The No 5 flash hider is just pinned, but some of those pins can be very hard to drive out. Use a short punch and if possible make a concave cut in the end of the punch so you don't batter the pins in case you want to put the hider back on.
Just FWIW, the purpose of that flash hider is not to hide the flash from an enemy, but to keep the flash from blinding the shooter. So if you might ever want to use the rifle at dusk, I suggest leaving the hider on. Without it, the flash is impressive, but if you fire cordite ammo you won't be able to see anything for five minutes.
Jim