While the theory would indicate that there would be more gas escaping through the larger touch hole than the small hole in the percussion nipple and therefore slightly lower chamber pressure, that the flint lock rifle would have a lower ball velocity than the same rifle with a percussion lock. I did some searching through some of the wonderful old "Gun Digest Black Powder Loading Manuals" that had the results of Sam Fadala's testing of many muzzle loading rifles. Surely there would be several instances of the same model with chronograph data. I found a lot of the identical data in the First Edition and the Third Edition. I also only found one instance of comparable flint lock and percussion lock rifles. The rifles were the Navy Arms Ithaca Hawken in Flintlock and the Navy Arms New Model Ithaca Hawken Rifle in Percussion lock.
Pertinent Data: I am only reporting the differences. If the measurements are the same, the second entry is not included.
Flint Lock Percussion Lock
Barrel Length 32" Barrel Length 34"
Barrel 1" across the flats.
Rate of Twist: 1 in 66"
Depth of Grooves 0.010" approx.
Projectile: 0.490" 177 grain Speer round ball
Powder: GOI (GOEX in 1995)
Patch: 0.015" Pillow Ticking 0.013" Irish Linen
Ignition: Flint with FFFFg pan powder CCI #11 percussion Caps
Powder measure: Uncle Mike's 0-120 adjustable
Powder Temperature 85 degrees F 80 degrees F
Powder (volume) Muzzle Velocity 100 Yard Velocity Muzzle Velocity 100 Yard Velocity
60 grains FFg 1499 fps 914 fps 1493 fps 911 fps
80 grains FFG 1717 fps 999 fps 1712 fps 1002 fps
100 grains FFG 1916 fps 1083 fps 1912 fps 1082 fps
110 grains FFG (max) 1977 fps 1111 fps 2000 fps 1120 fps
In the manual, the flintlock rifle was slightly faster with muzzle velocity until the maximum charge tested. All in all, for only finding one data point, any results don't have a lot of confidence. The same tables were in the first edition.