LRRPF52 Wrote:I don't recommend using a 6.5mm can on a gas gun, as while they are efficient at the muzzle, they push a lot of supersonic gas out of the vents in the bolt carrier.
Stick with a .30 cal can for 6.5mm gas guns is a good guideline to follow.
A bit of a generalisation there.
Depending on the volume of the can, especially the first vessel or expansion chamber. The larger the expansion chamber the less back pressure there will be. An efficient suppressor will have a large enough expansion chamber to mitigate the gas produced by 28 grains of powder. Barrel length is important here as that is also an expansion chamber. Shorter barrels will have more muzzle pressure the can has to mitigate. If your can is big enough then it is better to choose a bore which is as close to your calibre as possible. Manufacturers typically allow for 0.5mm clearance between the bullet and the baffles (a 6.5mm bullet travelling through a 7.5mm hole).
Over-barrel suppressors permit a decent expansion chamber whereas it's more difficult with muzzle forward. There will always be a degree of run-out the manufacturer has no control over that is introduced by the person who threaded the muzzle. The longer the suppressor forward of the muzzle the more exacerbated this run-out is, leading to bullet yaw, shift of POI and finally baffle strike. The longer the suppressor forward of the muzzle the 'heavier' the moment of weight will be from the balance point and also it just get ridiculously long. My point being, a decent expansion chamber is a luxury muzzle-forward suppressors don't have, especially when you also need length for enough baffles to peel/divert the gas.
Volume, is of course length by diameter minus whatever baffles and internal support the manufacturer has inside. In AR's, if you are not using iron sights your suppressor can be as wide as they come. Standard wide for suppressors is 1.75" diameter but on an AR with a 2.5" bore height to scope you can go way more than that. The bore height on an AR is 2.5" so you can even put a 4" oil filter on the end and still have clearance for the scope's cross hair. Fatter cans means shorter suppressors, as long as there is enough length to effectively peel the gas following the bullet. Makes me wonder why manufacturers don't make dedicated fatter cans for scoped AR's. Most recent I heard was on this forum where a manufacturer is making an injection-moulded titanium suppressor dedicated for AR's Its 2" diameter is wider than the norm but is level with the upper rail, obviously to permit iron sights. I'm thinking something even wider, around the 3" diameter mark.
The sound coming out of the receiver of a suppressed AR is not all gas system and BCG. It also comes out of the chamber. As you know the chamber is not hermetically sealed when in battery. The case expands, especially at the neck to seal so the case is doing half the work of containing the pressure. As the 'elastic' case starts to retract gas is forced around it and back out into the receiver. The bolt unlocks and you get spit and noise in the face (I'm a left-hander). I use suppressors on AR's both with and without pistons and even with pistons there is noise from the chamber and they spit in your face.
How much sound you mitigate at the receiver by going a larger suppressor bore is an interesting question. By doing this you are mitigating sound at the receiver at the expense of sound at the muzzle. I don't have the answer here but with respect, I don't think 52' has the definitive answer either.