Saw someone mentioning about how they could stuff an extra round in their mags. This is bad thing to do.
Let us use the AR15 as an example. If you load 30 rounds into a USGI mag it can be a little more difficult to insert that fully loaded mag when the action is closed. In some newer magazine designs, like the PMag, extra space is given so that it will be easier to insert that mag when fully loaded.
The problem becomes when you stuff that extra round in, you not only use up that extra space, but can cause binding or other issues. For another example, I found I could stuff an extra round in the Beretta 21A magazines, which would cause the pistol to not feed correctly.
If you want more rounds, buy a bigger mag.