So Many Items to Discuss--So Little Time.
Your msg raises all sorts of interesting questions. I don't have a Speer #13 at hand, but do have a #12. I see the very load you mention, 158 SWC with 5.5 Unique for 970 fps. Looks much like some of my chronograph work.
Can you furnish some more information about gear and technique? Type chronograph. Distance to start screen. Distance BETWEEN screens. Montana cast bullets--I'm not familiar with this brand. What kind (or color) of lube is in the grooves of these bullets? Is it easily dented/removed with a fingernail?
Cheapo raises excellent points about the smoke cloud causing reaction, rather than the passage of bullet. I would bet that the problem source will ultimately be found somewhere in this area. How were you oriented with respect to the sun? What time of day was it? Was battery in chrono fresh?
Nanaimo, you write that you used a borrowed chronograph. Did you check with owner to see if the electronics box has had any interesting impacts?
You gave us powder measure type, and this sounds fine. What kind of scale or balance did you use to set it? Did you zero the scale carefully? (Hopefully with check weights.)
Blackhawk and Vaquero revolvers--How long are barrels on these?
Also--and please, I intend no offense here--you say you are a beginning handloader and that the chrono was borrowed--might it be that results would have been different if a more experienced operator had been present to supervise? You mention "about 10-15 foot distance." Was distance identical for each string? Did you have the chronograph manual available? It is easy to make minor errors which combine to give grossly distorted results.
Archie---I must ask where you got the information that lead bullets give higher pressures than jacketed bullets? Not impugning YOU, sir, but your sources. I just don't believe this is the case. I've had occasion to remove several stuck bullets from barrels over the years, and it takes a LOT more force to move a 230 gr. FMJ .45 bullet through a bore than a very similarly-shaped lead bullet of same diameter. And, though I cannot quote sources here, I've read a formula to allow calculation of approximately how much MORE resistance there is with a jacket bullet than from a similar lead bullet, both pure and with a hard alloy.
You wrote, "Reading further in the book, I see a .38 Special listing for 4.7 grains of Unique giving an MV of 815 ft/sec, for a lead bullet. This is shown as a max load in the Special."
Wow--In the previous edition of same Speed manual, it shows same load at 913 fps.
Next time I go to the range, I'll try to duplicate this situation. I'll bet it has something to do with the lead bullet/Unique powder combination. This is a notoriously smoky combo (No, NOT particularly DIRTY, just smoky--) and one which may contribute to the erroneous readings. The more I think of it, the more I believe friend Cheapo may be on the right track.
NB, presuming you used proper care in assembling your loads--and the load is certainly a low- end magnum load--the problem nearly has to be in the chronography, either technique, operation, calibration, or disrepair of the instrument.
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Blog: Expert Witness