[CHW2021]

I must admit that I did not know Belgian gun laws were so strict, after the events earlier this century I would have thought the gun laws would be modeled after Switzerland. Oh well, learn from history or repeat it.

With regard to your magazine extension tube, would your laws permit you to have a machinist fabricate one to fit your gun? You could have the length you want and out of the material you want (steel or aluminum) with any weight you want to balance the shotgun in your hands.
Just a thought.

Also, could you have the barrel cut in length and threaded for choke tubes? Your behemoth can be a very practical sporting/defense gun with a little careful work.

 

[CHW2021]

Almost surprised you can own guns at all at that rate. Yes, you can swap barrels, if you go that route do try for screw in chokes if available. Why the 18.5 barrel?
Seems that you would have to file paperwork before using a gun for defense.... Just curious about your intended use.

 

[CHW2021]

http://www.gunpartscorp.com/catalog/Products.aspx?catid=11941

I guess I will still ask what you have in mind, 18.5" on a shotgun barrel is not the best bird gun of them all, 22-24 inches would likely be more practical for any field hunting. Screw in chokes help you maintain a pattern at varying ranges depending on constriction of choke and type of shot. How familiar are you with shotguns?
The 18 " barrel would generally be concidered a "riot gun" and not a hunting gun.
If there is a gun club about that you could learn at or practice at you may want to get more familiar with the gun and shotguns in general before going any farther; especially in light of the issues purchasing firearms in your corner of the world.

Find a few reviews and shooting tips here:http://www.americanrifleman.org/

Maybe this will be of help:http://www.americanrifleman.org/ArticlePage.aspx?id=1842&cid=12

 

[CHW2021]

Another suggestion is (provided there is a skeet club near you) to have your shotgun "fitted" to your build. Fitting a gun will not only make recoil more manageable, it will make the gun balance more naturally in your hands. In most bird hunting the 26" barrel is a standard, it serves for close/moderate range shooting and is acceptable in skeet as it is easy to swing. A gunsmith or armorer who is familiar with shotguns would likely be the best person to talk to, good luck.