Conventional wisdom is not to use monometal bullets such as Barnes-X in a double rifle.

I noted on the GS Custom website they mention their bullets can be used in doubles.

In reply to:

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All FN bullets employ our HV technology on the shank. This means that the bullet is bore diameter in the shank area with the thin series of bands at barrel groove diameter. The only stress imposed on the rifling by these bullets, is to cut the band and push it into the space behind it. The photos show clearly how this happens. This also gives superior gas sealing. All our bullets are moly-coated for further protection of the bore. When GS Custom FN bullets are loaded to normal speeds, recoil is reduced. When loaded to normal pressure levels, higher speeds than normal can be achieved. GS Custom FN bullets can in no way damage the barrels of a double rifle. The advantages are obvious.

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