To get started casting can I just get a mold, a dipper, and melt in an old pot? I know I can tumble lube... but what will happen if I don't size my bullets for the time being?
I would be starting with a mold for 38 special/357 magnum and have limited resources to get started.
I have a lead on some used 9mm molds but from what I understand those are slightly smaller than 38?
If I am reading your post correctly, your looking to cast for a 38/357, and hoping to use a 9mm mold?
If this is the case your probably not going to get the end result your looking for, with your loads, and would be better off going with the .358 molds.
Your best bet IMO is to simply pick out the bullet weight you want to shoot, the type bullet your wanting and get the mold in the proper caliber. This will put you ahead in the game from the get go. One of the biggest issues is fit, the next is alloy, then lube, and the last is load pressure.
Since your on a limited budget to begin with, I would look hard at the Lee molds. They are cheap and come in a variety of good profiles for the .357 caliber. They do however cast some very nice bullets even if they do have a few small issues. You want to be sure you size your bullets correctly to get the best performance. This also keeps form having to spend hours cleaning the lead out of the barrel of your revolver as well. It is REALLY better to slug your bore or have it slugged by someone to start off with, as well as miking the throats of each your chambers. Sometimes you will find that these are smaller than the bore of your revolver and in this case it is hard to get your loads to shot without leading as when you shoot the cylinder sizes them down to below bore diameter. This can easily be fixed by having them polished out a bit to the correct size.
Once you have your sizes noted, you bullet profile picked out, and purchase the mold, pour up some bullets and let them sit for a week or two before doing anything with them. They will settle out and by then not only will the hardness be more stable but they will have settled in size as well. Then you can mic them for size and if needed size them to the proper diameter to fit your revolver. The alloy you use will determine how this works out for you. Harder alloys take longer to completely settle it seems than softer ones. Your looking for something ranging from about a 9BHN through around a 13 BHN and doing a little research will put you down the right path to start off with. Standard WW alloy is great, just be watchful for the zinc ones and don't let them get into the mix.
Sizing, I highly suggest simply getting the proper die from Lee. They are so simple even I can use them. Screw the sizer into your press, slip in the push rod, stand one up on it and shove it through. You can lube them first or not. If not at least shoot them with a tiny bit of WD-40, so they don't smear going through the sizer. Once sized then do the tumble lube, and your set.
As for lubes, straight Alox, which comes with every Lee sizing kit, is as fantastic as it is messy and nasty. There are several different ways to work with it that help out a LOT. The best I have found is mixing up what is called 45/45/10 which is simply 45% Alox 45% Johnson's Paste Wax, and 10% Mineral Spirits. This goes on light, and dries within an hour or two and your ready to load or size depending on your routine. Me personally I lube before and after sizing. Another method is after lubing with straight ALox, to dust them down with a light coat of talc, or corn starch to kill the stickiness. When I lube I also use those cheap disposable nitril gloves to keep it off my hands. I lube in a zip lock freezer bag and can usually get several uses out of it before I chunk it.
When your looking at molds, don't simply look at the tumble lube ones as with the Alox or similar TL's you can lube any bullet design. If you want full .357 magnum loads then you might also want to use a GC design to help eliminate possible leading with the higher pressures. Straight target only or hunting loads of around 1000fps or less can easily get by with out them. This is where fit revolves around function.
If you know your revolver dimensions, you can usually pick up a new Lee mold, and sizing kit for around $40 depending on if you go with a two or six cavity mold, and where you get them from. The six bangers are a bit more involved, but there is plenty on the Castbooit site to get you up and running in no time. One thing to remember is the 6 cavity molds don't come with handles.
As suggested there is plenty of info and links in the sticky's located above and on Castboolits. Research is your friend and notes are your best ally. Good luck and hope this helps.