The 'grip' part (the second image) could be done with a single three part mold. One mold part line would run vertically around the piece with an additional vertical part-line perpendicular to the main portion at the plane across the aft edge of the 'flap' features (aft being towards the rear of the 'gun') and then around the exterior edges of the flaps. The interior space between the flaps would be filled by an insert plug that would have to key into the two other parts at the perpendicular plane (round 'peg & socket' probably best) to define the space between the two flaps. Clamping would be from the sides and from the forward end to keep the insert plug seated & minimize flash leakage. Of course you also must incorporate channels for pouring & air escape, probably by molding it inverted through the 'handle' portion and with a connecting 'sprue' to the plug cavity area from the cocking lever area with vents off the underside of the "slide" portion.
The other part, I cannot say as I cannot really visualize the configuration clearly from your renderings. Anytime you have 'trapped space', you must have an opening to use to extract any interior plugs, preferably with parallel sides (or at least a taper such that the opening is the widest area.) For example, think of a plastic egg. It is two parts, both hollow. A two part mold would work because the exterior mold is formed by a 'bowl' cavity while the interior surface is bound by a 'dome' shape. The domed half resembles a hat with a brim while the other a wash basin. If the 'basin' in on top, it should be stiff enough to resist sagging and touching the peak of the "hat" half. Inverted, the hat may sag and contact the bottom of the bowl part,. creating a flaw.and uneven thickness. So even a simple 'egg' shape can present challenges.
One thing about plugs: They can be of more than one part also. Imagine a typical round bucket shape where the opening is wider than the bottom (aka it tapers to the bottom.) This is easily molded same as the 'egg' example. However, what if the wide area is the closed area (tapers toward the top)? Then the "plug" cannot be extracted as before. However, if you insert a straight cylinder into the center of the tapered plug, when the cylinder is pulled, the now hollow, thin-walled remaining plug is flexible and can be deformed enough to remove from the interior of the molded part. In essence, a collapsing mold plug!
I hope this helps when visualizing your mold construction.
Regards, Robert
