My hornet was barely louder than a .22LR, definitely not as loud as the .17 Rem that my mate built on a ZKW465 with a corroded tube. That thing had a vicious bark to it, worse than the .17HMR and that's saying something. I never thought that the ZKW needed a can, and I've pretty much gone to cans on everything I shoot regularly. It was one reason I got rid of the ZKW to be honest, as I was advised not to attempt to chop it as the slim muzzle with it's fully banded front site is a bugger for relaxing and going sloppy if threaded.
I agree though there isn't much point loading a Hornet subsonic, you can't use them on public land with DOC pigeon rules and it's far more expensive than having a 22LR around with subs already in it. Part of the reason that the Hornet is as accurate as it is is the loading 'density', and the quality of the chamber. It's considered more accurate than the WMR and you don't get subs in that for the same reason as far as I'm aware.
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