If you really want to try, then you need to shoot your (cast) bullets at at least 2400 fps and make 1750 fpe. Or 1000 fpe for Roe.
This means a bullet of at least 137 grains. Or 78 grains for Roe.
Perhaps you might manage to get your bullets up to 2400 fps with usable accuracy and consistency. I don't mean shooting a 3 shot group with discounted fliers, but real consistent first-cold-barrel, day-to-day, year-to-year consistency. Such as we expect from jacketed bullets.
But you must also "design" your bullets to "expand in a predictable manner". Whatever that means.
If you know how to combine these conflicting requirements, good luck.
Even then, in Scotland (unlike England/Wales) the Deer Act gives the Secretary of State:
"power to make such order as he thinks fit regarding the classes of firearms, ammunition, sights and other equipment which may lawfully be used in connection with killing or taking deer, and the circumstances in which any class of firearms, ammunition, sights or other equipment may be so used."
Deer (Scotland) Act 1996
So if he/she doesn't like the idea of you using unjacketed lead bullets, they can ban it as they see fit, after due consultation. I don't think anybody likely to be consulted would regard homemade lead bullets as being a desirable alternative.
Fortunately here in England/Wales we don't have such restrictive legislation.
Personally I don't see the price of deer bullets per-se as being of any significance in my hobby. For target shooting yes absolutely.
Gad. How much cast bullet shooting do you do?~Muir