Use a base p90 no additions (you can use suppressor) and do not pick up anything, ammo, guns, materials. Do not use any items like drones or anything this counts as swap.
It has been a lil different for some folks but basically don't use extended mags and don't carry bodies (you re actually swapping to your sidearm). Drones didn't gave me any problems but it doesn't hurt to go for the sure thing