No4mk1 clamp on fron sight

[Chainsaw311]

What length is the barrel from the font face of the receiver to the face of the muzzle?

21 1/4".64 diameter

[Surpmil]

21 1/4".64 diameter

If you want the original sight graduations to match precisely at all ranges you'll need to cut it down to No.5 Rifle ("Jungle Carbine") length and fit one of the 800 yard backsights made for those.

Reproduction No.5 flash hiders / sight bases are still available I believe.

Otherwise, fit whatever you like as a foresight then mark the sight graduations appropriately after determining those on the range. If you grind off the original markings you will need to carefully adjust the back sight slide afterwards to ensure that is a close fit, unless you use a Mk.3 back sight.