Mossberg 835 barrels are overbored to almost 10 gauge sizes. They do not recommend that you shoot slugs through a 835 barrel, check the Mossberg site for more info before you hurt yourself.
from Mossberg site: Can I fire slugs through my 835® vent rib, Accu-Mag choke tube barrel? No, 835 field barrels (those with a ventilated rib) are "overbored", the extra inside diameter may cause the projectile to "wobble" producing unreliable accuracy. A projectile may wedge inside the barrel causing an obstruction. Obstructions of any kind can cause damage to the firearm resulting in personal injury. Dedicated smoothbore or fully rifled slug barrels are available for the Model 835. Other 835 Accu-Mag Choke Tube notes: Buckshot and steel-shot loads are not recommended for use with an Extra Full Turkey tube installed.
A little something else: Can I shoot slugs or buckshot through a "smoothbore" field barrel with Accu-choke tubes? (500/9200/88/695 models) Yes, and with Sabots or rifled slugs, the more 'open' the choke tube, the better the accuracy ("Improved Cylinder" recommended). The "Full" or "Modified" choke tubes are preferred for buckshot loads. Slugs, buckshot and steel shot loads are not recommended for use with an Extra Full "Turkey tube" installed , due to the tube's tight constriction. Do not fire any load without a choke tube installed, doing so will damage the internal choke tube threads.
Check your owners manual for other safety guidelines, if you don't have a manual contact Mossberg for one.
Bob