Weighing Factor in QL?

[C Valentine]

Can someone experienced in QL explain the Weighing Factor to me? I notice that the default is 0.5 on typical bottle neck cartridges, i.e. my .308, but on the .223 it defaults to 0.6. On my .223 load, a 0.5 factor gives me lower pressure (by 1211 psi) with a slightly higher (14fps) velocity than the default 0.6 factor. I read that the factor is an estimate representing the fraction of unburned charge and gases that move forward with the bullet, but that doesn't clarify much for me. Is it because the charge is just smaller in a .223 so a little more of it gets burned instead of moving forward? Is there any situation where this could or should be changed to 0.5?
Thanks for helping me learn!

 

[Brians356]

Since charge weight is a significant variable (in fact, it's simply combined 1/1 with the bullet weight to become 'M') of the common recoil velocity formula, that implies most if not all of the charge moves forward. How far it moves before being converted to hot gas is another issue.

Correction:

Recoil velocity:

V = ( b*v + c*p ) / W

b=bullet weight
c=charge weight
v=muzzle velocity
p=the average velocity of the escaping propellant gases
W=rifle weight

Given this, the charge weight could have a smaller effect on recoil velocity than bullet weight (depending on 'p').

But a common estimate (NRA) for gas velocity 'p' is about 4000 fps for smokeless powder, so in that sense it becomes a constant. And it then actually weights charge weight proportionally higher than bullet weight for any muzzle velocity below 4000 fps.

The upshot is that the powder charge does move forward, and it seems obvious a lot more of it than just 0.5% does. Unless my analysis is far wrong.

 

[mikecr]

Look at weighting factor as a setting for designed case efficiency.
For efficiency think relatively short, fat, high shoulder angle, smaller bore. This allows more of the powder to burn INSIDE the chamber. The only other way to head for this efficiency is high starting pressure combined with very high peak pressures from quickest viable powder(ala underbore -6PPC, 30br, 6.547L).

Opposite of standards in efficiency are cartridges like a 30-06 or 223. Viable powder for these funnels right down the bore to become mass added to the bullet. Then the unburned powder flashes off at the muzzle to slap across bullet bases(high muzzle presures).
Even where it burns further down the bore(long enough barrel), it's burn is at lower pressure and so likely a lower speed dirty burn. It's not that they're bad cartridges, but they're in no way 'improved' either.

I chose .45 for my 6.5WSSM, and this ended up perfectly in cal when results were measured across a full incremental spread.