1Shot has the right idea.
A really hard bullet alloy is not necessarily the answer. Its all about
"load balance". Bullet/style/weight/quality, powder type or burn rate/charge/load density, seating depth/concentricity, and, rifling twist rate for the velocity desired/achieved.
Shooting cast bullets with smokeless does not necessarily require a hard alloy.
With the proper bullet diameter for your bore, properly applied ,quality bullet lube, and a very straight/concentrically seated bullet that will enter the rifling smoothly, will give the load combination the basic building blocks needed for accurate cast bullet performance.
For instance, In my .308Win target rifle:
I shoot straight wheel weight bullets (air cooled). (Lyman Bullet Mould #311299,200 grain) I size my bullets .002" over my "ACTUAL" bore diameter (sized to .310").
I use either Hornady copper gaschecks, or my own .015" thick brass gaschecks.
Lyman super moly bullet lube.
I Seat the bullets around .010" into the lands/rifling to give the bullet a chance to start down the bore perfectly straight. A straight started bullet, and good ignition pressures aid in a
well sealed/contained initial combustion. A cast bullet
"lightly" engraved into and supported by the lands/bore slightly increases the initial ignition pressures, allowing for more consistent load standard deviations.
Cast bullets
DO obturate/bump up proportionately to the pressure created, and relative bullet alloy hardness. (If too much pressure is applied to the bullet), the bullet alloy
tends to "YIELD", ( and
doesn't return to original dimensions after the pressure drops), and the damaged bullet continues for the trip down the bore destined to ruin accuracy! A damaged cast bullet traveling down a bore will leak propellant gasses erratically, spray bullet lube from the grooves ahead of the bullet, and creates the perfect conditions for leading and bullet instability. It is all about finding the perfect balance of pressure/load for the alloy you have casted.
It can be allot of work to find this load, but pays off with great performance.
As load pressures increase, some component of the load recipe will eventually start to fail. (The accuracy will be your best indicator with cast bullets). Knowing which/how these components are failing, (lube/alloy/or both), aids the cast bullet shooter in assessing the problem and adjusting the loading components to overcome the problem/failure.
I have some loads that will shoot this 311299 bullet no faster than 1700-1800fps, without accuracy falling apart. (Faster powders)
With a powder change, I was able to increase the effective/accurate velocity up to 2000-2200fps. (Slower powder, IMR4350)
Again, with another change in powder, I can get 2300-2400fps ACCURATELY. (IMR4831), (2-3" groups at 200 yards).
One of the biggest contributing reasons I can achieve this velocity is the twist rate of the barrel. (1:12"). faster twists like 1:10", tend to induce more gyroscopic stress on a cast bullet at identical velocities. the faster the bullet is spun, the more dramatic the inherent flaws in a cast bullet are exposed. Sort of a velocity threshold, so to speak.
Get yourself a new copy of the Lyman cast bullet reloading manual. It is a wealth of information.
Good luck on your load development.
Allow yourself time, and a few load failures. Cast shooting is challenging sometimes, and it can take dedication/determination.
When it all comes together, its very satisfying indeed!
I hope this information helps you.
