There are three ways to handle this. I have tried all three and say they all work very well so it is up to the shooter to decide what works for the individual shooter. I am writing this aimed at a right handed shooter. If you want a lefty perspective let me know.
I am starting my description when the gun is at slide lock or the shooter is ready to perform a retention or tactical reload.
1. Use the index finger of the off hand to slightly rotate the gun in the strong hand so the strong hand thumb can reach the mag release. (The index finger applies slight pressure to the outside of the trigger guard to help the gun rotate in the grip.) Reload with a full mag and readjust grip of strong hand back to the correction position. Advantage was that for me this technique was faster then technique #2.
2. Maintain grip of strong hand and use thumb of support hand to release the mag. The advantage is that the grip is not changed/compromised. This can be pretty fast as well.
3. My favorite and I believe the best is only possible if you have a gun that has an ambidextrous safety or one that can be changed to the right side of the gun. (Mag safeties are on the left side.) Switch the mag release to the right side and use your trigger or middle finger to release the mag. Advantages is that the grip is not compromised and it is fast! I use my middle finger as I can keep my trigger finger straight along the barrel while reloading. This techniqued has allowed me to take a full half second off every reload.
I hope this answer your question. If anything I wrote isn't clear drop me a PM.
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