What if any hazard does unexplored API, I, apit,  or even tracer rounds  have after they are fired into the backstop?  Could they go off days, weeks, or years latter if  someone digs them up or steps on them, picks them up or something strange happens,.

They are even less dangerous after being fired than they were sitting in the crate or loaded into a magazine, because there's no powder around to create high pressures if it somehow got contained.

"incendiary" rounds involve no explosive compounds - they're kind of like the striker on a match head, just something that will flare up if pounded into a hard surface with enough force. No they can't "go off" like a hang-fire or a piece of unexploded ordnance. If you want a fired incendiary round to spark up you'll need to get it moving a few thousand feet per second and smack it into a hard surface.

Tracer compound is like a small road flare and it's stuffed into the hollow base of a tracer projectile. Even a .50 tracer only has a small amount of the compound present, it'd need a fire to set it to burning, and then it's just a little baby flare burning itself out in a fire. I guess if you were trying to melt down recovered, unlit tracer bullets to reclaim the lead for casting, you'd need to exercise a little extra bit of caution as you loaded them into your crucible for melting. They wouldn't blow up, but might conceivably flare a little. Certainly no worse danger than accidentally putting wet things into a cauldron of hot melted lead, though.

the reason I was asking is, I have several hundred APT,API-T and i. bullets  that i shoot at the local range but is only 100 M and a dirt back stop. these bullets do not go off when hit the dirt. I wanted make sure that there wouldn't be any danger to people latter on if they picked up one of the bullets or if they dig out the back stop for repairs. I have also shot a few hundred on my nephews farm where the bullets would go trough large trees and keep going. i wanted make sure there wouldn't be danger if someone would cut up a tree that the bullet may be stuck in beside damage to the saw.

I also use tracer and spotter tracer they are neat it digs a hole and you can watch the tracer compound burning in the hole for a few seconds.