Powder compression is incidental to accuracy for the most part

[Chill Wills]

The evidence is mounting. I have been a competitor in the BPCR game for 30 years. In all the load development of all the lots of powder and many different rifles, have come to think that the much talked about "compression" and "what powder brand likes "how much compression", plays very little in the accuracy of the load.

Quality of powder, bullet choice and fit, bullet hardness in some cases, and primer choice all have much more to do with accuracy than compression. What the compression ends up being is incidental to having the right amount (weight) of powder to hit the accuracy node.

A few days ago, in preparation of shooting an upcoming 300y, 500y, and 600yard mid-range match, I shot my 40-70 SS accuracy load, set up in cases three ways.
Full length cases
Lyman neck-sized cases
and LEE collet die cases (neck sized)

All resulting in a different case volume, and, by extension, different compression.

They all shot the same at 600yards accuracy wise. Interestingly, the LEE sized cases grouped about two minutes higher with all else being the same. This load holds the 10 ring in the vertical. Winds of course stretch out the group left and right.
Fouling control was with a blow tube as the mild spring air was cool enough to not over heat the barrel.

[RedneckRob]

Interesting to say the least.

[Don McDowell]

Compression numbers someone gives is mostly a matter of sounding good on the internet.
As you laid out here and other places, it's powder charge weight that matters, compression is a by product of seating the bullet to that rifles preferred depth.
Interesting that the Lee die sized cases had that much difference in poi. But it's not uncommon with smokeless cast loads to see similar results when using an M die to expand. My Ruger 1a in 3006 shows a similar thing, it will group better with the same powder charge , bullet etc when loading using a 40 year old set of Lee dies than it does when using a set of RCBS half that old..
It all falls back to the same old "let the rifle and target" determine what works best.

[Gunlaker]

That's what I've always believed, but then I've never tested the theory. Thanks for the info Chill

Chris.

[indian joe]

That's what I've always believed, but then I've never tested the theory. Thanks for the info Chill

Chris.

yeah thanks ....Don too for reinforcing the message.

[Lead pot]

Chill,
This is a good discussion, this compression comes up in a lot of posts. Most of us that compete with black powder develop loads to get the best accuracy.
When developing an accuracy load I start with zero compression and work up from there. Most generally my best accuracy will show up between .08-.130" using Swiss and .200" + a few using OE.
Now the head scratcher,
The accuracy gets better as I add a grain more to the load till I get a peak that might take 2,3,4 grains more and start dropping off. But a lot of times the accuracy comes back as more powder gets added but a lot of times that first peak is the best.
Now, adding powder increases the compression and accuracy than drops again after adding more. Is it the compression or other factors causing this??

Case prep, primers, bullet design/length/diameter, neck tension all play a large part. But I think so does the compression that changes the burn rate of the powder. Sometimes it might burn more or less with the same amount of compressed powder.
When I get a new lot of powder I go back to the development process.
I been with this black powder for 70 years and still looking for a better way.
Kurt

[martinibelgian]

Kurt,
Ladder tests show the same effect when increasing charge weight, also with nitro.
Barrel vibrations and exit nodes. It' s reasonable to believe the same happens with BP.

[Bad Ass Wallace]

More important is the type of powder used; short fat cases like 50/70 and 577 burn cleaner with 3FG powder than 1FG and is more consistant with velocity. conversly longer cases like 45/90 and 50/90 work better with 2FG and 1FG powders.