It's true. C4 needs an electric charge to be set off. Not going to happen with a primer!
No if it is experiencing pressure it will go off while being ignited.
For example a small piece of C4 can be lit on fire and will burn hot but not explode. However if you smashed it with a rock while it was on fire you would likely cause it to explode.
During the Vietnam era, there were reports of Navy Seals booby trapping AKs with C4 packed ammunition, and a warning was issued that any found AK not be fired with the ammunition that was in it.
It was more than a rumor, there was a formal program called first called 'Project Eldest Son' designed to cause the enemy to mistrust his ammo made in China.
Part of the intent of the project was to have the enemy fear their factory made ammo, so the program was intended to remain a secret otherwise the enemy would fear ammo US personal may have been able to sabotage, but not factory ammo.
So the remaining ammo could not be inspected and the true reason figured out they typically only sabotaged/replaced one round per magazine or one mortar in their box.
However they still went through the trouble of making the powder inside the sabotaged rounds look as close to the normal smokeless used as possible.
The sabotaged mortar rounds were certainly the most devastating, easily killing or maiming those all around the sabotaged round when it went off.
The limit in Vietnam was carrying such sabotaged mortars around was heavy, limiting how many they could place far behind enemy lines.
I have seen video of enemy mortar rounds simply detonating in the Iraq war that caused me to consider a more recent similar program a possibility as well.
I could see something like that being used against the insurgents in Afghanistan as well, make them mistrust their weapons and ammo, or have opinions like thinking 'khyber pass copies" were so inferior they were unsafe to use. Likewise the insurgents that infiltrate the ranks of the Afghan forces could do much the same thing.
I recall the story of one of the victims of a sabotaged rifle round in Vietnam. The high explosive caused the bolt of the rifle to be shot back through receiver and the skull of the shooter aiming the shouldered weapon.
Such sabotage of rounds is certainly an effective strategy to reduce morale and cause distrust of ammo, discomfort shooting weapons, and could certainly be used to kill or injure firearm users if someone so inclined managed to get such a round into a gun owner's ammo supply.
Eventually the enemy learned of the project, and it became less effective, the enemy realized the factory ammo was trustworthy, and it was simply ammo that could have come into contact with any Americans or their allies that was not to be trusted.
I would expect similar such programs in many armed conflicts, especially rebellions and civil wars and other forms of unconventional warfare.
Anyone that cannot provide or insure their own ammo, powder, etc is of the intended strength would be an easy target. Unsafe ammo allowed onto the black market rebels were buying from for example to help them enter circulation.
Or apparent stockpiles of the military left exposed so they could intentionally be stolen by the enemy that thought they got lucky.
Likewise the military itself could have some of its own ammo sabotaged by insiders loyal to the insurgency. Artillery detonating in the tube, various rockets for launchers sabotaged, you could even have missiles exploding on the wings of aircraft when they attempted to launch them, etc
Ordinance exploding in the face of people that go to shoot it would be very bad for morale for either side in a conflict even if the total casualties were low.
