If you are talking about small arms ammo, like pistol or rifle rounds, they normally do not go off that easily. You'd practically have to hit them with your shovel, hard, before they go off. Old handgrenades or artillery rounds can actually become more volatile with age and I'd be very, very careful around them.
I'f you plan to detect at an old battleground...well...I dunno if I'd have guts to do so....maybe, but I'd dig veeeery slowly. At least if you dig a WW2 battleground you do not have the chance of hitting a gas grenade...
Voriax
ps. incendiary rounds go off at the moment the botton of the bullet is exposed to air or the front 'cone' breaks. So even a small arms round can explode/burn if you maul them just right and happen to find an incendiary.
