Thread: Pressure Variations and Load Data

Greetings All,
Most of you have noted the reductions in load data for some cartridges over the last few decades. This was attributed to the more accurate electronic PSI pressure measuring system revealing variations in pressure not showing with the older crusher CUP system. I often wondered if these variations were in individual or different rifles. I was leafing through a back issue of Handloader and reading a Pet Loads article by Brian Pearce on the 7mm Rem Mag. Brian quoted a conversation with Alan Jones who edited the Speer Loading Manuals who reported wide variations, both higher and lower, in velocity in sporting rifles in 7mm Rem Mag compared with a SAAMI pressure barrel. These were in the order of several hundred fps. Brian reported wide velocity in sporting rifles as well. so there was my answer. It was the rifles.
So what does the average handloader need to know about this. Some will have noted that the max pressure of 61,000 PSI is lower than other magnum cartridges and published maximum loads are below that so an effort has been made to keep most of us out of trouble but there is still the chance that there are some rifles that may develop excessive pressure. My suggestion is to work up towards the max charges given chronographing the loads as you go. Do not go past either the max velocity or the max charge of powder. This should keep you safe.
As I type this I can hear a faint chorus of those who believe they can divine pressure by any number of methods but after close to fifty years of handloading and reading about it I have formed the opinion that the only useful method of pressure estimation requires a chronograph and good pressure tested data.
Regards Grandpamac.