The thicker walls also mean that less powder is needed to reach the same pressures as a thin wall case with slightly more powder charge.
Simplistically, yes - but remember this is a dynamic situation. It's not a case of building the pressure to a particular point (like a balloon) and then the bullet pops out at the desired velocity. It's all about the pressure curve, or the rate at which pressure builds. S385 is the slowest of the Somchem powders (other than B12.7, which is not widely available) which means that it reaches it's Pmax more slowly and also requires more mass of powder to reach Pmax, all else being equal. Pistol/shotgun powders being at the other extreme - burning fast and having a sharp pressure curve.
The smaller the internal volume of your (fire-formed) case, the faster you're going to reach Pmax for the same powder charge. Using S365 because it requires less volume is going to move you in completely the wrong direction; the reduced internal volume and faster burning powder has all the makings of excessive pressure.
SlushPuppy said:
I see the S365 Powder have very similar properties as the S385, and less grains of powder needed, so this might be a good alternative.
As per my answer above, yes and no. S365 can certainly get you to the same, or higher Pmax, but does so fast (it's faster burning than S385) so the risk of excessive pressure is that much greater. People in SA can and do use S365 successfully in the 300 mags, but typically for the lighter bullets. All the theory points towards S385 being better suited, especially for heavier (i.e. 200gr plus) bullets. For consistency, you aspire to 100% case fill or slightly over - that's not going to happen with S365.
SlushPuppy said:
What would be the best method of determining excess pressure?
The absolute best way is to avoid excess pressure!!! A well-known method that gets the message across first time, every time, is losing a thumb when the chamber blows out
![Eek! :eek: :eek:](https://cdn.jsdelivr.net/gh/twitter/twemoji@14.0.2/assets/72x72/1f631.png)
I've heard even the sloppiest reloader doesn't fail to learn from that method..
Best practical way is to chrony your loads - start fairly low and build your way up. Use Quickload to calculate pressures and keep you within Pmax. Quickload can also be used to get you onto an accuracy node, which is ultimately what you're striving for. Max velocity is academic if the load won't group - so you need to get onto your highest accuracy node within the Pmax limit for your calibre. Forget about chasing the highest velocity possible. QL takes into account barrel length, internal case capacity etc - remember garbage in = garbage out, so you would need to measure all the variables accurately to get the most out.
The best technical solution to determine pressure is using a specialised test rig with strain gauges etc but that's not practical for the layman.