The default MVC5 template has a css rule in Site.css:
input, select, textarea { max-width: 280px; }
I usually remove this rule.
It causes problems with Bootstrap v3 input group where the input box does not extend to its container's width.
Reverse the order of your elements and you get a gap:
Instead of this
<div class="row">
<div class="col-md-4">
<div class="input-group">
<input type="text" class="form-control" />
<span class="input-group-btn">
<button class="btn btn-primary">OK</button>
</span>
</div>
</div>
</div>
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