How hard does it need to be?

I've cast 500, 200gr semi wad cutters (Lee 452-200) and my mix of 50% coww and 50% pure lead with a little tin added, makes my mix of 3,2,95 with a bhn of 10.5. These are for the 45 ACP and I think should be about right. . . . Now I'm working on making a bunch of 300gr large flat nose (Lee 452-300 gc) for use in my 454 Casull's. They air drop at 12.5 bhn and water drop at 18.5 bhn. Since they are gas checked . . . . How hard do they really need to be? Since the gas checks take care of one of the reasons harder bullets are needed, would the air dropped bullets be ok . . . Or do I need to strive for the harder bullets? I usually load somewhere between mid range and full house for the 454. I think I'm pushing them around 1350 to 1450 fps. Any advise is appreciated . . . Thanks

I recently drove air cooled COWW 175.5gr GC saeco .30 cal tapered boolits in a .308 bolt gun at 1800 fps with 17 grains of 2400 and 7 shots went into .339". Even at 23.0 grains and some 2200 fps I was still getting around 1.50" groups. I would say to follow the group sizes and the effect that powders and their charges have on them. Gas checks insulate the projectile and give them a uniform shape, from slug to slug. Experiment and see what results.

First! I water drop from the mould all of my cast boolets for any caliber handgun or rifle!
For the 45 acp and all handgun boolets shot under 1,000 fps i use ww alloy .
All bhn is before water dropping ! after WD about 28 to 30 bhn

For my FA 454 casull's which ive been shooting about for 25 years i use a alloy hardness of about 16bhn from 1,100 fps is what i usually shoot in the off season using 9grs of Tightgroup or Bullseye powder. no checks and up to 1,450 with gas checks for full house i use linotype and gas checks . the 1450 fps load is comfortable to shoot and you can shoot as many as 100 rounds per range , perhaps my most accurate cast load is the LBT300gr gc LFN mould using 28 gr of 296 @1,450 fps, I have shot several 1/2 " group's at 50 and 2.5 " at 100 yards from a bench using a 2x7 pistol scope for accuracy testing. I rarely go over 1,450 fps except when hunting with jacketed bullets the now discontinued Nosler 260gr partition has worked excellent for me and is super accurate ! still have about 200 in my stash, which will last me for the rest of my hunting days .

I go by fliers. When a few are wide of the group, I harden more.
If I use 50-50 I oven harden and they still need a GC to be accurate enough.
I shoot straight WW PB boolits at 1350 when water dropped.

It's been my experience that 95% of my shooting can be done with lead that BHN' s between 9- 14 usually around 12 BHN. You'll have to experiment to find out what works best for you.

I'd go with AC (air cooled) for your 45acp should be fine.

I would go with WD (water dropped) for your 454, just for more consistency and ease of load development.

I'd go that route also.
I seem to cut off between air cooling and water dropping at the point I add gas checks.