A couple more powder cleanliness comparisons. The barrel had been brushed after the last session, but just to be sure it was clean I JB'd it before shooting today. The JB patch kept coming out black, and I could feel roughness in the usual spot about 8" from the breech. The roughness went away after pushing the JB patch through several times. In other words, I suspect there was carbon in that spot that had not been removed by brushing.
H4895 was cleaner than average.
H335 was starting to show some dark streaks, plus there appears to be lead in the corner of the groove.
Brush and solvent suffice to get this section of the bore clean, but the Russian borescope can't reach the problem spot" 8" from the breech without removing the barrel from the action.
UPDATE: I unscrewed the barrel so I could examine the breech end with the Russian borescope. The usual "bad spot" 8" from the breech looked good -- no carbon, no lead, and no visible pitting. But I could see a tiny bit of lead in the corners of the grooves, starting at the throat and continuing for several inches. The lead doesn't show up well in the photo so you'll have to take my word that I can see more detail with my eye than through the camera. Yes, I am picking nits -- normally I am a slacker when it comes to barrel cleanliness as long as the barrel shoots satisfactorily, but in this case the barrel is not shooting satisfactorily so I am trying to understand why.
I screwed up the powder charge for H4895 such that it was 2417 fps instead of the intended 2300 fps. H4895 had shot MOA on March 19
but that was at leisurely 2239 fps. According to Quickload, the 2417 fps load generated 51 ksi while the 2239 fps load generated 41 ksi. In general, H4895 seems to be a good powder for cast bullets as long as you don't push it too hard.
Because I was running out of bullets, I had to use two different batches of ALR bullets for the WC844 target. The two batches had slightly different seating depths, which you wouldn't think would make a difference, but as it turned out one batch impacted lower than the other batch.
I was disappointed with these groups, even allowing for the powder charges not being optimized and using mixed batches of bullets. This barrel strikes me as being awfully fussy with cast bullets.
One gas check parted company with its bullet but I didn't notice it until I had finished shooting, so I don't know which shot was responsible or whether it landed within the main group.Thoughts:
-- some powders are definitely cleaner than others, but cleanliness doesn't correlate to accuracy as long as you clean the barrel after every group.
-- I keep thinking this barrel is very fussy with cast bullets. Yes, it can shoot 1 MOA with its pet loads and a squeaky clean barrel, but accuracy drops off a cliff with powders it doesn't like, with too much velocity, too much pressure, or with a dirty barrel.
-- Previously I had talked about setting up a comparison test with a Krieger 18" twist barrel, and I may still do that, but I don't need a comparison test to tell me that this Shilen is a fussy barrel.
-- I would not rule out the possibility that the barrel has become "shot out." I don't keep a tally of shots fired but would guess it's up to 1000 - 1500. The mild mannered 30 BR has a reputation for letting barrels last 2000 - 3000 rounds, but every barrel is different. Update: a borescope inspection of the the breech end showed no visible erosion.
-- I'm kicking several options around but I want to think about it for a while, and perhaps remove the barrel from the action so I can borescope that "problem spot" 8" from the breach.
-- my latest theory is that I am "slapping" the bullets harder than they care to be slapped. My goal of a 180 - 190 gr. bullet at 2300 fps requires 40 - 45 ksi in the BR case. By comparison, most CBA competitors are launching 190 - 200 gr. at 2100 - 2200 fps with VV135, which requires only 35 - 40 ksi. It may be that I either need to 1) reduce my velocity to 2100 - 2200 to keep the pressure below 40 ksi or 2) use a slower powder or 3) use a larger case like the 308 Winchester.
-- I have no interest in the slow heavy bullet approach -- yes, it works, but it's already been done a million times before.
-- So I'll try a slower powder or two, and contemplate rechambering to 308 Win. The 308 could hit my goal of 2300 fps with only 35 - 38 ksi, assuming you could find a powder that burns well at that pressure.