@ 10:32 pm (GMT)
Nathan FosterHi Drew, yes thats enough to drive anyone nuts.
OK, lets get to it:
If the lugs have not been lapped, I can pretty much guarantee only one locking lug is touching, direction of thrust off center to the bore. Solution, lap lugs.
Check the bedding is not pinching the action in place, especially at the recoil lug. Many smiths have a poor understanding of bedding- critical points of contact versus critical points of relief, there are even entire aftermarket companies who do not understand critical relief. The aftermarket tight fitting recoil lugs for Tikka T3 rifles are an example. I have put guides to inspecting your bedding job / rifle in the long range hunting book.
King screw tension as Jim said- 55 inch lb on an ali chassis would be ideal (but not critical).
Bore, please tell me about the rate of fouling. Please study the barrel break in article.
Scope base should be glued and screwed as per the long range hunting book. Screws need to be thread locked in place
Please address all of the above before looking at hand loads.
I wouldn't mind if you would try experimenting at a COAL of 3.200. You'll note the cannelure of the bullet will be just inside the case mouth. The trouble with the current COAL is that not much bullet parallel is in the case, only .255". I want you to try and seat just past the cannelure, giving the bullet body about .315" bearing surface in the case mouth.
The reason why some cases are tight necked and others are loose will be due to some cases being soft and others work hardened. Annealing is optimal but I think before you do this, yes you need to look at neck tension. If the die has an expander button, it may be opening the case necks up too wide. The button most likely needs sanding down (mounted in a drill). If you want to experiment without sanding, size and decap all of your cases, then remove the expander assembly and size them again without the button at all. This interim test may prove helpful. Neck tension will effect velocity and ES a big deal. The tighter the better in my book.
H4350 is a temperature stable powder. In welter weight barrels, after a high round count on hot days, velocities can increase by 75fps- but this has as much to do with constriction of the bore. If you were starting out, I would recommend Varget for 150gr bullets but it sounds like you are up in the zone regarding velocities.
Chronographs will give different readings in different weathers and also are susceptible to orientation such as north south versus east west. Best days for readings are overcast days. make sure the chrony is set at its recommended distance from you- no closer. repeat this distance each session.
What you are going through is frustrating- yet entirely normal. Sometimes it can be fixed, other times it is a case of binning the barrel. For now, all you can do is go through the steps. I hear you.
You have a lot of work to do, good luck.