Go to the the BP_MasterAttachment and create a new Scene component called MuzzleEffect then set its Z rotation to 90. It should look like the image below.

Next go to the BP_Silencer and select the MuzzleEffect component and move it to the end on the silencer as this will be where the muzzle flash effect will be located when the silencer is attached to the weapon. It should look like the image below, If you have any custom muzzle attachments you will also want to do this in there bps as well.

Then go to the locations shown below and add the highlighted nodes, locations at the top of the screenshots.