I'm using an aluminum clone of the Lyman 314299 mould with pure wheel weights and water dropping them to add a little more hardness. They drop .315" (wheel weights increase in size when water dropped) and are a touch over 200gr. I put on .308 gas checks and size them to .314". I tumble lube with Lee liquid alox; it's messy but it works for me. I'm probably only in the 1200-1600fps range with mine as I'm cheap with the powder but I have pushed them closer to 2000fps. Currently I'm using mostly H4198 and SR-4759 (discontinued) but do use some Unique for really light 50yd loads.
It isn't necessary to slug your bore but you have to be lucky for it to work without knowing your actual bore/groove diameter. With my Mosin Nagant I just got lucky. I tried .311" gas checked bullets since that's what I had on hand for other rifles and they worked great. Still haven't slugged it as it shoots great and I don't see a point. I also had a Lee Enfield I tried to get cast bullets to work in without slugging it and it just tumbled and/or leaded badly. After slugging it I found out it was .315" which explains why my .311" and even .314" wouldn't work past ~700fps.
There are various videos on YouTube about how to slug a bore. Found this one which is informative:
https://www.youtube.com/watch?v=ErFaJlUVs1Y
One issue I have is he uses a caliper to measure the slug. Calipers should be assumed to have a tolerance of +/- .002" on the reading it shows. Many are better, a few are worse, but I still don't like trusting a caliper for any tolerance of .002" or less. A cheap micrometer will do the trick and can be bought for $20-40. The one I keep at my reloading bench was around $30 from Lee Valley and can measure to .0001" but I'm sure the tolerance on that is at least +/- .0002".
If you don't want to buy a box of lead balls you can cut open a 00 buck shotshell. 00 buck pellets should be .330" and if it's a cheap shell it shouldn't be extra hard or copper plated.
You want a cast bullet .001" to .002" over your groove diameter but I often go .003" and have gone as high as .004" and it works with a medium hardness alloy (my bullets are probably around 12-16bhn).
There is a velocity limit with cast bullets and it's common to use powders with a faster than normal burn rate for the cartridge in question. For .303 British I use a lot of H4198.
It could also be a good starting point to use the Hodgdon reduced recoil data. They say you can use any data with H4895 (might also include IMR4895 but I forget) and reduce the starting load to 60% of what's listed (multiply the starting load by 0.6) and it's a safe, lower velocity, reduced recoil load. From there you can work up to the normal max watching for barrel leading or erratic accuracy. You are pretty much guaranteed to have leading or accuracy issues before reaching max pressure. The upper limit of normal cast bullets with gas checks is 1700-2200fps. There are ways to go higher but things can get weird (like zinc bullets instead of lead).
You may also want a neck flaring die (Lee makes a cheap one that works from .20 to .50 cal) to add a touch of flare to start the bullet easier and prevent the case mouth from shaving your bullets.
In summary:
-slug your bore and find cast bullets 1-2 thou bigger
-find reduced velocity/recoil load data to use as a starting point (the Lyman Cast Bullet Handbook has
loads of data)
-load the ammo as normal except flare the neck before seating the bullet