A US handgrenade has a 4-6 second fuse, and if you count that out in your head, it's really quite a long time. A grenade could easily roll down 4.5m of barrel in that time. I would expect some nasty scrapes in the rifling, but it would probably still fire unless the blast damaged the firing pin. Really not a reliable method, but if I was in the shoes of a rifleman at the time, I would have damn well tried.
if I were in the shoes of a rifleman on those times, I'd give a damn of that 88, smash its optic with my rifle butt. Then I'd ask a help from a fellow rifleman, lift up a 88 shell and load the gun and then drop a grenade in to watch what might happen. It might work best with a HE shell kek kek, can't be that hard to open the lock, I mean your intention is to break it anyway so it doesn't matter if you fail couple of times to open the lock
