[dragoon500ly]

It is called a combat lock. In its simplest form it is a piece of metal that is moved into position to physically block the movement of the latch, eg the
M-113's rear ramp hatch lock. The upper grill doors on a M-48/M-60 are combat locked with a simple one-inch bolt that is screwed in.

On a tank you have to squeeze a locking lever to unlock and then rotate the physical lock to undo the hatch. In addition, most tank hatches do not have an external handle to pull open, the sole exception being the loader's hatch, which is the normal means of entry into a tank.

On older tanks, such as the M48/M60-series, the loader's hatch has a mount for a periscope to be mounted. If the hatch is locked from the inside and the periscope is not mounted, a hammer can be used to pop the 'scope cover open and then someone can reach in operate the latch.

As for the Hollywood staple of someone pulling the hatch open to chuck a grenade in....IRL our hero would be busy trying to stuff everything back in after the self-inflected hernia.

But there is a way to get a grenade into the turret. Tanks, when "buttoned up" have a serious lack of close in vision. Tank commanders are normally trained to ride with their hatch open and their head sticking out so that they can better see what is going on around them. A couple of infantrymen in the right spot can shoot the TC and then throw a grenade into his hatch. But what do you do if the TC is buttoned up?

The best answer, is to use one of the various man-portable rocket launchers, LAAWs, Vipers, Carl Gustav, RPGs etc, to get a shot into a vulnerable spot, prime spots would be the track, the rear of the tank hull (engine shot), or if you are above the tank, a round into one of the hatches. If you lack a antiarmor weapon, a satchel charge would be the next best choice. Wedging one of these in the gap between the turret and the hull will distort or even lift the turret right off of its turret ring, jamming the turret at the very least and maybe, getting onboard ammo to explode (before trying this in IRL, please get the maximum amount of insurance and list me as your beneficiary!). Tankers, needless to say are very watchful for anyone carrying a satchel charge.

Another old stand by is to use a flame weapon such as a Molotov Cocktail or even a flamethrower. This might have worked in WWII, but modern tanks are designed with run-off points to drain the flaming material away from the tank. When I was at Fort Knox, part of the officer's training course included a demonstration where we dropped a couple of Molotov's onto the back deck of a M-60A1....it was an utter waste of time and fuel.

Hope this helps!