Avoid going online and using "BASE" guns/characters/vehicles/etc that have not been leveled up yet.
Trial and tweak ALL settings inside portal against Bots before playing the upgraded items in online mode.
Instead play "Breakthrough" solo game mode first, select the "Hourglass" map that herds A.I. bots at capture points. (Note if you want to upgrade an RU or US item, you will have to select a solo map that places you on either the RU or US side when the solo match starts...took me a while to figure this out...🤔)
Place a respawn device hidden near the objectives (out of reach of the A.I. to destroy, for example inside a building at second Hourglass capture point and shoot bots through the windows...plus quit match when the dust storm arrives and hides all the bots to save time) and you can easily kill 80-120 bots in less than 15 minutes, which will upgrade all game items from T5 level to T3 level QUICKLY.
Note solo mode does not allow any leveling past T3 level, so once you get all the upgrades around 250-360, stop playing solo at T3 level and level higher playing online AOW and Portal game modes instead...