BP Grain size 3.5F? 4F?

[Old Ironsights]

Ok all you (low) tech weenies...

At what point does 3F become 4F where it becomes too dangerous to shoot as a main charge?

3FG = .84mm —.29mm / .0330in - .0114in
4FG = .42mm —.15mm / .0165in - .0059in

Note that there is a fairly large overlap in grain size from the middle of the 3F range to the middle of the 4F range.

Remember when I sifted a variety of powders? I was using a 30-mesh screen (.0233 hold) which lead to things like 20% to 80% "failure" - i.e. up to 80% of the granulation went into the "4F" pile.

Admittedly, that meant that all grains from nominal .0232 to .0166, which "should" be considered 3F got sifted out, but still...

Now a 40 mesh screen will hold down to .0120 - .0150 depending on (practical) wire size.

Given that 4F can be as large as .0165, would you consider .0120 "3F" (still in the 3F granulation range) safe as a main charge? (This is assuming a large volume case OLD gun or a muzzle-stuffer.) :confused:

[montana_charlie]

Mister Ironsights,
(seems kinder than using your first name)

To answer the question that you asked, I would say that it doesn't matter enough to worry about. And the reason is...

I have recently had something of an epiphany concerning the practice of using 'too fine' black powder in 'too large' cases.

Dr. Dick Gunn ran a thread elsewhere which sought peak pressure data for black powder loads. In his opening post, he described a case failure in a 40-82 cartridge loaded with 4F...but it was subsequently discovered that the cause of the failure was the length of the case, not the powder charge.

Since Dr. Gunn was not trying to destroy rifles, I assume that he expected a large 4F charge to provide the pressure data he sought - with a reasonable degree of safety for gun...and Gunn.
His experience in all aspects of shooting far surpasses mine.

Dr. Gunn does not recommend the practice, and does not imply that 4F is in any way suitable for large cartridges. But, his willingness to subject fine rifles to the experiment makes me conclude that it is not as 'inherently destructive' as I once believed.

So, while you will likely get a 'dirty burn' and inconsistent velocities, I doubt that you will wreck a reasonably strong gun when using 'too fine' powder in reasonable charge weights.

But, you won't find me trying it...
CM