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Those magnum pellets are the lighter ones, correct? PBA?
They are usually less accurate.
Some springers can be damaged by using those light pellets.
Accuracy Tuning almost always results in lower velocity with a heavy pellet.
The heavier standard pellets slow down the process and dampens the spring action.

Springers are not quiet guns. You can tune it and it gets much quieter.
Better spring, good grease to dampen it, and a properly fitted spring guide.
Much of the noise is the spring. Much violence takes place in that receiver tube.

My quietest backyard gun is a Stevens 22" bolt gun shooting CB shorts.
Considerably quieter than my Springers.