The Strun is a standard issue shotgun for all Tenno. Reliable, versatile and deadly.
The MK1-Strun is one of three starting Primary weapons available to new Tenno, which can be picked up during the Vor's Prize tutorial prologue. New players who want a shotgun's close range killing power can choose the Strun as their first weapon.
The Strun appears to use a tubular magazine, presumably the cylindrical protrusion near the rear above the stock - the Warframe will push this section while reloading.
Assuming the Warframe is adding shells into this tube, or replacing it, it would imply that the shells used by Strun are extremely small, which is quite puzzling given the enormous size of the barrel diffuser.