Quoted:
It is very important that you set the friction rings correct and don't over lube the mag tube. These lead to cracks in the forearm (very common on Model 11s and A5s).
Since the gun is operated by recoil, losing barrel mass can affect the cycling. I've seen riot versions with 20" barrels. 18" my be too short unless you add a barrel weight. The famous picture of the guns collected from the Dillinger gang contains two sawn off A5s/Model 11s.
Either way, congrats on the purchase. I have an '86 Mag 12 Auto 5 that i love to death. Going to get a belgian light twelve to have next to my japanese gun.
+1
Download an A5 operators manual so you can see the workings under the forearm, and how to properly assemble it for the correct loads you are using.
I've heard that you can go down to around 15" and still function reliably. I have a stock 30" barrel on mine, and since I only gave $125 for it, I didn't want to purchase a new barrel to cut down.
A class 3 dealer I spoke to at a gun show had a 15" SBS and an 18" non NFA M11 that he said both functioned flawlessly. They both definitely looked awesome.
However, just off the top of my head, if you remember the tales of the gun "Gangster guns", they have a M11 on there with ~15" bbl that cycles fine.