Questions
You have a lot of questions as we all do.
Most of yours are covered in a postit approved by the safety committee or the owners manual of the gun you buy (or online or by mail from the producers of most guns). BP is really dangerous stuff and you really do need to be careful with it in addition to all gun handling safety issues.
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1. BP is measured by volume Not by weight. It is usually transferred from cans or plastic jugs to secondary metering devices - powder horns or flasks. Many of the secondary metering devices have removable spouts of a specific volume. I will use your example of a 44 revolver loading sequence. So one fills the flask with an appropriate powder. Most pistols shoot fff, a granulation or coarseness measure. You have selected Pyrodex Pistol because it is available and that is what the clerk at the sporting goods store recommended. I am going to assume that you have a brass cylindrical flask with a spout. You have read your manual and have a 20 grain spout on the flask. Then charge all cylinders that you plan to shoot. To charge a cylinder, one holds the flask down with your finger over the open end of the spout. You then activate the fill or release lever and charge the spout. Next I lift the spout up, remove finger, and make sure that the spout is full. I repeat this process for all cylinders that you plan to shoot. Then put a wad in each cylinder and next seat each wad. I do the wad and then seat it so that I don't skip or double wad any cylinders. Next one selects the proper size ball, a matter of much controversy. Since your manual says .454 we will go with that (yes 44s are really 45s - sorta like 44 mags are really 43s - but I digress). Next I peek and make sure the cylinder that I am going to put the ball in has powder and a wad. Next position the cylinder that you are loading in the Next up position and place the ball on it. Rotate the ball under the rammer, release the ram arm, and seat the ball. It should shave a neat little ring which insures a correct fit. I have a pick handy and make sure that I remove all shavings - less crud flying around when the gun goes off - I have had the unremoved lead shavings jam the gun. Repeat for all charged cylinders. When I am done I check the depth of each cylinder with the ram. If I have left a cylinder unloaded I make sure it is the one without powder in it. (the ram test should have alerted you to this condition but not always). Last a wad goes over the loaded ball. Many people just use grease (Crisco - Bore butter - Whatever) over the ball.
This is how I do it. some people do the sequence differently. I have seen people completely charge and load each individual cylinder. I believe that slow and methodical are the most important. Get a sequence. If you change the sequence, change one thing at a time. Slow and the same every time are important.
2. 3/4 full by eyeball will work (go bang) but your accuracy will suffer.
3. Leaving a gun charged - opinions vary. I live in a very moist climate. BP substitutes are hygroscopic - draw or attract moisture. I tried the "leave your gun loaded" test with a ROA and an 1862 Sheriffs model. I ruined the Sheriffs model and took 2 hours to straighten out the ROA. I do not leave my guns Loaded. I clean my BP guns three times. Once the day I shoot them, Once the next day and the third time a week later. I also clean all of my BP Guns once a month weather they have been fired or not. (Remember I live in a climate with extreme moisture issues.)
4. Ox-yoke wadding is great stuff albeit expensive. Many people make their own wads, leave out the over powder wads and just put a wad over the ball, use Crisco over the ball or just use powder and ball. I am sure that people will have comments on my even mentioning some of these options. There are many threads about all of these options - complete with pictures of chain fire results.
My recommendations are get and read an owner's manual. Read Postits on this site and watch what other folks do. Remember all of the many safety rules of BP guns are in addition to the safe gun handling practices for all guns.
It is never OK to be looking at the muzzle of a gun. Always recap and remove All powder from the line before fireing any muzzle loaders. Ears and EYES - always.
Welcome and do have FUN.