Bryan Litz's article on longer CBTO and the reduced pressure that comes with longer OAL has me thinking. The concept makes sense, but can it be quantified?
For example, how much more RL15 can I run at 2.45" OAL instead of 2.39" for comparable pressures?
I'm loading up some experiments that are both longer and charged more than book recommendations and I'm trying to get a feel for how hot is too hot for a longer-than normal OAL.
From what I can tell, another 0.050" of OAL can really drop pressure a bunch. If the pressure can spike due to something like just having a thicker wall case, then surely more volume from longer OAL would work in the opposite direction.
Putting some science to this:
Accurateshooter.com lists case capacities for various makes of .223 brass as being anywhere from 30.6gr H2O from some old Lake City lots to as low as 29.8gr for some especially thick/heavy cases. That is a variation of 0.8 grains.
How many "grains" of case capacity is equivalent to 0.010 more OAL?
With a .224 bullet diameter, the volume of a cylinder of diameter .224 and length .010" contains the equivalent of 0.0317 grains of water. Thus, going 0.1 longer on OAL would be the same as having a case with 0.32grains more capacity.
I've loaded some rounds that are 0.110 longer OAL than the load book shows. The math suggests this is the same as having 0.35 grains more capacity.
The Hodgdon data for .223 uses a Winchester case which has a typical capacity of 30.1, same as Hornady.
But if I was using Lake city with more like 30.4 capacity AND seated very long (2.5")? My long OAL gives me another 0.44gr capacity relative to mag length (2.26"). In LC brass, I essentially have 30.84gr capacity.
Hence I propose a rule for .223: each 0.10" over book OAL is roughly 1.5% more case capacity and charge weight could likelwise go up 1.5% or so.
Not a huge amount for .223 as we're talking about 2-3 tenths of a grain of powder for most powders, but it's not nothing and could very well let you push more speed safely.
For example, how much more RL15 can I run at 2.45" OAL instead of 2.39" for comparable pressures?
I'm loading up some experiments that are both longer and charged more than book recommendations and I'm trying to get a feel for how hot is too hot for a longer-than normal OAL.
From what I can tell, another 0.050" of OAL can really drop pressure a bunch. If the pressure can spike due to something like just having a thicker wall case, then surely more volume from longer OAL would work in the opposite direction.
Putting some science to this:
Accurateshooter.com lists case capacities for various makes of .223 brass as being anywhere from 30.6gr H2O from some old Lake City lots to as low as 29.8gr for some especially thick/heavy cases. That is a variation of 0.8 grains.
How many "grains" of case capacity is equivalent to 0.010 more OAL?
With a .224 bullet diameter, the volume of a cylinder of diameter .224 and length .010" contains the equivalent of 0.0317 grains of water. Thus, going 0.1 longer on OAL would be the same as having a case with 0.32grains more capacity.
I've loaded some rounds that are 0.110 longer OAL than the load book shows. The math suggests this is the same as having 0.35 grains more capacity.
The Hodgdon data for .223 uses a Winchester case which has a typical capacity of 30.1, same as Hornady.
But if I was using Lake city with more like 30.4 capacity AND seated very long (2.5")? My long OAL gives me another 0.44gr capacity relative to mag length (2.26"). In LC brass, I essentially have 30.84gr capacity.
Hence I propose a rule for .223: each 0.10" over book OAL is roughly 1.5% more case capacity and charge weight could likelwise go up 1.5% or so.
Not a huge amount for .223 as we're talking about 2-3 tenths of a grain of powder for most powders, but it's not nothing and could very well let you push more speed safely.