..at least unregulated ones. As pressure drops, the valve releases a larger volume of that lower pressure air, and that larger volume makes for more noise. May be more noticable on the P-Rod as there isn't a whole lot of shroud volume, but it happens to all of them. May get exactly the same speed at the top end of the sweet spot from a little "blip" of 2800psi air as you do at the bottom of the sweet spot from a longer "blurrrp" of lower pressure air, but can hear the difference.
Got a size consideration. Going fatter would increase volume, but block the fill nipple. Going longer would increase volume, but get into length and alignment issues.
I drilled six very small holes at the back of my Prod shroud. These were in the neighborhood of 1/16" to 1/8" in diameter, and evenly spaced around the diameter of the shroud.
This helps reduce the pressure of the air blast at the muzzle and noticely reduces sound. This also works on the Mrod, if you need to lower it's already quiet report.
I wrapped a strip of masking tape around the base of the shroud, about 1.5" from the breech, and measured out six drill points equi-distant from each other. Using a small hand drill, I carefully and slowly drilled out each hole, using a new sharp drill bit. This put the holes at the very back of the shroud, yet clears the threaded breech connection to the shroud. I touched up the holes with a permanent black fine point marker.
My results have been quite satisfactory.
I noticed Eduard does this to his Edgun shrouds, and other premium airgun shrouds are also relieved in this manner.
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