The .243 has some drawbacks. The 1-10" twist, so it won't stabilize the heavies, is one. And the other is it's right on the edge of "overbore". Using fast hot loads and shooting long strings will burn barrels out quickly with them. Normal loads and shooting isn't so hard on them. Improved and modified cases abound in 6mm/.243" because of this. Seems like everyone loves them until they go through a lot of barrels. The trick then is how to not shoot them hot. As mentioned above, keeping pressures down and burning a slower powder for a longer push, the poster uses H1000. I do the same. in most of my cartridges. I also try not to go near max with the load no matter what the powder is. Lower pressure equals barrel life. The high BC bullets nowadays take a lot of the work out of wind calls. They retain speed so much better than the "plugs" we've used since the '50's. More speed=less time of flight=less drift.
If I were to get another .243 today, I would make some changes. Then again, the new crop already has. Throated for long bullets means the lands aren't as close to the powder plasma as it leaves the case. Less to catch on, less to get deteriorated by flame. Steeper shoulder. The 6CM (and most other new 6mm's) have a 30 degree shoulder. This is still good for feeding and in 6mm it's more efficient in internal ballistics. I would NOT go with a 40 degree shoulder. Too steep and restricts flow. I worked with a lot of AI cartridges and it's the same with all of them. The pressure curve at the top is steep. All it takes is a really warm or hot barrel and you go right over the top, flattening or blowing primers. The one thing an AI shoulder will do is mitigate case growth. But, to make them work right, you gotta keep pressures low. 30 deg. shoulders are the best mix of flow vs. pressure efficiency.
The one thing I would've changed with the new cartridges is I would have made the case geometry the same as the .308 based cases. Granted, you lose a tiny bit of capacity, but next to nothing in useable capacity. the 6 and 6.5CM cases are a bit too flat and would feed better, if their case shape was exactly that of the .243/.308.
The little bit shorter case allows seating the bullets out as well in short actions. You can still use all the long bullets in .243, just not seat them out as far. If you consider doing this, a well designed and long throat is must, IMO.
