While the concept is still a thought experiment, define your goal of casting for big bore. If your goal is to cast bullets to reduce the cost of range practice with your rifle, then this write-up can offer some light on a road already travelled. I am casting for .375 H&H. .416 Ruger and .500 Jeffery. It is possible to cast bullets for your big bore rifle at a small fraction of the cost of commercial bullets. At the time of writing, I can cast a bullet for three percent the cost of the commercial factory bullet. It is also possible to get the cast bullet weight and velocity, or power factor, sufficiently close to factory ammo so that in the field with factory bullets, the difference is barely noticed. It is a very tricky business to get lubricated lead bullets to perform adequately, ie : matching lead hardness to the pressure of application for accuracy purposes. A library of books have been written on the subject. You will not succeed on the first try. There are as many ways to cast for big bore as their are bullet casters. I will describe the approach I took, and which worked for me.
The fundamental considerations are :
My criteria for the mould was that it should be sufficiently over-bore so that after shrinkage, it should still be 3/1000 of an inch too large for the bore. The plan was to powder coat, then size to 2/1000 bigger than bore size. The only mould I could find commercially available that would give me the necessary over-bore size as-cast, was the Lee .515 mould, that I use for making .500 Jeffery bullets. For the .375H&H and the .416 Ruger, I had moulds custom made. Oversize as-cast lead is no problem, any bullet sizer can reduce a lead bullet by 5 thousands of an inch without too much effort required. My .416 bullets are cast .426 at the base. I size in two passes, the first reduces to .421, the second brings it a shade under .417 . The other very important point is the bullet weight. I chose to go heavy, so that even at reduced velocity, I approximate the power factor for the caliber. The reason for not chasing velocity is to reduce the possibility of the bullet stripping in the rifling, which will happen if the velocity is too high and the lead alloy too soft. For reference, black powder rifles shoot under 1400 fps to prevent the bullet stripping in the rifling. A good investment is a custom bullet mould made for your purpose. The cost of the mould can be recouped from the price of 50 factory bullets for your big bore.
With the advent of bullet powder coating, the important aspect of bullet lube is handled well. It is necessary only to match the bullet hardness to the velocity for good accuracy. I am using a polyester powder coat, and I powder coat twice. I use lead wheelweight, which hardens requisitely.
Load development with these bullets was with soft lead, BHN 8, with a single coat of polyester powder coat. I got minor leading in the barrel, there was a lead smear on the leading edge of the grooves, which cleaned up easily. I cast the next batch from a bucket of reclaimed range lead, used two coats of polyester powder coat, and they were BHN 12. There was no visible leading in the barrel after 15 rounds, but the first few patches came out grey, and the barrel was clean soon after that. Lead hardens over time. Let your lead bullets sit for a few weeks, and then check hardness. My as-cast, water quenched 12 BHN was 16 BHN a couple of weeks later. I suggest that if you want to exceed 2100 fps, you should use wheelweight lead or similar, to get the BHN to 16+ . But if you have a sufficiently heavy bullet, you don't need to go any faster.
Because of the powder coat, lube grooves are not required, and hence a lube sizer is not necessary. My requirement of sizing to +0.002 inches meant shopping around. I could order custom sizers from Lee, but they take several months. I found what I needed in the Lyman lube sizer dies off Ebay, but I was not going to buy a lube sizer press as I prefer the push-through method of bullet sizing. So I made a lube sizer die holder to be able to use the lube sizer dies in a push-through configuration on a single stage press. This is just a threaded bar with die body threads, drilled through then lathe turned to accomodate the diameter of the Lyman / RCBS lube sizing dies. It has a step at the top to prevent the die from moving up and out. The one Lyman sizer I could not get was .418, so I tried to make one using silver steel bar, and it sizes to a shade under .417 right now.
The powder cast bullets are sized, and Lee Liquid Alox used as the sizing lube.
The benefit of the system is with custom sizes, a custom size insert can be made. A holder for the inserts is easy to make from a threaded bar that is equivalent to a threaded die body.
When using powder coated bullets, care needs to be taken to ensure the case neck does not shave the powder coat off. Any brass that was previously crimped will shave the powder coat, so the crimp has to be removed. This is done by slightly expanding the case mouth. It only needs a very slight case mouth bell, if you can feel it in your fingers but not necessarily pick it up with your eyes, it's enough. Then chamfer the case neck to smooth on the inside. Case anneal is advised after every couple of firings as a preventative measure against case mouth cracks. Brass for big bore rifles can be expensive. Treat them carefully to get the most firings possible. For .375 and .416 case belling, I found that an old RCBS 38 special decapping rod assembly worked very well. Grinding the end to smooth round, and the steps in it bell the .375 and the .416 brass when fitted to a RCBS 458 case mouth expander die. My RCBS 500 Jeffery dies come with a case expander.