ORIGINAL: elmer deer
I was doing some load development in my 7mm mag using RL22 and 160 accubonds, I checked a few books and started at 65grs and went past max at 67grs. So my velocity at 65gr was 2843 5 shot average, 65.5 gr shot up to 2990av. could not figure out why? Any Suggestions? Next 66gr back down to 2886 av. and my group shrank to.250" shots, great!Then went a touch over max to 66.5grs and speed was 2933 and it opened up some, next went up to 67grs and my group for 3 shots went into .153" excellent again but the other 2 shots opened a bit, my error I think. I see that most everyone pushes their speed to 3050 alot of times,is it safe for me to go a little higher like to 68 grs of RL22? Have not seen any pressure signs like sticky bolt.My barrel is also 26 inches long and the gun is a Sauer202, What are your experiences with the first situation of a jump of velocity? And is velocity a safe way to gauge if your getting near max? Because book velocity does not match my numbers.
Thank you
Elmer
First let me quote wise old Bob Hagel to you: "All rifles are individuals. What may be a maximum load in one may be quite mild in another, and vise-versa."
The "maximum" load you find in a book is a maximum load in the rifle in which it was developed, and may be under or over maximum in yours. This is why it is INDISPENSABLE to work up loads in each rifle on an individual basis. This is also why book velocity won't match anyone else's figures, except by accident!
Naturally, you must keep your eyes open for indications of excessive pressures as you work up a load. PersonallyI stop increasing a powder charge when I get to an acceptable accuracy level with no excessive pressure indications, or at 5% below the load level that started making my primer pockets accept new primers too easily-in other words, at the first indication of some primer pocket expansion I stop and drop bvack 5%.
How many rounds did you fire at 65.5 grains? Is that 2990 an average of five shots? If so, I agree that you should redo the velocity test with 65.5 grains. I'd be surprised if it will average 2990 next time! Although when the "crossover point" of a given powder is reached, it is entirely likely that greater charges will give lower velocities (accompanied by HIGHER PRESSURES!!), increasing the charge further after the crossover point will NOT result in additonalincrease in velocity. So I do not think you had arrived there yet, since your velocities did increase after you passed 65.5 grains.
With a 26" barrel, you should be able to break 3000 FPS with a 160-grain bullet, but you might have to go to a slower powder, such as IMR 7828, to do it. I know I had to go to IMR 7828 with a 175-grain bullet, although I had been getting over 3000 FPS with 175-ghrainbullets when using Norma N205 and MRP, and RE 22 is supposedly "the same as MRP"!