Colin Farrell is bringing out the big guns ... in more ways than one!

The sexy 35-year-old actor was spotted on the set of his new movie, "Dead Man Down" (right), today in Philadelpha, PA -- where he showed off his new, more muscular physique.

The Irishman flaunted his buff arms while holding an automatic rifle, looking more built than he did when he played the vampire-next-door in last year's "Fright Night" (left).

According to IMDB, "Dead Man Down" Farrell plays the right-hand man to a crime lord ... and is seduced by one of his boss's victims, a woman seeking retribution.

See more stars who have gotten buff for a film role in the gallery above!

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