Can I safely use .380 ammo in a pistol chambered for 9X18 Ultra?
Yes. I would also recommend the use of brass-cased ammo, exclusively. Steel-cased ammo use I will not recommend.
What might happen if using .380 ammo with a 9x18 U magazine is that occasionally, it will misfeed the first round. For some strange reason, the remaining rounds feed just fine. That problem can be avoided by using a .380 magazine.
Works just fine with the Walther PP Super, Benelli B-82 and Sig P230. The proof's in the pudding with those three models and is as simple as this. Get a good working model. Take the slide off. Insert a cartridge of .380. See how it is chambered. Extract and insert a 9x18 U cartridge. Notice any difference?
Then slide a .380 under the extractor. Try to wiggle it around. Repeat with a 9x18 U. Notice any difference?
Simple as this. You have to have the three models, .380 ammo and at least one 9x18 U cartridge. Then you'll know for certain.
Btw, if the rounds wiggle under the extractor you need to have it tuned. It's not then an issue with the ammo, rather, with the extractor.
Fwiw, a loading of a 123 gr. bullet traveling at 1050 fps. can be achieved with the 9x18 U. For that I recommend using a trimmed .38 Super COMP case. No .380 is built to handle that.