Actually, #5 is the largest "offered" in the catalog (damn few of which were produced), which isn't to say someone insisting on a #6 would not have had his fantasy fulfilled.  (Don't know that such a monster has ever been reported.) No reason, either, to think that the half-round option couldn't be applied on any wt. brl. if the customer paid for it.

Val I believe there was a barrel recorded from the factory being even larger. I just need to figure out where I read it and I do have both of the Winchester Books.

I had a High Wall in .22 Short with a 30" #4 weight full octagon barrel. That was a tank, but it sure did shoot well!

I have a 1885 with a #5 full Octagon barrel 32" Long. The Receiver looks to have had the inside lip modified so the Octagon corners would clear. It is a Blued Rifle and I have not removed the Forend to examine it closely. It is a 38-55 with as far as I can see a bright Shiny perfect bore. I have not shot it but have plans to when I cannot heft a Rifle this heavy and wish to shoot from the Bench. Regards, FITZ. OLD TUCK.

redsetter, well let me tell you of an experience curve. My first really nice Schuetzen is a Hi Wall in 38-55 with a #4 full Octagon barrel. In my excitement to look it all over I wanted the Forend off. Deluxe Checkered. Removed the Screw and 
tried to pull it off. It was stuck, grasped it tightly and pulled. It came off and the Forend collapsed in my fingers. Bunch of Fancy Checkered splinters. Been really conservative ever since. I still have over 40 years later pieces of the Forend. Used it a few years ago to have another one I bought repaired. 
It had a piece split out at the tip. Gave both of them to a guy that does really nice wood work. He sliced out a piece from my old one and fitted it into the new one so well you cannot see the repair on the outside. You can see it on the inside. SOoooo! when I need to see what has been done for the #5 to clear I MIGHT? try to remove the Forend  I was young and working as a Toolmaker Machinist. Hands were strong. Have learned to handle Fancy wood very carefully. Regards, FITZ.
OLD TUCK.

One thing I have come to understand is that Wood not on an Action or Barrel will shrink over time. I have had  Octagon Forend's that do not want to go back onto the Barrel they came off of. Same thing with Butt Stocks. so now I try to keep them on where they came from if I can. Not necessarily bolted up tight but in place about 90%. FITZ. OLD TUCK.

Yes, marlinguy. That is why the Sharps Rifle Co. preferred to use English Walnut for their Long Range Rifles. The English knew the History of the Wood they sold. From when and where
it was cut, and it's Drying time. Sharps felt it was the most reliable Wood available at the time. Not sure about other makers at the time but Sharps specifically called it out in their literature. I have 2 1/2 Sharps Long Range Rifles. The original wood is nearly perfect with respect to splits or cracks. the 1/2 rifle is a rifle I bought at Auction because it was all Long Range from the Face of the receiver to the Butt Plate. But had been 
Re Barreled to a .219 Improved Zipper. I had an Original Sharps barrel in .45 X 2 4/10 with a bright Shiny Bore. So mounted it. Have an Original Long Range Fore End with that wedge mount have yet to make up the stuff I need to have to complete the restoration. Will never fool anyone as the Caliber
is stamped on the Barrel and Long Range Rifles were not. And the English were also wise to the Benefits of Straight Grain Wood. FITZ. OLD TUCK.