I had the exact same thing happen to me just over a month ago. Great example of an M48A, just had a clean crack almost in the center of the top handguard, from the tip where you slide the barrel band over, then about 3" back. It wasn't split all the way back to the second barrel band.
In the end, all I did was try to get out some of the cosmo (did a poor job by my own standards), then took some carpenter's wood glue and applied it to the crack on the inside of the handguard. I just used a popsickle stick and very gently opened the crack to apply the glue, as I didn't have a Pin Point syringe, which would have been the bees knees! Once the glue was applied, I squeezed the wood together with one hand, then wiped away the little bit of excess glue that squished out with the other. After maybe 7 or 8 minutes of holding it together, I let go and inspected my work. Unless you were inspecting the rifle for flaws, you'd never see the crack!
One tip, when applying the glue, less is more!



























