and aim it by eye. You do not need to hold it tightly but you do need access to the landyard in order to fire it.
Nothing is gained by firing it while gripping it to your chest.
A medium mortar (81mm is current British Issue -
http://www.army.mod.uk/equipment/pw/pw_81m.htm) has a seperate baseplate and bipod. in which the barrel is held to aid accuracy of subsequent shots - the quicker a mortar can be brought back to the correct aiming point, the quicker the rate of fire and more efficient. The Bipod for a medium mortar doesn't allow for a very high firing angle because the rounds could land back on the mortar crew.
The baseplate is approximately 60cm in diameter and is designed to spread the recoil of firing out from the base of the barrel. The base plate on a medium mortar is not considered "bedded in" (ie won't move more) until a couple of rounds have been fired. Rounds are fired by dropping the round down the barrel, where the round meets a fixed firing pin and is fired immediately.
Hypothesis - The Mirbat mortar crewman removed the medium mortar from its bipod and aimed it by hand in order to beat off the attack, relying on the protection of their pit from fragments.
But hey, I wasn't there. Somebody please post a picture or scan of a manual showing how a 2" mortar should be used (nearly) horizontally then this part of the thread can be put to bed.
The round from a 51mm Mortar is a number of times bigger than a 30mm Grenade or even Rifle Proppelled Grenade, which are fired from equipment designed to be fired from the shoulder. I'll say it again - please take my word for it that to fire a mortar from the shoulder or any other part of your body will cause injury - eg mashed bones rather than bruises.