Easily the most frustrating and common mistakes that I see shooters make revolve around an improper and/or inefficient grip. We have covered at length the advantages that a biometrically correct and ergonomically efficient proper grip will provide you, but we haven't really provided insight as to the common mistakes that people make. Let's take a look at some of these grips, and why they are insufficient and sometimes downright dangerous.
The Tucked Thumb Grip
This is probably the most common mistake that I see in regards to an incorrect grip. This is when the shooter grabs the grip frame of the handgun with their shooting hand, and has their thumb diagonally draped across the panel where their support hand should go. Inherently, this prevents the "meaty" part of the palm on the support hand from making maximum contact with the grip. This provides a path of lesser resistance on the side of the support hands, which results in a movement upwards and towards the support hand side during recoil. This negatively impacts the speed of front sight acquisition during the follow-up shots. Furthermore, since we don't have maximum contact with the weapon- we also don't have maximum recoil control. To compensate for this, we are left with the poor option of simply squeezing our hands harder. This can result in hand tremors which will negatively impact sight picture. Moreovoer, since you are having to exert more muscle and less technique to control the firearm, you will have a harder time gripping the gun consistently with this method. This means that you will have trouble producing repeatable results in regards to your point of aim/point of impact. In short, your grip sucks. Fix it!
The Tea-Cup Grip
An outdated yet not dead grip brought about during the days of the snub-nose revolver's hayday, the tea-cup grip has been a long time Hollywood favorite. Basically, the shooter will place their support hand underneath the magwell and hold the firearm and shooting hand as if they were a "tea-cup" in a futile attempt to add stability to the gun. While I guess one could argue that this grip is more stable than shooting one handed, I would argue that it's only slightly better. You aren't doing yourself any favors in regards to recoil control or providing faster follow-up shots, and you aren't providing any advantage to basic weapon manipulations. In short, your grip sucks. Fix it!
I think we can all agree that if you can't clearly see your target, you shouldn't shoot at it. I think we can also agree that it is very likely that a violent encounter will occur in low-light (or no-light) conditions. That is why, in my personal opinion, guns should come with lights. At least all guns designed for fighting and personal defense should come with lights...
I have participated in multiple low-light training events where many of the experienced shooters suddenly realize how dramatically defficient they are at shooting in the dark. They don't have good experience or acceptable skills to be able to effectively manipulate their offhand light and their weapon at the same time. Their times go to Hell in a handcart, they fumble their basic weapons manipulations, their shots and follow-up shots are slow and inaccurate, and their light seems to act as a similarly polarized magnet to their front sight and pushes it off track. They focus on the center of their light's beam only to find out that their front sight is nowhere near the target. Then they correctly get an acceptable sight picture only to discover that their light is now pointed somewhere off in the distance. They repeatedly "paint" themselves and their location as they try to manipulate their handgun during reloads, malfunction corrections, etc. This foreign object (flashlight) somehow destroys their skill sets that they thought they had mastered.
Then there's the other guy... The guy on the line that is shooting at his usual pace, correctly manipulating his firearm, reloading at blazing speeds, etc. He's performing up to par, and he's scoring good hits in the process. The multiple people in the first group are wondering "How in the Hell is that guy so good?" Then they look down to find that he has the difference maker... He has a weapon-mounted light. Yes, this guy who is performing within tenths of a second of his usual par times has a light mounted to his firearm. He makes only minimal adjustments to his grip in order to manipulate his light. He still grips the firearm with two hands. He isn't fumbling around with a flashlight and a magazine in the same hand during a slide-lock reload. He has the advantage.
In a gun fight, why wouldn't you want this advantage?
You have the option to employ a weapon-mounted light with a minimal additional investment. Instead of purchasing another "safe-queen" gun that you're going to fire 250 rounds through (total), make the investment to purchase a quality weapon-mounted light and a holster that will securely fit your firearm with the light. Is it a little extra weight? Yes. It is a little harder to conceal? Yes. Could it honestly make the difference in being able to engage your target effectively and not being able to fight because you never train with a flashlight in your hand? Absolutely.
Note: this doesn't mean that you should neglect training your freehand flashlight techniques. It doesn't mean that you shouldn't be very competent and familiar with weapons manipulations with a foreign object in your support hand. It simply means that in a gun fight, we want every possible advantage that we can get. Every firearm that I have that I plan on possibly using in defense of myself or loved ones will always have a weapon-mounted light, night sights, and plenty of rounds that have been through it during training. I recommend that you do the same.
In virtually every beginner's shooting course that I've ever taught, at least one inexperienced shooter will show up with a very small handgun that they have rarely (if ever) shot. Generally, these firearms are a snub-nose revolver, a Ruger LCP, a S&W Bodyguard (.380 ACP), or something very similar in size. Inevitably, the student will rave about the gun upon their arrival and prior to the live shooting portion of the course. I always ask them what it was about that particular firearm that they made them want it, and they will always (without fail) tell me that they "just wanted something small that would be easy for them to shoot and handle."
STOP. RIGHT. THERE.
Small handguns have several advantages in regards to concealability, comfort when carrying, light-weight, etc. However, if they are anything, they are not easy to shoot or handle. In fact, they are quite the opposite. Their trigger pull is always terrible. Many, if not most, of these models are double-action only; meaning that the trigger pull is going to be about 10-12+ lbs out of the box. (To put that in comparison, that is about double the resistance of the trigger pull for a striker-fired handgun out of the box.) But their atrocious trigger pull isn't the deal-breaker for me... These micro-handguns also have a frame that is tiny and lightweight. While that is seemingly an attractive feature- let's consider the science that is at play here.
When you fire a handgun, you are creating and containing a small explosion in this handheld device. That explosion creates enough pressurized gasses to expel the projectile out of the barrel at upwards of 1,200 feet per second when it leaves the muzzle. In short, there is a lot of energy that is displacing the space immediately around it with a fairly violent reaction. Now consider this: the only substance separating your hands from that explosion is the frame of your handgun and any springs/components of the firearm that move and react when that energy is displaced. Therefore, the gun absorbs a good bit of the energy, and any leftover energy is transferred into your hands, wrist, arms, etc. Therefore, the larger the frame, the stronger the springs of the firearm, and the more energy that is absorbed within the contents of the gun. This equates to less energy being transferred (in the form of "recoil") into your hands. In short, the more comfortable it is to shoot without your hands getting beat up from the recoil of the gun. Bigger gun = Less perceivable recoil (assuming that the caliber and ammunition has remained constant).
I can't tell you how many times I hear this excuse why people don't want to take a formal training class (or begin with a Level 1 course): "But I already know how to shoot... I grew up with guns... My grandfather taught me how to shoot when I was 6 years old... etc... etc..."
What if I told you that your grandfather didn't know the most ergonomically efficient ways to grip a handgun? What if I told you that shooting for self-protection and the protection of others has literally nothing to do with your grandfather's deer hunting adivce? What if I told you that six year olds don't always hear and retain all of the fundamentals that they are shown- so you have theoretically spent the next 20+ years engraining bad habits into your "shooting abilities?"
I know... You can hit a gnat's ass at 25 yards with your deer rifle. You can even hit a milk jug at 30 feet with a handgun. You can do it all day long... You still don't know how to fight with a firearm. What you have demonstrated is a mediocre ability to employ marksmanship at the above distances. You haven't demonstrated any weapons manipulation skills. You haven't demonstrated any competency for problem solving with a firearm in your hand. You have merely demonstrated that when everything is going right with the world and you are standing in a static position and aiming at a static target that you can put shots down range given an unlimited time frame. You still haven't proven that you know how to shoot.