Aguila SSS ammo is a 22LR length a made with a case very similar to a 22 short with a 60 grain cast lead bullet. It resembles a parabolic nose rebated boattail though the boatail is not tapered, it just fits into the case. There is very little room left and there is just enough moderatly fast powder to drive the bullet to around 1000 fps in a 10" barrel. The bullets are was coated so the bore friction is low and they don't loose much velocity in a long barrel.
There is a seller on Gunbroker who is selling 29" 22LR barrels which fit a Ruger 10/22. They're $159.00, with the breech end tapererd like a standard 10/22 barrel, then nearly straight to the muzzle. The manufacturer is at:
http://www.urbanriflesupply.com.
The barrels are blued carbon steel, appropriate for a 22LR.
I bought one of them to see how well they work. Shipping was prompt, everyting went smoothly, and the barrel had a good fit and finish.
The seller claims they are "very quiet" and I suppose the are compared to a typical factory 10/22 barrel but there is a major problem with the concept. That problem is that the breech block on a 10/22 opens before the bullet is very far down the barrel. There is a fairly loud report which comes from the breech from the propellant gas escaping still under pressure.
I also have a CZ 452 Lux 22LR bolt action with a 27" barrel (bought for the same reason) which is much quieter overall. How much I can't say exactly but I'd guess 20 to 30 db quieter. Muzzle blast and bullet flight noise is probably the same for both rifles, both of which are much quieter than the breech blast on the 10/22. The 10/22 with the 29" barrel is quite enough to shoot without hearing protection but that's not saying a lot.
I'm just giving a bit of warning for those who might consider buying this barrel. The above website has video clips of various rifles shooting various ammo but it doesn't give a realistic comparison of the sound. I doubt it's intentionally faked, just the limitations of a typical camcorder's audio performance.
I''ve not made velocity or accuracy comparisons. I just shot the 10/22 for the first time today.
Another warning is that both barrels have 15" twists. The 60 grain SSS ammo may not be stable in dense air. I shoot at over a mile elevation in Arizona and have no stability problems in any 22LR that I've shot the SSS ammo in.
The SSS ammo in any closed breech action starts to get quiet with barrels around 24". I think that's about where the propellent gasses expand enough to be subsonic out of the muzzle. The expansion ratio of the CZ Lux rifle with the Aquila SSS ammo is around 100:1
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