Making button go full-width?

Why not use the Bootstrap predefined class input-block-level that does the job?

<a href="#" class="btn input-block-level">Full-Width Button</a> <!-- BS2 -->
<a href="#" class="btn form-control">Full-Width Button</a> <!-- BS3 -->

<!-- And let's join both for BS# :) -->
<a href="#" class="btn input-block-level form-control">Full-Width Button</a>

Learn more here in the Control Sizing^ section.

I simply used this:

<div class="col-md-4 col-sm-4 col-xs-4">
<button type="button" class="btn btn-primary btn-block">Sign In</button>
</div>

Bootstrap / CSS

Use col-12, btn-block, w-100, form-control or width:100%

<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" rel="stylesheet"/>

<div class="row no-gutters">
     <div class="col">
         <button class="btn btn-success col-12">
             class="col-12"
         </button>
     </div>
</div>

<br>

<div class="row no-gutters">
     <div class="col">
         <button class="btn btn-primary w-100">
             class="w-100"
         </button>
     </div>
</div>

<br>

<div class="row no-gutters">
     <div class="col">
         <button class="btn btn-secondary btn-block">
             class="btn-block"
         </button>
     </div>
</div>

<br>

<div class="row no-gutters">
     <div class="col">
         <button class="btn btn-success form-control">
             class="form-control"
         </button>
     </div>
</div>

<br>

<div class="row no-gutters">
     <div class="col">
         <button class="btn btn-danger" style="width:100%">
             style="width:100%"
         </button>
     </div>
</div>

use

<div class="btn-group btn-group-justified">
  ...
</div>

but it only works for <a> elements and not <button> elements.

see: http://getbootstrap.com/components/#btn-groups-justified

You should add these styles to a CSS sheet

div .no-padding {
  padding:0;
}

button .full-width{
  width:100%;
  //display:block; //only if you're having issues
}

Then change add the classes to your code

<div class="span9 btn-block no-padding">
    <button class="btn btn-large btn-block btn-primary full-width" type="button">Block level button</button>
</div>

I haven't tested this and I'm not 100% sure what you want, but I think this will get you close.