thanks for the help every one . I have been reloading 9mm , 45acp and .270 for a couple years now but .303 just seems like a whole different critter
OP may already know this, but I'll type it anyways for those who may be interested:
If we start with fired brass:
When the rifle fires, the neck of the case expands outwards against the walls of the chamber. Due to it's elasticity, it shrinks back down a bit (about 0.002") after the end of the pressure pulse. So the post-firing outer diameter of the brass neck is about 0.002" smaller than the chamber neck of the rifle it was fired in.
The inner diameter of the brass is then the chamber neck diameter, minus 0.002", minus 2x this thickness of the brass at the neck.
In order to get a good grip on the next bullet, you need to make the inner diameter of the brass about 0.002" smaller than the actual OD of the bullet.
The way the dies do this is that they first squeeze the outer diameter of the case neck in a lot (see below), and then pull an expander "pill" up through the hole. The pill is about the same size as the bullet. The case neck stretches out allow the pill to pass, and then shrinks back down to about 0.002" smaller than the pill. Now, when you insert the bullet, the case neck stretches around it, and it's elasticity grips the bullet into place.
They have to do it in two stages because the neck thickness varies significantly depending on the manufacturer of the brass, and the die manufacturer cannot control that variation.
Here's one of the complications:
A thin case neck means that you need to squeeze the outer diameter in lots to get the inner diameter ready for the pill.
A thick case neck means that you only need to squeeze the outer diameter in a bit to get the inner diameter ready for the pill.
If RCBS makes dies that squeeze in the outer diameter too little for thin-necked cases, then the pill just glides through, the neck ID is too big, the bullet is not retained, and customers complain.
If RCBS makes dies that squeeze in the outer diameter too much for thick-necked cases, then the brass gets worked unnecessarily, the case necks crack early, and customers complain.
Also, the dies wear over time, which reduces the squeeze-in they provide.
The manufacturer tries to balance these effects against each other. The dies you have, when new, were designed to work with a certain range of case neck brass thicknesses.
Now, through a combination of wear making the dies bigger (which reduces squeeze-in) and the neck on these cases being thinner than the expected range (which requires more squeeze-in), you are no longer getting enough squeeze to prepare the inner diameter for the pill.
To solve the problem, I would suggest you first measure the pill size. If it is equal to or smaller than the bullet diameter then the pill is not the problem, and it means that these dies cannot be successfully used with this brass.
I would not expect dialing the dies in further down to help with this, as the neck portion of the die is not (significantly) cone-shaped.
You will need to buy new dies or use thicker-necked brass.
Personally, I use home-made squeeze-only dies and only neck size (because I
can control the neck thickness). It reduces wear and tear on the brass, plus it's cheaper (and more fun) for me to turn down, ream, harden and lap a bit of old S-10 torsion bar on the lathe than it is to order dies.
More hours than dollars, as they say.
Oh, 0.002" is nothing magical. It is a ballpark number for the "elastic" range of a thin brass ring of c.340" diameter. Stretch it by less than this and it will return to it's original shape. Stretch it by more than this and you will make a permanent change.
