I have found that using heavy for caliber or bullets requiring a tight twist to make sub-sonic reloading difficult. All my sub-sonic reloads ended up same or very close to that bullet weight which I normally shot in that rifle. I also found that in bottle neck cases I ended up using same propellant as standard loads.
In straight necked cases I used S265

I am very weary of trying to make subsonic loads using necked down type cases - .308 - 243 for instance.

What will this load application be ?

I don't want to say exactly what it's for, in case I run afoul of the bunny huggers.

Problem animals up to 20kg body weight.

I'm not concerned about reducing the load and using dacron.

I've used dacron in a .243 before when testing different powders and I didn't like the amount of air in the case.

I'd prefer to use 100 grain cast bullets, unfortunately the moulds are extremely rare and it's not worth my while having a custom mould made if the 87 v-max is accurate at 1100 fps.

Trail boss has as equivalent VV N320 and MS200.

https://hodgdon.com/wp-content/uploads/2020/07/subsonic-data-final.pdf

I must try explain my little wad inserter to you. Uses wife's make up applicator.

I have some serious misgiving about trying to load an overbore cartridge (including the 243 Win) to subsonic velocities.  There are many opportunities for mishaps here.

Generally, subsonic loads like bullets that are shorter (lighter) than what works at normal velocity levels.

My suggestion would be to go with reduced capacity cases, to prevent all the possible dangers that can damage your rifle.  Unfortunately, these are expensive.

A 303 or similar rifle converted to a handgun case (e.g. 32 S&W Long, 32 H&R Mag, 327 Federal?) and loaded with +/- 160gn cast bullets should give you many hours of quiet plinking and varminting pleasure at a very low cost per shot.
I suggest 303 as you can use the original barrel and just set it back and re-chamber.  This should keep the cost manageable.  I imagine you are too much in a hurry to go through that process now?

My own low-cost low-noise rifle is a 44 Mag built on a Lee no4 action salvaged from a scrapped 303.  The 44 case head fits the 303 bolt face perfectly.  Probably a bit too much punch for your purpose.

Hodgon would not publish subsonic data if it was a danger to the user. A heavier projectile is easier to get below speed of sound without having a very low charge of the pistol propellant. This ensures a better initial pressure and less sensitivity to a variation in charge. The more momentum will help with penetration if that might be an issue. Loading S335 down to subsonic levels is not on, as you can then have detonation being possible.

I have some serious misgiving about trying to load an overbore cartridge (including the 243 Win) to subsonic velocities.  There are many opportunities for mishaps here.

Generally, subsonic loads like bullets that are shorter (lighter) than what works at normal velocity levels.

My suggestion would be to go with reduced capacity cases, to prevent all the possible dangers that can damage your rifle.  Unfortunately, these are expensive.

A 303 or similar rifle converted to a handgun case (e.g. 32 S&W Long, 32 H&R Mag, 327 Federal?) and loaded with +/- 160gn cast bullets should give you many hours of quiet plinking and varminting pleasure at a very low cost per shot.
I suggest 303 as you can use the original barrel and just set it back and re-chamber.  This should keep the cost manageable.  I imagine you are too much in a hurry to go through that process now?

My own low-cost low-noise rifle is a 44 Mag built on a Lee no4 action salvaged from a scrapped 303.  The 44 case head fits the 303 bolt face perfectly.  Probably a bit too much punch for your purpose.

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From past experience, I just would not even try down load to sub sonic with necked down cases, or magnum over bore cases. I think the perfect sub sonic cases are straight walled cases or second best necked up cases, like the  8mm 06 or 6x45mm (6mm in a .223 case) these cases are just so much easier to load for. We tried with MP-MS 200 and I think it was 321 in 243 cases and although subsonic was easily obtained, absolute explainable 120 fps jumps in velocity and shot report made me think that we were perhaps playing  beside devils and demons on some shots.

https://youtu.be/QXnYAjPpHMI

https://youtu.be/HQOEy6bofKg

https://www.thetruthaboutguns.com/charlie-sisks-sub-moa-subsonic-243-win-load/

Hi gents
New user here based in PE. Looking to learn and share.
Any further developments on this topic? Would be interesting addition to a 243.
If the target animal is not too big a plain subsonic silenced .22 is hard to beat.
Regards

A .22 can really surprise with its killing efficiency, or let you down terribly. That even with HV ammo having a bonus of 300fps over the subsonic.

A 6mm 90 to 100gr bullet will have double or more momentum than a 40gr .22 bullet and will therefore give better penetration. This will be of benefit when a skull has to be penetrated.

Lately I've been researching reduced loads for a 7x64. For good overall knowledge as well as calibre specific information, try the following:

https://www.google.com/url?sa=t&rct=j&q=&esrc ="s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwipjJf7x737AhW4QkEAHf_jBY8QFnoECA4QAQ&url=https%3A%2F%2Fimrpowder.com%2Fwp-content%2Fuploads%2F2016%2F11%2Fh4895-reduced-rifle-loads.pdf&usg=AOvVaw2NOvnZearGn4fjeTV0qqVd"

If the above link doesn't work, just Google Hodgdon H4895 Reduced Loads.

Also search for "Trail Boss Reduced Loads"

For local powders, search for an article entitled "Verminderde Ladings" by Hercules Louw.

Here is the file

This Hercules guy seems much clued up ?

Hercules Louw has been a long time contributor to SA Hunter magazine and is very knowledgeable about matters pertaining to reloading and shooting.