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The projectile in front of the bearing surface sits on top of the lands.

The bearing surface seals the gas in :)

Solid copper is lighter than lead so more is required for a certain weight, bullet is longer, usually increasing the bearing surface, resulting in more friction.

So requires less powder and/or...

No grooves, long bearing surface, hard projectile, gives more pressure & speed.

Combine that with soft brass, not starting low enough & ignoring initial pressure signs.....Nek Minit.....opps