I'm trying to find replacement barrel bolts to hold the scales on a fixed blade knife.
The dimensions, as best I can measure, are:
Barrel Section: 6mm diameter and 8mm long
Screw Thread: 4mm diameter and 6mm long
Heads: 8mm diameter and 3mm thick with 2.5mm hex sockets
I have done a fair amount of web searching and I have looked at several of the large online tool and fastener companies. Almost everything I can find has a much thinner barrel and wider head.
You know what they say about that... if McMaster doesn't have it, it doesn't exist.
Consider a standard part that you can cut to fit your needs.
Also consider Imperial sizes. 4 mm is almost exactly 5/32", 6 mm is almost 1/4" (roughly 1/2 mm smaller). You can much more easily change the length than any other dimension so find one that is longer than you need and cut to fit your needs.