I'm working on a project for a local company. They have provided me with two piece fiberglass molds for an artillery case/shell and two caps. I have been doing fiberglass layups with fairly good results with the downside being the mess and dry times. The caps are open ended and easily accessible to the inside and I will continue doing these in fiberglass for the time being. The shell/case is roughly 18" deep and 6" in diameter. These cases have to have a relative uniform thickness and be hollow to allow for "cutaway" displays.
I have been exploring multiple options to convert over to silicone/rubber molds and doing resin casting. We currently dont want to modify the current mold as its a one off and the originals are very rare. They tend to disintegrate when shot. They are wanting 100pcs done, so obviously my current method is not sustainable for production. I have pondered doing a 2 or 3 piece block mold with a plug. Im just looking for additional eyes to ensure I'm on the right course and some insight to materials. Based of other suppliers I would be looking at $163 for a single casting, which seems high.
You are definitely in the right direction using a silicone mold over fiberglass. Though to do the molds, the sheer size you're going to need WILL hike up the price but the upside is that you'll be able to do a lot before the mold breaks. Something on a much smaller scale that is similar to what you want is doing "pen blanks". The same principals would apply to making the mold(s) you want. I'd take a tutorial for pen blanks as a guideline and just scale up your final product. As for silicone, I'd go with either HS-2 (High Strength) or even HS-1 and be sure you use a sturdy mold box - also keep that mold box as it would be good to help support the mold since it's going to be so large. And defiantly at least a 2 piece with a plug as you're thinking.
Now as for a resin casting, that I'm not too sure about. Even though resin heats up when cast, it does become "soft" and will deform when heated back up after casting. But I also use mostly RC-3 and that's how it reacts. There might be a better resin for dealing with a high heat situation but I'm not sure where to point you to on that.