Where have you heard that a kettlebell is better?
The issue here is specificity. If you want a stronger grip for deadlifting (which, this being the powerlifting section, I assume you do), then you may find that training the grip in a way which correlates with your specific goal is most useful - ie, hold onto a heavy barbell, whether that means going strapless on deads, doing a ton of shrugs. That being said, other grip exercises may well be useful as an addition, or even as a replacement for heavy barbell work.
A few people on here, myself included, like to do their deadlifts (especially reps) with straps, in order to be able to train the movement without worrying about our grip, then train the grip separately if it's a weakness we need to train. George Leeman (and I think Andy Bolton) do this, although Leeman's weakness has always been his grip, so take that for what it's worth.
Specifically regarding the kb vs deadlift scenario, just because I find it interesting.
- Your hands are trying to remain closed against force.
- Force (in newtons) = mass (in kg) x acceleration (in metres per second squared - m/s^2) = (F=MV)
- This force comes from two places
- 1 - The natural acceleration provided to anything with mass by gravity (equal to g, or 9.8m/s^2), therefore F=Mass of BB/KB x 9.8m/s^2
- 2 - The acceleration you provide to the weight when you pull it off the floor/back through your hips - F=Mass x How quickly you accelerate the implement
When you deadlift, the bar moves in a straight line up. Therefore there is a constant downwards pull of gravity on the bar that your hands are fighting against. With your 400lb/180kg deadlift, this means that the force of gravity is roughly 1800 Newtons (using g=10 for simplicity).
Assuming that when you deadlift you are able to accelerate the bar constantly (you likely won't be able to, but this simplifies the maths), you can add another 180 Newtons to that (assuming an acceleration of 1m/s^2 - I have pulled this number out of nowhere, again to simplify the maths). You get a total force of around 200 Newtons.
The kettlebell swing is different. The kb moves in an arc, so only when the kettlebell is hanging straight down from your shoulders is the line of action of gravitation acceleration through your hands equal to 9.8m/s^2. Elsewhere, the effect of gravity on your grip is lessened (although it is always trying to pull the kb down to the ground, it is not always directly opposed by your hands in doing this). This is difficult to explain without diagrams, but I hope the point is coming across - the point being that with your 100lb/45kg kb, at one point in the movement, force due to gravity = 45kg x g = roughly 450 Newtons. Elsewhere it is considerably less (at the peak of a kb swing you could release the kettlebell and it should float "weightless" for a split second).
Where the kettlebell swing gets big on force is the point where it comes back between your legs at speed (you should be actively pulling it down from the top) and you quickly reverse the movement. At that point, you not only have to generate a lot of acceleration to move the kettlebell up, you have to generate acceleration to overcome the acceleration of the kettlebell in the opposite direction. This is different to a deadlift, where the barbell is stationary on the ground between reps.
The combination of the fact that you have to overcome "negative" acceleration, and the fact that you create a lot more acceleration with a swing vs a heavy deadlift means that the swing can also create a large amount of force through your hands.
For example, you bring your 45kg kettlebell down at speed (maybe 10m/s), you then have to generate force to decelerate it from 10m/s to 0m/s, then back up to 10m/s. That's a net change of 20m/s, and you probably did it in less than half a second. Acceleration = change in speed/time, so your acceleration here is 20/.5 = 40m/s^2.
Therefore the force created at the turnaround point of this swing = 40m/s^2 x 45kg = 1800 Newtons. Account for the force of gravity at that point being somewhere in the range of 0 - 450 Newtons, and you have comparable total force at that point in the swing to a deadlift.
The consideration you would take here is that the deadlift's force curve through your hands will be fairly constant, whilst the swing has its greatest force at the bottom of the movement and is considerably easier to hold onto at the top. That's the part I felt compelled to spill my brains about.
You can also take into account that kettlebells have thicker handles, no knurling, and often it's hard to fit both hands onto the handle, so your grip is comparatively weakened by that.
What you should actually do is ignore all the sciencey stuff above and consider my first paragraph a bit more strongly. Get a stronger grip, most methods will work to be honest, but I like holding onto heavy things. I know Pwnisher likes CoC grippers, and a fair few people will recommend Kroc rows too. It's all good.