The US president’s eldest son can be seen smiling as he shows of the firearm, including a magazine that featured a drawing of the former presidential candidate.
Mr Trump Jr also posted several close-up pictures of the rifle, which was adorned with the US flag and a label reading: “Made in USA.”
It also featured a medieval knight’s helmet with a Crusader cross above the magazine.
“Nice day at the range,” Mr Trump Jr wrote on Instagram. “@rarebreedfirearms and @spikes_tactical adding a little extra awesome to my AR and that mag…”
His caption was followed by several emojis of flames and faces with tears of joy.
The post received over 183,000 likes within a day of being posted.
President Trump’s supporters repeatedly chanted “lock her up” during his campaign to become president in 2016.
The chant has been repeated at recent rallies where Mr Trump has mentioned his former opponent.
Donald Trump Jr and his brother, Eric, are avid hunters. During their father’s presidential campaign, photos resurfaced of them posing with an elephant, a buffalo and a leopard they had shot on safari.
Last month, Mr Trump Jr shot an endanagered argali, the world’s largest living sheep, in a remote region of Mongolia.
He did not have permission to kill the animal at the time, and only received a permit after the hunt.
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