what causes FPS variations

[Jeffery8mm]

I loaded up several loads for test this weekend for the 30-30. Powder is WC846, boolit is Lee 170gr FP GC.
I loaded 5 rounds each of differing charges. In one of the groups I had 1920,1944,1899,1938, then a 1725 reading. Is this a chrony mistake. All charges were hand weighed on a pact digital.
Just wondering if other have a good FPS string going only to recieve a largly different reading.
Thanks
Jeff

[felix]

Must pay attention to projectile impact location and felt recoil. If not the same, then the chrono is probably correct. ... felix

[montana_charlie]

Are all of your cases the same age and from the same manufacturer?
Do you ever anneal case necks?

CM

[AZ-Stew]

There was a recent thread on chrono errors. Most possibilities were discussed. See: http://castboolits.gunloads.com/showthread.php?t=62787

Regards,

Stew

[Jeffery8mm]

All are RP cases same age same # of loads. No annealing.
Jeff

[lead Foot]

Some powders become erratic with reduced loads. Try more or less powder to find the sweet spot.
Lead foot;

[Russ in WY]

Disregarding the 1725 reading ,were the rest of the strings similar in ES & SD ? If so I would surmise that the low reading was a chrono mistake/error. Now for what reason I have no clue. I have seen partial sun across the chrono windows give some strange readings at times.. If that load was the top or low powder load it may be an indicator of some sort. Good Luck on your search. My 2¢ Russ.

[fourarmed]

There is no doubt that clear, sunny days can produce erratic readings with sky screens. However, such velocity excursions should not be dismissed routinely. I once tried some loads in 7x57 with charges of H-380 that did not completely fill the case, and got velocity spreads of 250 fps in a 5-shot string. I suspected the chronograph, but when I shot another string, pointing the muzzle upward and tapping the receiver before each shot, the ES dropped to less than 100fps.

[runfiverun]

thats why a lot of guy's use fillers.

[Jeffery8mm]

Bright clear day!! The load was 26gr of WC846 in RP case with above stated boolit. WC846 has given some promising results with this combo
Thanks
Jeff

[largom]

thats why a lot of guy's use fillers.


Fillers have cut some of my extreme spreads in half!

Larry

[Denver]

Here's a couple targets I shot using WC 846 with and without a filler. When I've used ball powders for reduced loads I generally see a tightening of the group and a small velocity increase with a reduction of ES if I use a filler. I use a half to three quarter grain piece of fiber fill material. The boolit I used here is the Ranch Dog 311-165 cast from WW with a couple ounces of lead shot added and water quenched from the mold. Don't know what the hardness is, but I damn near have to put a cheater on the handle of my Star sizer to size them.