Using a 50gr bullet in the .223 and a 180gr bullet in the '06, H335 is a better choice. For the .223, H335 is an ideal choice, actually, while H4895 will require a compressed load to even reach starting pressures. For the '06, H4895 would be a better choice than H335, but the latter will do the job with about an 80% load density. If you choose a lighter bullet for the '06, H335 becomes a more appropriate powder, but it's still a compromise.
If you really must have only one powder for both, H335 is the answer.
Never tried H-335 in my 30-06 but it is listed in the manual. With lighter bullets in the 30-06, book velocity is right there with the other powders. When you move up in bullet weight it seems to not make the best velocities.
I know H-335 is great for the .223 and in mine produces great groups with the heavier 69 grain bullets even though it's usually better with a lighter bullet.
I would have to agree on H-335.
The "one powder" mission for the two cartridges in question is a reasonable goal, depending on bullet weights. In fact, the bullet selection might even be the focus.
If you liked 60gr Hornady HP's, and 150 Nosler partitions, for instance, either powder would be just fine, and would have a very wide range of suitability. Of the .223's I've owned, H4895 has been a top accuracy powder in every one, with 50-60gr bullets. The same powder has always been suitable for several '06's, especially with 150/165gr bullets.
At the moment, if you can find either powder, you should get out your wallet.
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