I've just bought a Pelletgage from Intershoot and was surprised at my results. The Pelletgage is a plate with 10 holes in it varying from 4.46mm to 4.55mm in .01mm increments. You drop your pellets in the holes until you find a hole where it will not go through. If it doesn't go through the 4.5mm hole but goes through the 4.51 then it is a 4.51mm diameter pellet. Obviously there is some leeway, if you really wanted to you could force the pellet through the 4.5mm hole but the method is relatively easy and not too time consuming.
I have just measured about 100 JSB express 4.52 pellets and the results are interesting. I found no pellets at 4.52mm, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, 10@ 4.47.
I did some testing from a bench at 50 yards indoors with my Weihrauch 100 but cannot say conclusively at this early stage whether there is a quantum leap in accuracy by sorting head sizes, I guess if I shot some 4.51's next to some 4.47's I may see a large group appear. Shooting unsorted straight from the tin I tend to get say 3 pellets in a small cloverleaf and maybe one or two outside this small group annoyingly opening it up to about 20mm c-c. The good groups are about 10mm c-c.
I really need to have a good test session and sort through the latest batch of exacts which seem to shoot well in my gun so need to make time for that.