I recently purchased a chronograph to try and better my reloading tecnique as well as watch for pressure signs etc. At first I put the chrony about 6 feet from the muzzle and that gave me a wide range of FPS so I did some research and found that it should be 12' or better from the muzzle. So now I have the chrony as far as my data cord will allow, which is about 12'-14' from the muzzle.
My problem is that I am getting variances from 500 to 800 FPS. I try to load all the ammo the exact same way but I don't understand why I'm getting such a variance. Any ideas?
Here's some more info. Single stage press's. All use the same dies, primers, sizer/decapper. ll are processes on an RCBS trim mate. I chamfer and debur the necks. I apply no crimp as I'm shooting from a single shot bolt Cooper model 21 in .223 with a 1:12 twist and a 24 inch barrel. I hand weigh on a high precision scale with no drafts or temperature issues and I'm positive the charges are as identical as they can get.
Do you suppose neck tension could be the issue? If so, how do I improve it? What is the normal variance range in FPS for handloaded ammo?
I shot my pistol (.40 s&W Sig) through the chrony and had variations of only about 15 to 20 fps so I don't think it's the chrony.
I'm sure i've forgotten to provide some important info, so ask anything you need. What do you guys think might be the problem?