While they technically can, according to the rules, it is something that almost never happens in practice.
A 50 tonner with an XL engine has two internals, so if you get hit internal for one point, and take an ammo hit, with CASE you take 1 more internal hit which destroys you.
Above 50 tons, you have 3 internals with an XL up to 75 tons, with 80 tons and up having 4 internals.
It is mathematically possible for them to survive if they take ONE internal hit and then take an ammo hit. Mathematically. However, in all the games I have played in BattleForce for the last year, I have never seen it happen. Usually, we salvo fire on one target to KILL IT NOW and the crits often don't even count anymore because the object of our affection is DEAD RIGHT NOW. ;) And I have yet to see an assault or heavy 'Mech take that ONE and ONLY hit to the inside and have it be an ammo explosion and have the 'Mech survive.