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Why the big charge difference

 
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  #1  
Old 01-14-2007, 02:15 PM
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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.
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Old 01-14-2007, 06:52 PM
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Re: Why the big charge difference

Often the big manufacturers use minimum dimension high quality test barrels for their data developement and then put that info into a reloading manual. They may get those velocities but often we won't in the real world. I'd be really surprised if your 280 is a 26" barrel...I'm thinking 24" and maybe even 22".
I ran the 59 grain load (re19)past my quick load program and its guessing you would be doing about 3033fps at 62000psi. That would definately be a max load. And, your chrono is saying you are at that level. You could run there but might want to back off a tad to about 2950---especially if there is more accuracy there. It's also possible that your throat has some wear from that many shots that could slow down the bullet slightly.
I had a friend years ago that was telling me how his 7 mag was doing such and such a speed cause he read it in a manual. I took the chrono over to his house and he was terribly unhappy to see 150 fps slower. He went back into the house and came back out with some new shells 15 minutes later. His next round was within 50 fps of the manual but.....the funny thing was his bolt now wouldn't open!
It's good to get a couple load manuals and cross reference loads and projected veloicty. I find the hornady manual to be a bit more realistic than most.
Don't worry about trying to get book velocity.... if you get anywhere near with great accuracy you've done your job.
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Old 01-14-2007, 09:00 PM
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Re: Why the big charge difference

Thanks for the info Kraky, is there any reason that you can think of why I can get above the maximum with RL19 while can not get anywhere close to the max with IMR 4831 or RL22. Is the data just particularly light with this powder, and not the others? I backed it down to 58.4 around 2900 fps and it is really grouping well. Now if I could just get my magnums to shoot this well all would be good. Thanks again.
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Old 01-14-2007, 09:18 PM
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Re: Why the big charge difference

You might try running some known good factory loads across the chrony just to see what you get. You may be surprized how slow they are. I once checked some factory 25-06 for a friend--his 30-06 factory loads were faster!!

Also, when I question my chrono, I use some 22LR match ammo to check the chrono. I recorded the 22LR velocity right after I bought the chrono. I have kept several boxes for this purpose. Also, my brother has a chrono like mine--we use them to cross check.

Jim
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Old 01-14-2007, 10:15 PM
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Re: Why the big charge difference

Great idea keeping some known factory loads as a gauge...I will do that.
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Old 01-15-2007, 08:15 AM
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Re: Why the big charge difference

I wish I could tell you why Re19 is doing better. I can say from experience that RE19 seems to be getting to be a slower burning powder...almost on the heals of RE22. And RE22 has had alot of lot to lot inconsistancy.
What pressure signs are you seeing that make you think you've hit the wall on some of the RE22, I4831 loads??
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  #7  
Old 01-15-2007, 08:46 AM
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Re: Why the big charge difference

Only flattened primers, not really any cratering, a slight extractor ring on the brass. I am pretty conservative and usually back off at the first pressure signs. I never got heavy bolt lift. I know that flattened primers are not the best indicators, got a little nervous abut the slight extractor marks.
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