Also the trigger is the most prone to breakage part and the very first thing people want to make move even as you look them in the eye and say "please dont pull the trigger its solid and you will break it", they do it anyways. If you know how to tear the gun down and rebuild it, remove the trigger and mold it separately. They are simple to install on a casting with a pin, screw, carpet tape or buytl... anything that will hold it in place. That way when its broken it can be replaced.
The hammer is another area on many guns that is broken due to the same with the trigger. People just want to make something move that doesnt.
Some handguns will have details in the grips that could result in air bubbles on the surface. This is normal with something like a .45 Colt 1911 or similar. When possible mold the grips separately as well. It is more work but very simple work. You will thank yourself compared to the potential parts failure and wasted material of air bubbled parts.
For anything molded as a solid piece (revolver or automatic where you wont mold the slide separate) I assume youll be pouring material into a closed mold. The pour spout needs to be on the tallest part of the mold where air pockets have the least chance to build and material high enough to fill any voids. This is typically the barrel tip. You can make a smooth and flush spout by either casting in a dowel into the barrel and out a few inches or removing the silicone material with a sharpened hollow metal rod when the mold is completed. The sharpened metal rod is the same principal as a hole punch yet will be sharp as a knife. You can sharpen brass tube with an xacto knife (inside not the outside), fingernail file, bench grinder... I also polish the exterior tip so it doesnt get caught up on deep cuts.
You will need to add another small vent for air to escape or else your going to get burped. The vent doesnt need to be very large but just enough for excess air to escape when pouring material in. Can be off the trigger guard or lower slide/frame.
Its a small learning curve with any mold. Start with the most simple one you have and work up from there.