How to keep your gun in hand

Consider the following:

Flint stood, gun in hand, at the door.

What you get from this is that Flint is in a state of readiness. What's going on syntactically is that he is performing the action of "standing, gun in hand".

Now this:

Flint stood at the door with his gun in his hand.

Somehow this suggests a hint of trepidation on the part of Flint. As if he's about to defend, rather than attack. Syntactically, 'his gun in his hand' is an ablative complement introduced by 'with'.

Finally:

Flint had his gun in hand when he stood at the door.

There's nothing grammatically wrong with this, but I suspect this is a failed attempt to reproduce the syntax of 'gun in hand' of the first example. Because it doesn't mean what the author wanted it to mean. "To have something in hand" is an expression meaning that it is taken care of, under control, or in the course of being done:

Preparations for Flint's surprise birthday party were well in hand when I arrived. We were concerned that the children would give away the surprise but Molly kept them in hand. Needless to say, Flint got the wrong idea, and as his surprise turned to alarm, the situation rapidly got out of hand.

"Out of hand" is the opposite of "in hand".

Misuse of "in hand" is getting out of hand. Rather like the whole "look at/look to" confusion, this isn't the grey area you might think it is. We often need to differentiate between "in hand" and "in his hand"; we often need to differentiate between "look at" and "look to".  Useful differentiations like this are what tend to armour usage against evolution.