Seating depth and pressure

[Stonewall2]

crittergitter
Check the headspace of fired and unfired brass with a shoulder gauge or Precision mike.
If the case head has enough room "to get a run at the bolt face" it will leave an ejector mark on the brass -rather like the way the cases are made. I have seen this firing new Weatherby brass in a factory .30-378 chamber that had + .006 " clearance on the belt before firing.

Clean your chamber with rubbing achool on a mop before firing. Wipe the cases down also and put greese on the back of your lugs.
Now tell us what happened.
Glenn

[crittergitter]

Wow, lots of suggestions to reply to! I seriously doubt that the cases or chamber had any friction reducing contaminates on them. I've been paranoid about that after seeing smeared case heads and disintegrated primers result from someone moly coating their chamber (not me). Nevertheless, that is the most likely variable that I would have overlooked. There was a thorough cleaning before and after each group that was followed by an oil patch and a dry patch. I generally swab the chamber with alcohol before hitting the range, but I may not have fully stripped the chamber of oil this time. I'll give extra attention to that step next time out.

Cases are new, and I am bumping the shoulder with the die because factory inconsistency within the lot is enough to prevent a case from chambering without excessive force every now and then. I sized the second set of cases on a different day, so I suspect that it is possible that I might have had the die set too deep with the first round, although I don't think so. I cannot measure any difference in the headspace on a newly resized case and a fireformed case, and I have not adjusted the depth of the die. I did remove it from the press and reinstall it.

I'll post again after my next outing. If I have a positive experience, I'll assume that there was some cleaning error or other fluke variable that I overlooked. Thanks for the advise.

[crittergitter]

Update on load development for the 338/ 300 RUM. I have worked up to 105 grains of US 869 and have barely noticeable (in the old days I'd still be working up) pressure signs. The 300 SMK is cruising at 2957 fps and fired a 3 shot, .6" group at 100 yards on a squeaky clean bbl. The second 2 shots punched the same hole unmeasurably close. ES is + or - 20 fps. I think I'll stick with this load for a while.