Aside from the mould, you'll need a pot for the lead and a heat source, a means for sizing the cast bullet if neccessary, and some lubricant, as well as a dipper of some sort if you opt not to purchase a bottom-pour melter.
If you're just looking to dabble, the Lee mould will be OK, if it works? Big ? I know that many people have used Lee moulds to what they consider good effect, that said, I think they are crap and not worth the aluminum ther are made from. Nothing wrong with aluminum moulds unless made by Lee. In your particular situation, the Lee may be very suitable, provided it matches you bore size. If the accuracy of the rifle is in question, due to bore size, by all means go with the Lee mould. Lee provides a very great diversity in moulds of different shapes, sizes, and configurations. You may be dealing with a weapon with a far less than perfect bore and bore uniformity to start with. This would be mainly the result of age and care for the firearm. I normally shun Lee moulds in all instances, based on limited but personal experience, but this may be a very valid use for them.
If you want to start off cheap, I'd get a Lee Precision Melter(nothing wrong with their lead pots), the suitable mould, and a pouring device of some kind. They make a very simple ladel for about $4. You can hand lube the bullets with a soft lube, such as SPG, and shoot them as-cast without sizing. This may actually be the most accurate way to shoot cast bullets from your particular firearm. If your shooting needs outstrip your prodiction rate with the simple, but very economical, tools listed above, you can upgrade to whatever suits you. Either way, it's best to find out if the bullet shoots and you like casting before you invest too much money.
One thing you'll want to consider with this type of mould and rifle is alloy. I would discourage the use of wheelweights and go with something as close to pure lead as possible. You can find it, and I beleive it will perform better in the questionable diameter of you bore. The Hollow Base mould you mention should accomodate any bore variances with this rifle at the limited velocities you are dealing with. Unless the bore is in a very bad way, or you don't mind dealing with leading, I'd go with as close to pure lead as you can find.
LEE molds ain't the best made by far, however they will make excellent projectiles ifin' ya lube em per the instructions and preheat and smoke the mold before castin' with it.
The Lee 459-405HB is a very close copy of the original 1873 government bullet that the 45-70 Springfields were designed ta shoot. Spence Wolf had it made up custom, and Lee added it to their line of bullet moulds. The 45-70 Trapdoor Springfield has a unique bore that needs the hollow base bullet ta allow for engraving of the lighter 405 gr bullet.
(The 500 grain 1886 bullet has enough inertia to engrave without a hollow base.) Original alloy was 20 parts lead ta one part tin.
Fer the price the LEE castin' pots are a good deal and with this big a slug the bottom pour is real nice.
For the most part agree with what KciH and DoubleJK say, but would add one thing else as a real biggie! Get the Lyman Reloading Manual and read it. Read chapter and verse on casting and reloading cast bullets, and then when you get done, go back and read it again. If that isn't big on your list find an experianced caster and ask to watch and learn. Better yet, do both! As to Lee molds, I have had good, bad, and very bad, but mostly good. With Lyman, OHAS, RCBS, etc. the results have only been good. The biggest mistake IMNTBHO that new casters make is failure to read, ask questions, and go slow. Lastly, the advice on soft lead for starters I think is excellent, and my advise is to start with a minimum load listed in the Lyman manual for a bullet of that weight.
Best of luck.
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