Don't believe I've ever heard of an 1861 Colt Navy in .44 caliber. Perhaps an 1851 Navy? Or an 1861 Navy in .36 cal? Or an 1861 Army in .44?
At any rate, if it's truly a .44, there are three possibilities: .451, .454, or .457. I suggest trying the .454 first; if it leaves a full ring of lead upon loading, then stick with that.
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