Here is a portion of one article Comparing .45 GAP and .45 ACP +P Factory Ammo Let's be very clear that the .45 GAP was originally developed to emulate “standard pressure” .45 ACP velocities. Therefore, it is perfectly fine and legitimate to compare .45 GAP ammo against “standard pressure” .45 ACP ammo. My sources have stated that Speer factory 45GAP ammo typically runs between 19,900 psi and 22,500 psi (slightly under to slightly over the standard pressure 21,000 psi maximum for the .45 ACP; maximum +P ACP limit is 23,000 psi). GAP handload data runs between 16,400 psi and 22,500 psi. The GAP was not marketed to equal or outperform .45 ACP +P ammo, even though the .45 GAP can dip into the general ballpark of +P rated .45 ACP velocities. In fact, Double Tap .45 GAP ammo brings the velocity up to the same level as +P .45 ACP ammo velocities. Remember that while .45 ACP has more space for powder, +P ACP brass still only has a 23,000 psi SAAMI pressure maximum just like the .45 GAP. So 45ACP can be handloaded dangerously over-pressure if one is not careful, especially since modern powders don't need as much space to perform as the old time stuff. Side Note: It takes two cases of .45 ACP to handle both standard pressure and +P pressure ammo, while the .45 GAP uses one and only one strong case for all of its loadings.