First off, it could be the barrel threads or can threads. The run out could be too extreme and that'll cause a problem regardless of the mount. Second, AAC says to NEVER fire frangible ammo or a bullet that could experience jacket separation (which can occur when firing, say, 90gr. bullets through a .300WinMag for instance). Over accelerated SMK's and others can do it too if the jacket is really thin. Similarly, the bullet needs to be stabilized fully upon exit from the bore, not just downrange, so twist/bullet geometry is important. I always check any ammo using white paper up close before I settle on a load, suppressor or not. I called AAC, and while I couldn't get a "clear" answer from them, ballistic tips are okay but soft points aren't. Factory FMJ's are all they truly recommended. Third, I've had problems with AAC and the SDN6 in general, so the following rant is also applicable. If your can fits tight with no wiggle, it's the run out with the barrel threads or the can itself or an ammo/twist issue. If it fits loose, it's that goddamn 51T and I'll bet that's the problem. There's a fourth one too: it's possible to attach that particular can wrong and think it's on right, but that'd be more rare. Be careful with that though, I read of one guy that had that happen.
I have an SDN6 and several mounts and RMW is spot on. The SDN6 has an extremely poor design for the mount despite how good it looks. They just don't seem to be able to maintain the tolerances necessary for a design like that, or else they don't QC worth a shit. AAC claims it's only rotational movement, but I can tell wiggle when I see it and it's more than enough to see with the naked eye and no dial indicator needed. I have one mount, for instance, on an FAL and when the can is attached it's clear that the bore aperture is eclipsed. The mount is on straight, I don't know the run out, but I DO know that if the damn thing locked up right that it fixes the problem. As a result, I've never used the can on that weapon as I KNOW it'll strike. One day I'll get around to finishing AAC's job on that part. I have minute wiggle on my 5.56, but I don't think it's enough to be problematic (though now you have me concerned). On my .300BLK, it does worry me, but it does lock up a little better on that one. On my 6.5G, it locks up butthole tight --ONLY because I fixed it myself. The wiggle also affects accuracy. I got extreme spread with my Grendel --.33MOA to about 2. I ground the mount so it locks tight and it's less than 1 now. It also seems like it shoots lower the more wiggle is present too, from my experience, and cans shoot low in general (which I think is just due to having weight on the end of a light barrel).
You can fix the mounts so this isn't a problem, but it voids their warranty so you can't let them know it. They are a piss poor company to deal with when it comes to these 51T mounts too, and they'll do what they can to make it your fault or make it feel like it's your fault. That they act like you're wasting their time is also a non-starter.
I USED to speak highly of them --until I got more mounts and started having problems. What I thought was good CS when they sent me a free mount and told me to keep the other was actually them admitting the problem --they didn't question for one second that it was fucked up and I get the idea they've gotten lots of calls about this. The attitude of the guy spoke volumes about that. Later calls were met with "send in the whole rig". I don't need to, I can see the problem. How about you send me 20 mounts instead and I'll pick the one that matches the can you sold me and send you the rest back?
I will never do business with AAC ever again. When I get my new barrels from Satern, I'm gonna get a nice thread on. Problem is, AAC makes a damn good sounding CAN, just a shitty mount, but had they taken care of me better I'd have bought a thread on from 'em anyway. They do sound nice. But I just can't, there are some places I don't deal with: CTD, Midway and AAC are at the top of the list, and in that order. So that's the one good thing I can say about AAC: cans sound great, materials are bar none. Engineering/QC and CS is shit though, and that's just as important to me.
Liberty has taken damn good care of me so far, so has Elite and I've never heard of any bitching about those two. No bullshit, they aren't in a hurry to get you off the phone and they don't blow smoke up your ass. In fact, Elite is likely going to be the one making the DTA HTI QD cans, and he told me personally that if DTA doesn't give him license to make it right, he won't do it at all (it's still over a year out). He doesn't want shit products tacked onto his name. He also doesn't claim .50BMG cans sound like .22lr either and he and his wife are straight shooters. IMO, they're kinda like the Satern family of the suppressor sphere. Liberty is similar in ethics --they'll tell you no before selling junk.
There's a good informative video on here somewhere of a guy that took a few cans and tested them against a perfectly threaded barrel. Almost zero run out. Found out many manufacturers are selling cans that have run out well in excess of what it would take to cause a strike. Was VERY interesting and I'm dying to see their followup. They only tested a few cans, but not an AAC one, but the results were kinda shocking. They were illustrating the problem may be endemic to the entire industry as a whole, but that those particular cans could also be outliers.
Oh, and I don't think AAC warrants any END CAP strikes (which is almost inevitable with their damn mount). OP didn't mention baffles, just the end cap. I'd have to look, so don't hold me to that statement but I think that's in the warranty card. Handloads are okay according to it in one sentence, but in another they recommend factory ammo. Doesn't say it voids the warranty, but it doesn't say use 'em either. Go figure.
Good luck, I feel for you and I know I have a good chance of being in your shoes one day.
