Symbol on buttend of M-3 knife

The bursting bomb is common in heraldry.It just represents a capability.

Tom and Ghost are both right. It began as a symbol of "fusiliers" and moved on to be a fairly universal symbol of "Ordnance." It is often used in heraldry to indicate service in the Royal Fusiliers.
You will also find this mark on MANY military arms, both edged and projectile weaplons.

I cut it, and I cut it, and it's STILL too short!