For instance, most .243 Winchester loads top out at 100 grains and few .243 rifles have a twist rate (1:9″ and 1:10″ are very common) to stabilize bullets heavier than that.

On the other hand, 6mm Creedmoor rifles usually have a faster rifling twist (1:7.7″ in the case of the Ruger Precision Rifle), so the cartridge is commonly available with 103 grain, 105 grain, 107 grain, 108 grain, and 112 grain bullets.

The 6.5 Creedmoor commonly uses 120gr, 140gr, and 143gr bullets.

Note: while the powder capacity figures listed below do give a good indication of the differences between the two cartridges, exact case capacities vary slightly according to the brand of brass used.

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).