In a jam (perhaps pun intended), .380 ammo CAN be fired in a 9x18. I'm not saying it's a good idea, reliability will suffer and it's not good for the gun, but it's not extremely likely to cause injury. I am NOT saying it's completely safe. It's nothing like shooting .38 in a .357, .380 in a 9mmMak is truly an ammo mismatch and is likely to cause damage to the gun and probably increases the likelihood of a dangerous incident.
What I'm saying is that if TSHTF and you have a Mak and all you can find is .380, then load up. There's really no reason to do this in a non-emergency situation.
Don't even think of going the other way around.
9x18 bullet diameter is 0.363"
9x17 (.380 ACP/9mm Browning/9mmKurz) bullet diameter is 0.355"
Also, some 9x18 ammo is loaded MUCH hotter than any .380 round. Putting a 9x18 round in a .380 is a real recipe for trouble.
If you want to shoot .380 in your Mak as a matter of course, you can buy a .380 barrel for around $50 and a barrel press for around $40. Swapping barrels isn't hard, but it is not nearly as quick as swapping barrels on an auto without a fixed barrel.
There are also a few Maks on the market that are already chambered for .380. There were some .380 Russian Maks that came with adjustable sights.