Hans von Hammer
Not all motars are spin-armed, some fuzes are armed by "Set-Back", meaning that the prop charge exploding and subsequent rapid acceleration (G-force) is what arms it.. Whacking the round's tail on the firing plate to arm it is quite plausable ... I don't how hard you have to hit it though, but I'd imagine it's a pretty good shock, more than can be generated by dropping it...
Again, the problem is, you are ignoring the 'consequence' of whacking that mortar round on the tail. When you fire a mortar, you drop the round down the tube. The weight of the round dropping onto the firing pin fires the shell in the tail, which propels the round out of the tube. So, if they are replicating that without the tube, as they are in SPR, what they are going to get is the same result - a round being fired, except that instead of it being enclosed (and directed) by the tube, it is being held in a hand. Granted, one result of that is that much of the force that would have been channeled upward by the tube will be dissipated. But there is still going to be a heck of a lot of force being released. As I said, even mortar rounds fired with a charge of zero (no additional propellant bags, just the force of the propellant in the tail) will travel quite a distance - I don't have my firing tables in front of me, so I can't give you exact ranges. But that doesn't matter - what matters is, when you whack that round on the ground, tail first, the primer will ignite, driving the firing pin (technically a 'floating firing pin') into the ignition charge, which will flash out the holes on the shaft of the round (that is how the increments - the propellant charges - are ignited). The ignition charge also expels force back down the shaft tail of the round, 'firing it' (agian - the zero increment situation I mentioned). So, our heros are replacing a 1/2" metal tube with their hands, and 'firing' a mortar round....
Here, I'll give you a way to simulate - go grab a shotgun shell, lets say 20-gauge, for arguments sake. Now hold it in your had and smack the primer against something hard until it goes off. Now, come back and report on how your hand feels.
Hans, you are correct about VT fuses - I was describing Mechanical Time (MT) fuses and in my haste, used the wrong nomenclature...
By the way, on the issue of fuses (and not to go all 0341 on you - that's the Marine Corps MOS designation for mortarman, by the way), but there are four basic types of fuses (five if you count dummy, but that is irrelevant to this discussion):
Point Detonating (PD) - rounds fused this way are called 'superquick' in that they will detonate when the fuze impacts on a hard surface
Proximity (VT) - as Hans said, these emit radio waves out the nose of the projectile and, depending on the setting on the fuse, detonates when the radio waves reach a certain 'intensity'
Mechanical Time (MT) - these fuzes work with a clockwork mechanism that delays their detonation to a specific time after firing - most commonly found on illumination rounds to set the height of the burst and effect the expanse of the illumination
Multioption (MO) - these have three settings: proximity, impact (aka superquick), and delay (round will penetrate to a certain depth, depending on how hard the surface is, before detonating)
If we are taking SPR as 'accurate', I'd guess they were using PD fuses... I've still got issues with the firing thing, though....