For some reason there were some loaded 74 gr. GC rounds leftover from last time, except with a half grain less powder to bring the velocity down to the 2700's, so I went ahead and shot them. Accuracy was about the same as last time so at least they're consistent.
This is my first barrel that would consistently do sub-2MOA at 2700+fps, so I'm pleased even though I haven't achieved the magic 1 MOA yet.
Today's main goal was to compare water based polyurethane coating to oil based polyurethane coating. I chose plain base bullets as the guinea pig because plain base are more dependent on the coating than GC. The plan was to start out at 2000 fps and increase the velocity until it started shooting wild, and see which coating failed first. However, Mr. Murphy had other plans and as it turns out this barrel does not like the 62 gr. plain base developed for the 6x45 project, so accuracy was lackluster no matter the velocity.
Also the original plan was to make both types of coatings 0.001" thick, but Mr. Murphy intervened and gave us 0.001" thick on the oil-based and 0.0018" thick on the water-based.
Anyway, here are today's targets with an ID# for each target that will be explained below. My labeling system is "TOP ROW #1-4" left to right, abbreviated at T1, T2, and so on. The middle row is M1, M2, and so on. The lowest row is L1, L2, etc..
From now on I will abbreviate oil-based polyurethane as OBPU and water-based as WBPU.
#T1: 74 gr. GC, HTRS, ~40 BHN, , OBPU, 2.196" COL, 26.0 gr. WC844, 2768 fps, 1.23% standard deviation, 10 in 1.68", mean radius=0.55".
#T2: 62 gr BB, ACWW, ~11 BHN, OBPU, 2.075" COL, 15.0 gr. RL7, 1845 fps, 2.9% SD, 10 in 3", MR=1.04. Note the high standard deviation suggesting the pressure was too low to make RL7 burn well.
#T3: 62 gr. PB, ACWW, ~11 BHN, OBPU, 2.090" COL, 16.0 gr. RL7, 1970 fps, 0.58% SD, 10 in 3.15", MR=1.10".
#T4: 62 gr. BB OBPU, same as other BB loads except 18.0 gr. RL7, 2202 fps, 1.34% SD, 10 in 1.82", MR=0.64".
#M1: 62 gr. PB, OBPU, same as other PB except 19.0 gr. RL7, 2344 fps, 0.96% SD, 10 in 2.33", MR=0.75".
#M2: 62 gr. BB, OBPU, same as other BB except 20 gr. RL7, 2439 fps, 2.25% SD, 10 in 3.35", MR=1.11". Odd that the SD was high, maybe it was starting to lead? I did not check with the borecam.
#M3: 62 gr. PB, OBPU, same as other PB except 21 gr. RL10, 2460 fps, 1.6% SD, 10 in 12.98", MR=2.83". Poor accuracy and high SD again suggest that it was starting to lead, but I did not check with the borecam because I was short on time.
#M4: 62 gr. PB, WBPU, same as other PB except 16 gr. RL7, 2015 fps, 1.52% SD, 10 in 2.53", MR=0.79".
#L1: 62 gr. BB, WBPU, same as other BB except 18 gr. RL7, 2212 fps, 0.83% SD, 10 in 3.91", MR=1.08".
#L2: 62 gr. PB, WBPU, same as other PB except 19 gr. RL7, 2329 fps, 1.0% SD, 10 in 3.61", MR=1.06".
#L3: 62 gr. BB, WBPU, same as other BB except 20 gr. RL7, 2448 fps, 0.84% SD, 10 in 1.95", MR=0.56".
#L4: 62 gr. PB, WBPU, same as other PB except 21 gr. RL10, 2520 fps, 0.67% SD, 10 in 1.99", MR=0.70. Note that both SD and accuracy were much better than the OBPU with this powder charge, suggesting again that the OBPU was leading, while this WBPU did not lead.Conclusions and Observations:
-- this was supposed to settle for once and all whether water-based was as good as oil-based, but instead neither shot well because apparently the barrel does not like the 62 gr. bullet.
-- that said, the oil-based went wild around 2450 fps while the water-based maintained 2 MOA accuracy to 2520 fps. But, remember the water-based was 0.0018" thick while the oil-based was 0.001" thick, so the added thickness may have been the determining factor. Aargh !
-- the jury is still out on water-based poly. This test also raises questions about the optimal coating thickness. Previous tests determined that 0.001" thick was more accurate than 0.003", presumably due to less imbalance, but that doesn't mean that 0.001" is optimal. Hmmm.....
Things To Try Next Time:
-- perhaps the barrel would prefer a 74 gr. plain base?
-- if I can find a plain base it likes, then I'll redo the water vs. oil shootout.
-- on GC, tweak nose-size depth and neck tension.