Quote:
Originally Posted by Paul Harm
From a Accurate Powder Co. reloading handbook :
The fundamental difference between a shotshell and a typical center fire rifle cartridge is, that the efficiently of the shotshell is 100% dependent on the round itself. By this we mean that all the " resistive forces " must be generated within the confines of the round itself. No assistance is provided by the gun.
The reason for this is that the Maximum Peak Pressure is reached long before the base of the shot/wad assembly has left the the case. [ In the case of a CF rifle cartridge, the peak pressure is achieved when the bullet is engraved, therefore the large contribution, as a result of leade/free-bore dimensions on the combustion process. [ ie. bullet/bore interface fit, bullet hardness, bearing surface, etc].
The reason for this is that the critical engraving force which is so important to the dynamic combustion process present in a CF rifle cartridge is totally absent in a shotgun.
This means that the efficiency re ignition and the subsequent increase in pressure, is totally controlled by the integral configuration and assembly of the round itself. These constitute the main internal mass [ shot mass ], the internal volume [ wad design ], the dynamic collapse [ primary expansion ] of the internal volume [ collapsible section of the wad], plus the displacing of the internal assembly and the unfolding of the fold/crimp [ secondary/final expansion].
The way this COMBINATION interacts, will determine the efficiency Pressure impulse [ profile and time-base ] and the Peak-pressure vs Velocity [ P/V ]. The resistive force, presented by friction in a shotgun is negligible.......
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I was just quoting the reference attributed to Accurate Powder, who surely has objective modern data to substantiate what is written. It's copied in part above; please especially note the last sentence. I have a hard time understanding a substantial increase in bore friction because of a 1/4" or so longer shot column OF THE SAME WEIGHT riding in or mostly in a plastic wad.
I certainly subscribe to the advice that all should stick to published recipes for bismuth shot by reliable industry sources.