777 has a higher ignition temperature than real black powder. However, ignition problems in percussion guns using regular percussion caps are rare (flintlocks are another matter - 777 is not recommended for use in flintlocks). If your gun is a percussion ignition system you should have no problems using regular percussion caps, but if you do they will be easily remedied by using magnum caps (not musket caps, which require a different nipple).
Regarding compression, 777 does not react well to heavy compression. In fact, the Hodgdon web site cautions to use only light compression. The issue is not ignition failure but rather inconsistent velocities and thus projectile trajectories.