Picked up an old 25 lbs. canvas bag of #9 lead shot at a yard sale this summer. The plan was to use it to cast round balls and conicals for black powder rifles. Not sure if it's pure lead or if it will be soften enough for this application. Will this type of lead be suitable for casting ?  -Thanks

The bag is looks like this :

You should be good. From Ballistics Research, which sells lead shot:

About Lead Shot:

Lead shot is classified in very general names that refer to antimony alloy content. The two types of lead shot are known as either chilled or magnum. Antimony is the alloying metal used to harden lead. Antimony is a brittle flaky, crystalline metal with a density of 6.7 grams/cc. Pure lead's density is 11.34 grams/cc. Therefore, when lead is alloyed with antimony it becomes a harder pellet, but as more antimony is added, the overall density of the pellet is reduced.

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Chilled Lead Shot is softer and slightly denser than Magnum lead shot. Many shooters, like Don Zutz, prefer the increased density of chilled lead for hunting loads and the manner in which it deforms when it strikes game. (Think of why hollow-point bullets so effectively transfer energy. It is because they deform quickly when they hit the target. The process of expansion and stopping is what transfers energy.) Chilled lead shot contains approximately 2% antimony.

Magnum Lead Shot is harder and slightly less dense than Chilled lead shot. Harder shot is more prone to retain its spherical shape during the firing sequence. Some shooters prefer Magnum shot's hardness advantage because it generally patterns more predictably than softer shot. Magnum lead shot generally contains approximately 4-6% antimony.

That lead is good for casting bullets for pistols and probable rifles.

For muzzleloader balls and Conicals you might want softer pure lead.  Off top of my head muzzleloader balls are around 8 hardness.

Check local scrap yards for lead.

Make friends with a plumber, or go visit a stained glass shop. Scrap yard can be hit or miss.

Lead shrinks as it cools.  The molds are designed with this in mind, depending on it is to be an alloy or pure lead.  The balls may end up oversized and worse with the conicals as they may be too hard  in addition to being oversized.  This is especially a problem with ones such as the REAL or the TCs which name I can't remember.

I also have a bag of #9 shot I was thinking of melting down for round balls.
I would think it’s pretty soft stuff. Ever bite into a piece of shot while eating game? The lead deforms pretty well. Hard shot would break a tooth.

Do fishing weights make good casting lead for muzzleloader round balls ?

Depends on how hard they are.  

You should be able to scratch the lead with a finger nail fairly easily.

Fishing weights can be made from any sort of mystery metal, purity has no effect on how they will function. For muzzle loading you want dead soft lead. I’m a plumber, I’ve got a lifetime supply of balls cast, now I just cast ingots and sell them. Facebook marketplace in your area will likely have ingots for sale.

I have a chance at some 00 buck shot in 25 lb bags.
Was thinking this might be good for melting down into .490 balls.
Most buckshot I’ve ever seen is pretty soft.
By the way what is the Brennel hardness of the typical black powder round ball?

Preferably 5-6, I've used some that were 9 with luck, but a touch stiff loading.