BARREL LENGTH plays a role in this as well. While +P loadings may not be required in most cases, they can compensate for short barrels and the resulting loss in muzzle velocity. For example: In 9mm, the 124gr Gold Dot is a good choice in barrel lengths of 4" or more. In compact guns of 3.5" or less, a higher-velocity loading would be advised. This is Dr. Roberts take on the issue:
Actually, all our testing has traditionally been done in 4" barrels for 9 mm, .40 S&W, and 4.25" for .45 ACP, although recently most of the organizations we test for have been asking for 5" barrel data for .45 ACP. There is really no difference in performance between a 3.5" and 4" barrel in 9 mm and .40 S&W. In .45 ACP, we see a reasonably significant change in performance going from a 5" to 3.5" or less barrels. Since almost all viable pistols in 9 mm and .40 S&W use 3.5-4.5" barrels, there are no "short barrel" worries for serious end-users. Likewise, almost all .45 ACP platforms worthy of hard use use barrels greater than 4", so again, the short barrel question is moot...
Please be aware that if you venture into guns with a barrel length of less than 3.5", you're in uncharted territory.