for 243can I use varget? How much grains?
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Wanting a load that does say 58grn heads and one for heavier loads at 100grns.
You will not find one powder which works well for both bullet weights, they are too different.
Assuming you want the 100 grains for Scottish deer, this will be the challenge. You will struggle to meet legal requirement of 1750fpe (i.e. 2807 fps for 100grn bullet) with anything but a well matched powder. You may struggle full stop if you have a short barrel. Its no secret that it can be challenging to get a .243 to shoot 100 grains plus at deer legal energy.
Varget is a poor choice, its better suited to a lighter bullet. On their website Hodgdon publish a maximum load of 33.7 grains for 2838 fps for 100 grain bullet, which only just makes the requirement. At the start load it only makes 2674 fps i.e. 1587 fpe. Whether you will actually achieve 1750 fpe in your rifle is questionable, and it may not be possible to use a maximum load, it may not give good accuracy, and it will be stressing your rifle barrel and brass to their maximum. Typically manufacturers use long barrels to measure their data, to give the most flattering results, and it can be difficult to replicate these in real rifles.
I would be looking for a powder which is published to deliver at least 2800 fps velocity at the starting load.
E.g. Hodgdon data shows six powders which can do this with 100 grain bullets. The best one even gives 2927 fps at the start load, and 3046 fps at maximum. Thats 1902 - 2059 fpe which is terrific for a .243. Unfortunately it is Winchester Supreme 80, which I don't think is commonly available here.
Once you have identified some powders which can definitely work legally at 100 grains, then see if there is data for them with your lighter bullet. If there is any, I think you will find that it will not give velocity anywhere close to the better powders.
By the way, your Varget looks as if it will work very well for 58 grains, so I'd stick to that for the .222 and the .243 58 grain bullet. Then select a better powder for the 100 grain bullets.