By 5 round extension, do you mean an extension that ADDS 5 rounds to the 4 round magazine tube?
That would mean 10 shots total with the chamber loaded, and that tube would stick out awfully far.
A standard 18" barrel is usually fitted with a +2 extension which means a total of 7 rounds with the chamber loaded. That would not stick out beyond the muzzle.
If you get a 20" barrel you can use a +3, or total of 8 rounds with the chamber loaded without sticking beyond the barrel.
The long 10 shot extensions are only for 26" barrels. Putting one of these on a 18" or 20" barrel and there is a chance of the shot damaging the extension.
Plus, there's not enough support for that long an extension on a shorter barrel. A not-to-hard bump will either bend something or knock the extension off.
In either case, the gun's original magazine tube is usually the part that gets damaged. This means a trip back to Remington to have a new magazine tube installed.
Best advice: If you have a 18" barrel buy a +2 or 7 shot total extension.
If you get the 20", buy a +3 or 8 shot total extension.
Whatever you get, BUY AND USE A BARREL CLAMP. The clamp prevents bending or knocking the extension off. How strong the extension is doesn't matter. The GUN'S magazine tube is the weak link.