I have a bottle of T/C #13 and have used it on rare ocassions. Never had any flash rust with it. The #13 is all natural. I do not see how it could cause flash rust. Grante if this was all you used to protect your rifle from rusting, you are going to rust the gun. But as a swab compound it will work just fine (if you like that kind of stuff). Just be sure to run at least two dry patches after a swab of #13. I've had lots of misfures. That's why I swab with isopropyl alcohol.