How do barrel shrouds work?
by B.B. Pelletier
An announcement before we begin. After speaking with the manager of the Firearms Division of the Michigan State Police, Pyramyd Air has changed their policy regarding sales to addresses in Michigan. Unfortunately, she is the same person who gave out erroneous information 9 months ago! When they spoke to her this time, they quizzed her about the legality of certain airguns, such as the .50-caliber Dragon Slayer, and she replied that the federal government regulates those types of airguns! In fact, she even stated that federal law prohibited felons from owning airguns.
Of course, the federal government does not make specific laws concerning pellet and BB guns. So, Pyramyd Air has changed their policy for shipping to addresses in Michigan to reflect what they were just told, and that's how they'll be handling orders from now on.
On to today's blog....
A reader asked me about barrel shrouds after reading the Air Arms S410 series. He figured out how they work, but there are subtleties that I thought the rest of you might like to know about, so today I'm looking at shrouds.
The simplest type of shroud is a jacket that encloses the barrel and contains the violent release of compressed air from the muzzle. It lets the pellet escape but forces the air to use up much of its energy before it leaves the shroud, thus reducing the report. In effect, it acts like the simplest form of silencer, and the only difference is that the shroud covers the entire barrel, while the silencer is just a component added to the barrel. Let's look at the AirForce Talon SS, which has one of these.
There can be more to shrouds, however. For one thing, only a few guns have a frame large enough to use as a shroud. Others must install an actual jacket around the barrel. This jacket must be rigid so it doesn't move and hit the barrel. It must also have its end cap hole aligned precisely with the true muzzle, so the pellets don't touch the sides of the hole when they exit. And, it must look right on the gun. That drives makers to use a smaller-diameter tube (smaller than the AirForce frame diameter) and to attach it rigidly at the action. But that's not all.
Holes are the secret
To keep the shroud to an overall length that doesn't ruin the looks of the gun (and longer barrels are better for PCPs), the wise maker does everything he can to direct the flow of energetic air backwards after it leaves the muzzle, so it has to run the entire length of the barrel and back again before exiting the end cap. That way, the outer shroud diameter can remain small yet still have a lot of space for the air to expand (length instead of width). Some wise person discovered that if they allowed ambient air to exit the shroud at the rear, it wouldn't build a pressure wave and reflect the compressed air back to the end cap so readily. So, the shrouds on really advanced PCPs will have small holes just in front of where they exit the receiver.
There's one final consideration for a good shroud - materials. Use the wrong materials and the shroud becomes a gong, amplifying the sound instead of dissipating it. Use the right materials and deaden them further by installing vibration dampers at the right place, and you'll get a dead-quiet rifle that has nothing you can remove and install on a firearm. Yes, by machining the materials from a removed shroud, you might make an effective silencer for a firearm, but anyone clever enough to do that is better off starting with common PVC pipe from the hardware store.
The wrong material to use would be thin aluminum tubing - something I see a lot of hobbyists using to make shrouds. It's easy to get, so they use it, without realizing what it does to the outcome of their project. It buzzes and resonates with sound unless you take pains to dampen it.
As you can see, there's a little more to shrouds than you may have thought. Done well, they are as effective as silencers and can also be quite attractive - like the "barrel" on a TX200 Mark III, which is really an attractive shroud.