I considered these when they came out in 2015, but with a copper monolith designed for an inline you have to be concerned about their expansion characteristics in a flintlock, so I sent Federal an email about it.
Hi, interested in your bor lock muzzleloading bullets but was wondering what the minimum terminal velocity is for expansion?
Below 1400 fps is where you should expect the expansion to become unreliable.
If you are not trying to extend your range it might work depending on your muzzle velocity. In my gun I probably would expect about 1600fps mv if comparable to a similar weight powerbelt. The borlock is .168 BC so I would cross the 1400fps threshold at about 50 yards, 1259fps at 100, 1137 at 150. I want reliable out to at least 150 so I passed on trying them.
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