I have a forend for a highwall that is roughed out for a round barrel and I want to put it on my lowwall which has an octagon barrel. My intent is to start by squaring off the bottom then cutting the lower corners with a 45 deg router bit so that the barrel will bottom out properly. the width of the barrel is almost exactly the same as the width of the current barrel channel so I wont have to make it wider, just square if off. 

If there are any things that I need to "do" or "not do" which might affect accuracy I would appreciate a heads up before its too late. Its a nice looking forend and way better than the one I have, so I am looking forward to getting it mounted but I dont want to make any mistakes that could affect the way it shoots. 

Thanks!
Joe S

Thanks JKR!
I managed to cut the mortise for the lever spring with my chisels which I thought were really sharp but it was hard going!
A friend has offered the use of his milling machine and I think it would be foolish to not take him up on it so we will give it a try 
Can touch it up as necessary with the chisels if needed
Due to the fact that it is tapered the trick is to cut the bottom flat straight down the middle 
Will try to shim the forend in the mill vise to make a straight cut
Joe S

An Octagon calculator is helpful when doing this in a mill.  I like this one.
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Ues the smallest barrel measurement for your cutting calculations.  
Find center and make your first cut for bottom flat, with an end mill, to depth.
Then open up the lower flat to final width working both ways equally.
I then raise up and cut the side flats to just under barrel width.
Then use the 45 to finish the angle.
I hand fit from here.
Bob

Joe_S wrote on Feb 2^nd, 2026 at 7:28pm:


Yes, flat width.  M = barrel width, a=flat width. You also need the flat width for calculating the depth of cut for the side flat.
Bob

When I'm doing a forearm for an octagon barrel, I've got a couple of barrels that I use, I tape a sheet of sandpaper on the barrel and run the forearm up and down on it and it gives me a nice octagon channel till I get close then use scrappers for final fitting. Just another way to crack a nut as I don't have a mill or router to use.

I too have an old #3 weight octagon barrel that I use. I got it off of eBay years ago from John King. It is a 45-70 24” long with a beautiful blue finish and the worse sewer pipe bore I have seen. I think I paid $25 for it just because I could 

Jack