Being a bp cartridge newbie I have a question on compression, Why do some loads say compressed or not? I mean are not all loads compressed? If I fill a case to the top why can't I just use the bullet to compress the powder? how much compression should a load have and if you compress the powder you still need to seat the bullet and not have any air gap, doesnt that change the compressed load? also just looking for general answers on compressing.
Howdy
For many applications real Black Powder is pretty forgiving as far as how much compression is applied.
I have always found that for CAS cartridges, a compression of around 1/16"-1/8" works fine. Yes, I compress the powder with the bullet as I seat and crimp it. It is not too critical exactly how much compression. Some of the original specifications for Black Powder cartridges like 45 Colt and 44-40 listed the powder charge as 40 grains. Modern cases are more heavily built than some of the 19th Century cases and do not have as much interior capacity as the old cases. So it is difficult to actually fit 40 grains of powder into most cases without a lot of compression, more than 1/8". Some shooters have tried mashing the powder down a lot in order to stuff 40 grains of powder into their cases. It can be done, but there isn't much point, the amount of powder that fits into a modern case with 1/16"-1/8" of compression will do the job just fine and any more powder is just wasteful. Black Powder actually varies in weight from manufacturer to manufacturer. I load my BP by volume. The amount of BP that gives me the compression I want with 45 Colt or 44-40 is as little as 33 grains with one brand, as much as 37 grains with another brand. The ballistics of cartridges made with these amounts of powder will obviously vary, but for CAS purposes at close targets, it is good enough.
When I load shotgun shells with Black Powder I compress the powder after I seat a cardboard over powder wad. I simply lean on the wad with a piece of 5/8" dowel until I hear a slight crunch. Compressing the powder in this fashion does not have the mechanical advantage of using a loading press, so the powder is very lightly compressed. I have never measured it, but I'll bet it is less than 1/16". For my purposes in a shotgun shell, it is enough compression and the loads work fine.
Precision long range shooting is a different thing entirely. There you want absolute consistancy. You also do not want to be damaging or mashing the bottom of the bullet out of shape at all, so most serious long range shooters of cartridges like 45-70 and the like never compress the powder with the bullet, fearing that any distortion of the base of the bullet will damage accuracy. Many long range shooters compress the powder with a special compression die, then seat a wad and finally the bullet with no more compression at all. This takes very careful setting of the dies so that the bottom of the bullet just kisses the wad without leaving any air space at all.