How many claymores can I have out at a time

As you already guessed, underslung attachments replace your primary kit, just like the normal grenade launcher would, which would be, as in the example of the assault class, the medkit.

You switch to your underslung attachment by pressing what you normally press for the medkit, you will then switch to your grenade launcher. You can verify that by observing the slightly raised barrel stance of your rifle.

If you haven't chosen the underslung rail as attachment option, then you will access to the grenade launcher as a separate gun, which will cause it to take longer to switch to and back.

For more information, take a look at this Wiki page.

Some buildings can be collapsed completely. Upon collapse they kill not only everyone inside, but also falling debris kills anyone standing to close on outside. This are typically small, house-like, one or two level buildings.

For some larger structure whole façades can be destroyed, core structure remains intact, but whole front will collapse. These are typically high-rise buildings in city levels.

Some buildings have destructible walls, however structure remains untouched. This are typically either ones with metal structure or the unfinished construction type of buildings (like for example ones at construction site in Karkand or Gulf of Oman).

As for weapons:

• small caliber arms don't damage buildings
• large caliber machine guns (like eg. .50) chip away walls bit by bit
• canons, rockets, grenade launchers and C4 do significant amount of damage
• hand grenades and claymores do no damage