Thread: 303 Query

Originally Posted by John Duxbury

The dimensions of factory ammo are such so they can chamber in all different rifles.

.303 cases stretch more than others so neck sizing instead of full length sizing, goes towards reducing that. That is why they recommend neck sizing.

Neck size the cases until one day you have difficulty closing the bolt, then you have to full length size them, and then keep going once again with neck sizing. Keep you eye on trim length after full length sizing.

.303 fired cases all come out different, because the chambers are often oversized. This means you can only neck size using brass fired in your rifle. (There is a high chance that brass fired in another rifle will not chamber in your .303 and vice versa.)


There is no point in reloading and accepting velocities as low as cheap factory ammo. There is no reason why you cant get the same velocity as the military ammo it was designed for. I load my .303 with 48 grains of 2209 and a 180 Speer, for 2420fps. I will also load it with 42 grains of 2208 and a 174g FMJ for 2500fps. These loads are appropriate for all .303 I have tried them in. Both loads have shown excellent accuracy in several rifles.

Only problem with that idea is when you neck size a case
And need to full length size occasionally
Every time you full length size that ammo is different to the neck size and will shoot Differently
The best way is to anneal every shot and full length size that way your case is exactly the same every time not Different every so often because you had to size your brass

Not sure it will make enough difference to worry about in a shitty old 303 though
But having said that I wouldn’t trust a 303 chamber to be perfectly round ether which is required for neck size to work Reliably
So in an old rifle like that a full length size is the way to go

In a modern rifle a Partial full length size (Shoulder bump) is the way to go
90% of the world’s top shooters have gone away from neck size in favour of shoulder bump
Due to the above Reliability and consistency issues with neck sizeing