Due to the fact that it uses the 6.5 Creedmoor as a parent case and performs similarly to the .243 Winchester, the 6mm Creedmoor is most often compared to those two cartridges.

Aside from having very slight differences in their overall length, the only other difference between the 6mm Creedmoor and the 6.5mm Creedmoor is the fact that they shoot 6mm and 6.5mm bullets (.243″ and .264″) respectively.

The cartridges are otherwise identical up to the shoulder and both cartridges have the same SAAMI maximum pressure of 62,000.

On the other hand, the .243 Winchester has a longer case length, but a shorter overall length than the other two cartridges.

The .243 Winchester only has a maximum SAAMI pressure of 60,000.

Even though it has the longest case of the bunch, the .243 Winchester has a 20 degree shoulder, which is not quite as sharp as the 30 degree shoulder shared by the 6mm and 6.5mm Creedmoor cartridges.

All all three cartridges have the same rim .473″ diameter and they are all close enough in overall size that they are used in short-action rifles.

The other big difference between these cartridges is that the 6mm and 6.5mm Creedmoor were both purpose built as competition shooting cartridges and were designed to use the longest and heaviest bullets available for their caliber without intruding into the powder column.

The 6mm Creedmoor was also intended for use from the start in semi-automatic rifles like the AR-10 as well as bolt action rifles.

For these reasons, long, heavy for caliber bullets will fit in the magazine and chamber in 6mm and 6.5mm Creedmoor rifles. Since those rifles also typically have a relatively fast rifling twist, they’ll accurately shoot heavier bullets with a high ballistic coefficient.

This is not the case with the .243 Winchester (or other cartridges based on the .308 Winchester like the .260 Rem for that matter).