A tweet published by “Caesar” contains two pictures showing a “silent mortar” being fired in the Donbas conflict area by unidentified warfighters, possibly Russian or Russian-supported Ukrainian ones, as it seems to appear from the tweets that follow the pictures.
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(Picture source: screenshot published on Twitter by "Caesar")

This mortar enables its users to fire grenades without being spotted, thanks to the big sound suppressor fitted on the mortar tube. A silent mortar will prevent detection by artillery surveillance devices that require a listening post to activate that device (radar or sound ranging). But newer types are monitoring constantly with no requirement for a human to hear the mortar to activate them. “Sukhoi Su-57 Frelon” commented on the tweet by saying he had already seen a video showing a similar Russian mortar in action and it is “not that silent”.

(Picture source: screenshot published on Twitter by "Caesar")