Give the RE-45 and Sentinel Disruptor Rounds

The Sentinel would be able to activate Disruptors while you have the Disruptor Rounds hop-up but using 2 shield cells to charge it is still an option just in case you don't have the hop-up on your Sentinel and you want to activate Disruptors. This will make players use the Sentinel more knowing that if they find the Disruptor Rounds hop-up you won't need shield cells to activate Disruptors on your Sentinel. Getting the Disruptor Rounds hop-up will still need you to activate the Disruptors doing the charge up animation but you just won't have to use shield cells to activate it.

Disruptor Rounds gives a 1.7× multiplier to shield damage for the RE-45 making body shots do 20 damage to shields if this would be to overpowered it could be reduced to a 1.5× multiplier doing 18 damage to shields.

TLDR
You can activate the Disruptors charge up only needing the Disruptor Rounds hop-up for the Sentinel instead of using shield cells. This would bring back the Disruptor Rounds hop-up for the RE-45.