A few observations about the Fire and Brimstone tattoo reveal scene:

Watch it again and keep your eyes on McAllister the whole time.  You'll notice he's very controlled.  Keeps his arms crossed when PJ says 'one of you is RJ' (after covering his face with his hands...to smile, perhaps?).  ALSO has same stance when PJ says one has a tattoo, no change of countenance, and he KNOWS he has the tattoo.  So, same face with both accusations.
When Haffner stands and PJ gets his gun, McAllister is motioning with his hands very calmly.  He then tries to calm "Patrick" down with words. 
Some have said RJ wouldn't deny being who he is, but wouldn't you deny it to keep the game going?  If he's found out right there, PJ kills him, game over.  Also seems to me the writer would WANT the most dramatic tattoo reveal to be the real RJ.
Notice how Haffner and McAllister share a knowing glance as they're removing their shirts.  As do the 3.  Stiles protects McAllister from being shot by PJ.
But perhaps the most telling thing to me is McAllister's body language as he stands up and looks at Smith.  The camera angle shows just enough of his face where you can see him smiling behind Patrick's back.  Then he walks to Bertram and gives him a knowing glance.  Do the others with the tattoo know RJ's identity?  I don't know. 
Bertram certainly keeps someone updated (Strawberries & Cream and
F&B phone calls to reveal PJ's plans).  If they don't know, RJ has protected his identity by making them all have the same tattoo as him.
Others have said this and I agree: the bomb is probably hidden in McAllister's hat.  He still had most of his sheriff gear on and could have hidden a detonation device in it.  I saw no other suspect with any other belongings brought into the house.  Plus, he could have another type of weapon on his uniform that might go unnoticed.

Thoughts?