If the handguard is not completely cracked, I would not recommend trying to open crack any further and by all means, leave the coins in. Use epoxy reducer to thin the epoxy, use a syringe and inject from the back side with the handguard upside-down – gravity will do the work. Clean any bleed-through with vinegar. If the crack is not visible from the back side, time to get out the Dremel with a very small ball rotary cutter and cut a minute back-side trough. Fill the trough with additional applications of unthinned epoxy. Crack should be unnoticeable from the outside and the handguard will be quite strong.