The first thing I would do is determine the amount of bullet jump it takes for the bullet to touch the rifling in the throat. The maximum I can seat the bullet out in my JM Marlin is 2.550"
Any longer and you cannot eject a loaded cartridge, without removing the lever and breech block. After shooting over 500 rounds in mine, trying to determine the best loads, and never really getting the accuracy I wanted, I measured the bullet jump to the rifling and found that Marlin had reamed the chamber such that it takes 1/2" of travel before the bullet reaches rifling! Because of the length limitations of the port, there is nothing I can do to remedy this. Maybe your rifle is chambered differently. I wish Marlin New Haven was still around because they would at least hear some angry noises from me.
That bullet jump is why my Marlin will never shoot cast bullets worth a hoot
My testing revealed that my rifle is very sensitive to velocity extreme spreads. These lever actions are extremely flexible and vibrate so much that they will fling shots with high and low velocities.
Varget was very bad, a flung shot on every target.
Let me recommend, if you reload, to first of all, stay within factory velocities and don't try to push the bullet faster than factory velocity. I got blown groups and the action would unlock. These lever actions are not rigid, they are appropriate for factory pressures, but not more. I used exclusively 170 grain bullets. These bullets are fine within 30-30 velocities, everything I have read is that they will expand nicely at the slower velocities of the 30-30. I determined things were best when I pushed my bullets around 2150 fps. I did shoot 150's, and they came out around 2250 fps.
I can recommend as a starting point, IMR 3031, IMR 4064 and AA4064. My rifle is a 2 to 3 MOA thing at best. I never shot a ten shot group under 2 inches at 100 yards, and the group increases with distance.
My loads are way above manual, because my chamber is huge. The only real data of value are the velocities and group size.
Do note, what is in the popular, monthly, in print magazines is largely farcical. I have read any number of articles where the gun writer is shooting three shot groups and claiming sub MOA accuracy with lever actions. Until I see some 20 shot groups at distance, like this one
(this shooter is still active, now he is 73 years old, and shooting Master Class scores in Smallbore Prone)
I will consider claims of MOA, sub MOA lever action accuracy to be unsubstantiated, wishful thinking.
I did this with a match Garand, prone with a sling, with irons! Those high shots are due to me having to break position. My 30-06 rounds were in the ammunition box, bullet up. And I got so hot, and sweated so profusely during the string, that the rounds were slipping out between my thumb and forefinger. High power is a physically stressing sport! I had to break position, rearrange all slow fire shots base up, so I could get a fingernail in the extractor groove and lift the cartridge. That high shot was due to the change in position on the rifle, and so was the other high shot, until I figured out, make an elevation change on the rifle.
Now in terms of range, I consider the 30-30 fine out to 200 yards.
but look how much the bullets drift at 300 yards.
same with AA4064
someone could run a ballistic calculator, and I will bet, at 300 yards those round nose bullets are going so slow, that they have all the wind drift of spit wads. I don't know the actual distance they start to float in the wind, because Talladega does not have 225, 250 nor 275 yard targets. But somewhere past 200 yards, the bullet drop and bullet drift is excessive.
These lever actions are not for the Western, over the horizon, 700 yard game shot types. You plan to shoot from mountain top to mountain top, get something else.
In the eastern part of the US, where the woods be thick and deep, a 30-30 will do just fine.