pro·pri·e·tar·y
adj.
1. Of, relating to, or suggestive of a proprietor or to proprietors as a group: had proprietary rights; behaved with a proprietary air in his friend's house.
2. Exclusively owned; private: a proprietary hospital.
3. Owned by a private individual or corporation under a trademark or patent: a proprietary drug.
(taken from dictionary.com)
I think definition number 3 is what is related to. To say a barrel (or anything for that matter) is proprietary in or by design, is to say that it was designed/developed specially by an individual or company and being protected by special legal rights: e.g. patent.
The same example can be applied to computer electronics and software: Video Card utilizes proprietary technology from a company. Other companies may make similar products, but cannot replicate the design exactly. (This is why we have compatibility problems in the computer world)
To say proprietary is custom could be a fact, but the way I read it "custom" can refer to modifications done to something to improve design/structure/operation/accuracy etc. without having any special rights to it.