It's a beef that really came to a head this week when 50 posted a trash-talking post on Instagram essentially saying he'd wreck Rampage if they ever really fought.

Rampage responded with some trash talk of his own ... and now there are rumblings the two could actually be setting up a real bout.

Enter Sikora ... who's been working with 50 Cent on "Power" for years -- so, we had to ask if he REALLY thinks his boss can do what Chuck Liddell couldn't get done ... knocking out Rampage.

"I think he's serious," Sikora says ... "I got my money on Fif."

So, why the confidence?

Sikora says 50's in incredible shape and has some real accomplishments as an amateur boxer.