Testing slow powders, to see if reducing the pressure and acceleration will help accuracy.
I've decided that rather than using the barrel's most accurate bullet (the ULR or PLR) as my control load, I will instead use a bullet has shown potential but always had a couple of fliers spoiling its groups -- the 193 gr. Loverider. The 193 has typically put 8 shots in a decent group with 2 low shots spoiling accuracy. Since its velocity variation has been good, I suspect that the 193's low fliers are due to the heavy bullet's increased pressure causing some sort of deformation or damage. Let's see if reducing the pressure makes the 193's low fliers go away?
Before shooting I had JB'd the barrel to get it squeaky clean, then the barrel was cleaned between groups with a brush and Seafoam. Lately most fouling shots have been landing within the group so I did not bother to load a fouling shot today. Naturally, today's fouling shot's landed outside the group
, so I only counted 9 shots.
The control load with IMR 8208 was true to form with 7 shots in less than an inch but 2 low fliers spoiling the group.
IMR4350 shot one ragged hole. I was impressed. Even using a drop tube, this was a heavily compressed powder charge and velocity was only 2166 fps.
WW760 made a shotgun pattern. WW760 has always been a finicky cast bullet powder for me, shooting well in a few cartridges but poorly in others, with no rhyme or reason.
Was IMR4350's improved accuracy due to the lower pressure and lower acceleration, or due to the lower velocity? I vote for pressure and acceleration, but there is no way to know for sure.
IMR4350 left dark carbon streaks. The thin shiny streaks in the corners of the groove might be lead, though I can't be positive. Overall IMR4350 was "average" for cleanliness.
WW760 was cleaner than average, but the thin shiny streak in the corner of the groove might be lead, though I can't be positive.
IMR8208 left a thin grey haze but otherwise was pretty clean. The thin shiny streak in the corner of the groove might be lead, though I can't be positive.
The first shot of the day lost a gas check. It landed outside the group, but then again it was a fouling shot, and it is common for fouling shots to land outside the group.Conclusions:
-- the nice group with IMR4350 seemed to reinforce my theory that I need to reduce pressure and acceleration. Then again, the lousy group with WW760 does not reinforce that theory.
-- all of today's loads left a thin shiny streak in the corner of the grooves that might be leading. If so it's not bad leading, but then again there's no such thing as good leading. Any leading or any hard carbon fouling will deform the next bullet that passes over it. I'm puzzled as to why this Shilen is leading at 40 - 50 ksi while my Pacnor 7BR doesn't lead at 60 ksi? Maybe I just got lucky with the $90 Pacnor?
-- I'm still leaning toward punching the Shilen's chamber out to 308 Winchester, but I want to think about it some more.
-- the alternative is to lower the 30BR's velocity down to 2150 fps, but that would defeat the whole purpose of using a 14" twist in CBA competition. The idea behind the 14" twist is to allow the velocity to be increased to 2300 for less wind drift.