The other day I was out testing my favoraite load and come up with 120 FPS extreeme spread. The funny thing is this load will shoot groups in the mid .300's (5 shot) daily. Most of the velocities are very concistent, but 3 out of 10 got me thinking. What will this do at 600 or 800 or even 1k?? Is there something in barrel harmonics that will dictate good groups to 1k despite this 120 FPS differance? There is just a little air space in the loaded case. Is it possible for that little bit to do this? I am not sure what else could make 120 FPS of differance. Cases are matching and weight within 2 grains, bullets and powder are all off the same lot, primers too. All charges are the same and OACL is right on, necks are turned and tension is the same round to round. Should I be looking for a more dense load? Or am I beating a dead horse?
I recently had a similar problem. I traced it to neck tension. Just because the cases were neck turned & sized properly didn't mean that the neck tension was the same. When I acutally measured the neck diameter after sizing, it varied greatly. An anhealling job fixed it. Now my extreme spread has dropped to single digits with no other changes.
annealing is simply heating the case necks up to make brass more pliable[less brittle,softer,more workable].after repeated firings brass gets hard and loses some its ability to spring back.by annealing the cases u level the playing field by making the cases very close to being the same again.its very easily done.are u shooting these at short range?ES doesnt make a real difference at short ranges at long range it can be huge,depending on how far u are shooting,my-2-dave
308175, new prepped cases should size the same. If you have a case that has been fired many times, it may have different 'hardness' from work hardening than other pieces of you have. This can cause the actual neck diameter of the sized brass to be different from each other, causing neck tension to vary. The only way to know is to measure the neck diameter after sizing. I had brass that the sized neck diameters varied by nearly .002". After annealing, they varied by about .0003". You should be able to find info on the process by doing a search.
If your rifle will consistantly hold .3's at 100yds, at 600 yards it would be shooting no better than about 11.8" groups from the 120 fps spread alone, no wind.
If it's consistantly shooting better than this at 600, I'd check your chronograph out carefully to see what's wrong with "it", because it won't be holding .3's at 600 yards with some leaving 120 fps apart.