OK, two new strain gages mounted on the Marlin.
One is centered 0.400" from the receiver, about even with the base of the bullet. We'll call it the "base gage" for lack of a better name.
The other is centered 0.800" from the receiver, about even with the throat. We'll call it the "throat gage."
Both were installed with Devcon "flex" superglue. The throat gage worked fine from the get-go. The base gage gave suspiciously high pressures that got higher as more shots were fired. Back at home, while I was giving the base gage a look-see, one of the solder connections broke loose. Maybe that connection had been weak all along????? After resoldering, the base gage started giving reasonable pressures.Note to self: the next time a gage acts funny, check the wiring and/or replace the connector before blaming the gage or the adhesive.
Alliant publishes some PSI data for 2400 and unique, so I duplicated a couple of the Alliant loads as closely as I could.
"QL-vel" means the pressure Quickload predicted to reach the actual velocity with that powder, and never mind the powder charge.
"QL-pow" means the pressure Quickload predicted for the actual powder charge.
The base gage averaged 3159 psi or 13.2% higher than the throat gage. The bad news is that the two gages do not agree exactly. The good news is that they are in the same ballpark. I could live with 3000 psi uncertainty.
But when you compare the Alliant data, the Quickload data, and the gage data, the discrepancy is a lot more than 3000 psi. What's a reloader supposed to go by, if different sources of data aren't even in the same ballpark?
I don't know the answer. I'd like to see more 357 strain gage data from more guns and more loads. Maybe if we had tons of data, we'd start to see some trends and be able to draw some conclusions, but right now I am puzzled and I don't have a lot of confidence in the 357 pressure data from any of the sources.
I also fired a few firelapping loads with a 0.5cc scoop of Unique (approximately 4 gr. but I didn't bother to weigh it) pushing a 160 gr. PB. They clocked about 800 fps and the base gage averaged 7752 psi. Quickload predicted 5053 psi for the actual velocity or 8303 psi for 4 gr. of Unique. The itty bitty pressure curve looks cute.
Here's a typical trace from the throat gage.
A typical trace from the base gage.