It's one of the most fascinating topics on the boards.
Shooters are spending thousands of pounds on top end target rifles and scopes, spending hours practising, and yet what determines whether we hit that little metal paddle is that tiny pellet that we fire out of the barrel.
There are loads of threads about this and many many more recent than 2013.
The most popular theory is that the number relating to head size is totally pointless. Well respected shooters and posters state that they have measured tins of JSB and there are a variety of head sizes in that tin. They also state that they have measured tins that claim to be 4.52 and the average ( majority ) size has been smaller than tins that claim to have a smaller head size ( say 4.51 ).
From my own experience ... I've measured tins of various head sizes and at times have seemed to see a trend where the majority of pellets in one stated size ( say 4.52 ) are larger than tins that claim to be smaller ( say 4.51 ). Then the trend goes to pieces as I find quite the opposite. I've definitely seen several head sizes in each tin. I've never found a tin that has the head sizes anywhere near what it claims on the tin. They are always smaller ( say 4.52 on tin that are actually measured at 4.46 to 4.48 ).
People that weigh them also find a range of weights in each tin.
We have heard stories that the dies are made to certain head sizes and then stories that claim that is rubbish. We believe there are about 50 different dies and recently we learned that there are different 'probes' or 'males' that are used to form the inside profile.
What is generally accepted is that no pellet maker can possibly make pellets that are within a 0.01mm spec.
You'll also hear some people say that some pellets will go well in some barrels but not in others and visa versa. I personally think it more likely that there are good batches that will go well in most barrels and poor batches that will go poorly in most barrels. I've found batches that have say 2 out of 5 pellets that are poor ( bad tails or extra little blobs of lead etc ). These give poor groups where those 2 out of 5 give flyers. They've done that in all the rifles I own. I can't see that batch going well in any rifle.
There's another theory, and I sort of buy into it, that the most popular size ( labelled ) is 4.52. So the dies that are producing 4.52 are getting hammered more than the other sizes. So they get worn quicker. So the 4.52 labelled pellets are more likely to give you poor batches. Maybe that's changed now, if they have new dies, but I certainly found that switching to 4.51 labelled pellets seemed to give a better consistency of pellet.
The general advice is to find a batch that goes well in your rifle and then buy every tin of that batch you can get your hands on. People spend hundreds at a time buying up one batch.
It's frustrating when you are paying best part of 10 quid a tin and then find they are no good in your rifle.
Recently some chap on another board posted about the variations in shapes and profiles of the pellets. He was hit with a reply that suggested he would be getting a law suit.
A number of posts in very recent times seem to be saying that the quality of the latest batches is very good.
There doesn't seem to be an alternative pellet that can coax people away from the JSB brand. When you get a good batch of JSB/AA that suit your rifle they are very very good.
So ... you will keep reading the threads and you will do some testing of your own and you will come to some conclusions ... or maybe you won't ... but good luck with it.