I have seen lately a lot of defective new guns. My perception is that it is more than normal. You need to verify every gun you use for defense is working properly. With today ammo cost this may cost more than the gun. For someone that is already well trained and experienced it takes maybe 200 rounds or more of practice ammo to break-in/ get the feel of the gun. You should also practice drawing the gun from your holster. If you have more than one holster you might need to practice with all of them.
Testing carry ammo can be real expensive. Maybe you already have a brand that you like. You are still going to have to make sure it works well in the new gun. I recommend shooting ten shot groups at 3,5,10,15 20, 25 and 50 yards. Label and keep your targets. Refer to them often. It is common to see two identical guns have different point of aim, point of impact. Every gun needs to be tested.
Say you have a Glock 17 now you need 35 rounds to carry in the gun and your spare mag. I think you should consider also keeping a couple more mags loaded and ready. At the beginning of the winter season I shot up all the ammo I carried with me the last year. As you can see you may need 150-200 rounds of that real expensive carry ammo.