I finally got my Specwar 762 mounted on my Mod 70 300 Win Mag with sporter barrel...love it! I had the barrel cut to 21.5 inches (necessary to get the right shoulder for the QD) and have seen no impact to accuracy (though _I_ shoot a lot better now without the recoil or the noise) or velocity (numbers when I get a decent chrono). Some observations:
1) the thing is a beast (24 oz, about 10 inches) and lugging it on the end of the barrel, through brush and up Colorado mountains is not for the faint of heart. However, it is so easy to put on, I carry it my my pack and add it when I get to where I am hunting. I am really glad I got the barrel cut down for this. The QD brake would allow for quick shots w/o the recoil if I am surprised by a critter while moving (though I suspect I would not actually pull the trigger unless it was a once-in-a-lifetime trophy...sucker is noisy w/o the can and with the short barrel). In more open scrub (not ducking under branches) it stays on the barrel.
2) Recoil--about like a .223. WOW! Much nicer to shoot than my Rem 750 autoloader in .308 or Savage .243. Eye relief is no factor anymore...no scope-eye danger. Makes the .300WM wife-shootable.
3) Sound suppression: astounding. Basically ear safe, quieter than my 26" .17HMR. Deeper sound, and yes, you still get the sonic crack, but it is a joy to shoot. 100 rounds while working up a good load was no issue on either the shoulder or the ear (I still use plugs when target shooting, even .22s). Hitting steel with it is so much fun, and you can hear the impact on different types of targets easily.
Bottom line: I love this can, and would recommend it. Rated up to 300 RUM (one reason I got it), it works great on my AR-15 (with slidefire stock...what fun), and my Sig P516 (short barrel), though both are a bit louder than the .300WM due to the action cycling and smaller bullet/bigger hole. I have some hearing loss from hunting, flying, and a close call with an IED, so hearing is a big deal to me...I plan on never hunting without this again. Yes, I could have gotten a new gun with a heavy barrel, but this Winchester has taken about 10 elk and more deer than I can count. Also, it was in hand, which helps with the marital/financial side of the hunting equation.