Does anybody know what the chamber really is? I know I can use either whisper or aac ammo. Looking at the chamber reamer drawings of each, there is a difference, so I would like to know which reamer S&W used. I have already talked to customer server, and all they tell me is that I can use either ammo.

Reamer drawings: 300wb_diagram.jpg Photo by Idge0 | Photobucket

I would get the .300 AAC Blackout since it is the looser of the two designs. I'm not sure, when looking at the diagrams you provided, if .300 AAC would work well in a .300 Whisper chamber.

If you the S&W 300BLK models, It is chambered for the blackout. It is just a hair larger. The best way to think about it is like the whole 5.56 vs. .233rem chamber difference. The throat on a 5.56 chamber is about .002 to .003 longer than a .233rem chamber. about the same with the blackout/ whisper.

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15T,15-22,5946 & some not S&W

Yes, I have an M&P 15 the barrel is marked "300 Whisper/300 AAC Blackout".

Hey All, just ordered the M&P 15-300 whisper but I can't find magazines anywhere. Anyone have ideas? Thanks.

you use the same magazines as .223. the case is the same size in diameter.

What hanken said. I can't think of an upper that fits on an AR15 lower that requires a different magazine. Some rounds will reduce magazine capacity due to their size but the stock magazines can still be used. .458SOCOM is one that will definitely reduce your round count. Possibly 7.62X39 as well.

Only ammo using the .223 case as a baseline use the same mags (successfully).
6.5 Grendel, 6.8 SPC need a different mag. or at least modified feed lips to feed right.

This man speaks the truth.... same for 5.45 and 7.62X39.... the taper is so radically different that they will not load and run right in regular AR mags.... (5.45 will load about 10, then stick) 9mm also requires a mag and block system to run in an ar, of course.

I never got interested in the whisper because of JD Jones needing more than I could afford to play with it, but the 300 black makes me wish I had paid for whisper earlier.... it is a fantastic caliber out of an AR....