High Bulk vs. Low Bulk Powders

[Gary H]

Today at the range, a fellow shooter remarked that low bulk powders loaded in relatively large cases can result in poor accuracy. I can certainly understand why this might be so, but is it true? I have avoided these powders for fear of double charging, so have little experience, but have been thinking of using Clays with some of my pistol calibers.

 

[MonkeyMan]

The biggest problem I've encountered with low density powders is huge variations in velocity. I took some light .38 Special loads and shot them through my chrony using a barrel up and barrel down sequence in my 4" Model 19. Starting each shot from a barrel up position, velocity was fairly consistent and accuracy was OK. Starting each shot from a barrel down position and things got strange real quick. Velocity was all over the place and accuracy went out the window. I had pretty much the same results in my 9MM.

 

[Big_R]

Here's what I know and have heard:

Fast burning (low bulk) powders in pistol cases generally give accuracy on par with any other powder. That seems to change when you get into rifle cartridges.

If you have a charge that's less than say 50% of your case volume, and you fire the rifle horizontally (naturally), the powder can be laying anywhere from completely under the flash hole to ramped up over the flash hole, to ramped up on the bullet. This may be hard to visualize, but when the sparks from the primer ignite the powder, the powder may ignite at different rates based on how much powder is exposed to the sparks.

For this reason, most bench rest guys use compressed or full case capacity loads. This keeps the powder fully against the primer resulting in more consistent ignition. It makes sense in theory.

As a side note, I personally know a guy who blew up a very nice .220 Swift while working on a reduced powder charge. It is my belief that he had so little powder in the case that the charge acted like a shape charge and forced pressure up instead of pushing the bullet out of the bore, but that's my theory.

Ryan

 

[Quantrill]

I would agree with Big R about the pistol loadings. In an old NRA study on factors effecting accuracy in .38 spec. pistols, one of the items tried were 2 totally different powders. The 2 powders were Bullseye and PB. Bullseye is low bulk while PB (do they even still make PB?) was a high bulk powder. The NRA conclusion was that either type of powder was sufficient to do the job properly so that would indicate that (at least in pistols) different bulk powders would have little or no effect. Quantrill

 

[MoNsTeR]

Go ahead and give Clays a try, it's fast-burning and bulky.

 

[HSMITH]

Clays will fill a 9mm nearly to the top if double charged making a double obvious, a 45 will get nearly to the top too even with very light loads. 38 specials are a little harder to see with light loads, but it gets to about 80% fill with a double charge of 6 grains thrown.

Clays is a great pistol powder, plenty of accuracy and decent velocity in most calibers. 9mm loads are a tad slow, but shoot good.

 

[PaulS]

If low density loading caused inaccuracy then why is H4895 so accurate even with loads down to 60% of maximum? Some powders are very sensitive to volume while others are not. Unique in large cases for reduced loads perform with good results even 12 grains in a 3006 with no wadding, while reducing a load of H110 just 10% can cause squib loads.

I load powders that are typically just dense enough to fill the case with maximum loads because they tend to be the slower powders for the bullet/case combination and give the highest velocity. I seldom load maximum loads as I find accurate loads before I get there.

PaulS