Why the big charge difference
I just recently started rolling my own and have been concentrating on working up loads for my 300 weatherby and 7Rum. I finally got around to loading up some more .280 rems. The absolute first batch of reloads I ever assembled was for this gun back in December when I started, and it shot a one ragged hole 5 shot group at 100 yards. With these results I called it quits with the first load I tried, which was 53gr IMR 4831 pushing a Nosler 140 ballistic tip(this was the starting load and I didnt try any others, just loaded up 20 for hunting purposes). I am using winchester large rifle primers and seating to fit the magazine. I was happy with my initial sucess.
Until my chronograph arrived. I was running some 300 yard ladders with my 7rum and decided just for kicks to shoot my old reliable .280 through the chrono. The load date from Nosler said that it should be around 2850, which would have been fine with me. It only shot around 2520, nearly 300fps slower than the data projection.
I decided that I could probably work up a better load and bought some RL19 for this purpose. I loaded up 8 rounds jumping .3 gr starting at 53gr. I shot each of these through the chrono, no pressure signs, but nearly 250 slower than the listed velocities. Loaded up 6 more thinking I would find pressure signs on this batch...nope. Repeated this process shooting all at the same target until I was to 59 grains, and started getting a little nervous even though there are still no pressure signs and velocity is only up to 3000fps. The max load listed 57gr is supposed to be 3152fps. My questions are:
1. Why is my velocity so much lower using a load that is 2 gr over the listed max, but 150 or so fps slower?
2. Should I keep increasing my load until I see pressure signs then back off to the next lower charge that hits in the sweet spot, or just let it go?
3. Or should I just back off to the max listed data and let it rip?
This is being shot out of a twenty year old browing a bolt that probably has 1000 to 1100 rounds through it using factory ammo through the 26 inch barrel (pretty sure its 26). Out of the twenty two rounds(a couple were duplicates because of called pulls) fired working this up; 14 of the varying weight charges threw the bullet into a group that was less than 1 inch at two hundred yards; 7 were in a pretty distinct group about an inch in size but one inch left and 1/2 inch higher; the remaining 3 were just strung between these groups. (This doesnt happen with my magnums usually)
I tried the same procedure with RL22 and IMR 4831 and never got anywhere near the max load before seeing pressure signs (or velocity over 2850). All of this is being done in high humidity and nearly 70 degree. I hate to ask stupid questions. I usually try and search and find answers to my questions, but I couldnt think of any terms that were applicable here, and this has me scratching my head. Any guidance will be appreciated, sorry for the long post.
BTW This particular gun has always been a real grouper for a hunting rifle, always has shot 1/2 moa or better with nearly any ammo. I am pretty sure any load from the starting load up to the final load I tried will group well. I bought it used when I was in college and am not sure what other than a trigger job was done to the gun.
I admit that I know just enough to be dangerous.....but dangerous at ever extending distances.