Thread: 303 reloading

Originally Posted by zimmer

One point. The primers don't alway fully reseat in their pockets. The flatten and spread and develop a flat topped "top hat:. This is not unique to rimless and occurs with rimless as well. Usually due to pushing the shoulder back too far when sizing. Well covered in past threads.
Often mistaken for over pressure.

The last of my #4s, a 1955 Fulton regulated, gave me a big fright when I first fired it. Using S&B ammo virtually every case 3/4 seperated. Called it quits and checked using a friend's head space gauge. Was massively over head spaced. Changed to a more appropriate bolt head (reasonably available back then) and my problems ceased. The previous owner was a target shooter. Maybe he had no problem with club CAC??

Greetings @zimmer,
This is why I don't consider primer appearance useful for guessing pressure. The chronograph is much more useful.
A drizzly day in HB today with fine patches just long enough to get your hopes up so I did a bit of measuring with a comparator. I was looking for the extent of stretching common with .303 loads. The .303 does not have much of a shoulder so accuracy of measurements may not be great but here is what I found. New S&B and PPU cases measured around 46.4mm using the .375 chamber with the comparator. New CAC cases were around 46.2mm and DI 42 cases only 46.0mm. Fired cases measured up to 47.0 mm with one outlier going 47.5mm. With the rimmed case the amount of stretching is limited to the free headspace on the rim with the shoulder being fire formed the remainder. With the rifle I have measured this free headspace amounts to about 0.3mm (0.012") of stretching each time you pull the trigger if you full length resize with the die down hard on the shell holder with high pressure loads. Short case life is guaranteed. Granted some rifles will have less free headspace.
Most of my cases have been given to me as once fired brass so the original load is unknown. Some have needed the shoulder bumped back to chamber freely but none are FL resized. For my rifles with full power loads cases are FL sized with the Competition shell holders.
Regards Grandpamac.