First one needs to decide what amount of money they are willing to spend on the new gun. Secondly style needs to be decided such as pump, over/under, semi-auto, etc. Thirdly going and trying different brands and how they pull up and fit. Lastly going home knowing that you made the right choice.
There is a wide price range in shotguns today. They range from $300 all way upto $6,000. When choosing my gun I set my price to less then $1,500. Well with my trade in I ended up paying $700 cash. So I did stay under my price range. The pumps and single shots are on the lower end of the shotgun spectrum. Your semi-automatic is next in the spectrum and then the fine over/unders are on the top of the spectrum.
Next one must decide what brand and style of shotgun they like. The brand names out there today are Remington, Browning, Winchester, Benelli, Escort, Stoeger, Franchi and many more that I have not mentioned. Remington makes the 870 pump, 887 pump, 11-87 semi-auto, and the Sp-10 semi-auto. Browning has the BPS pump, Gold semi-auto, and the Silver semi-auto. Benelli has the Nova and Super Nova pumps, Super Black Eagle II semi-auto, and M-2 semi-auto. To name a few. Currently the only pump and semi-automatic shotguns manufactured in left-hand is Remington and Benelli. The gun I ended up getting was the Benelli M-2.
Go to your local gun dealer like Cabela's, Bass Pro, or your hometown gun shop is always a good choice. Pick different guns and pull them up and swing them. This is the only way one can tell if the shotgun fits. When I picked up the M-2 the pull up and swing motion were fluid. These movements should be an extension to your arms. If it doesn't feel right try another gun.
Lastly take your gun home and feel confident in your choice. If you think you may need to practice with your gun pulling up the do a daily excercise of pulling the gun up like you would do in the field.
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