One thing you can try is fitting the 51T mount to the can by machining or sanding down the beveled edge of the mount onto which the can seats. On the mount, right behind the coarse threads that engage the can, is a small beveled surface; this is what the body of the can butts up to when it is installed. Moving the face of that bevel back until the ratchet mechanism on the can just baaaarely clicks over to the next tooth on the mount will do wonders for getting a consistent installation of the can (which should in turn provide you with a more consistent POI shift).
I have since sold my SDN-6 and all the mounts, but the way I did it was that I had a mandrel made onto which I could thread the mount, and chucked it in a drill press. Test fit the can to the mount to see how far you have to move the beveled surface, and get the drill press spinning. Use 600 grit sandpaper on a block to dress the beveled surface on the mount, and re-check your fit to the can often. I never really shot for groups with that can after I did that, but we did it with a buddy's SDN-6 as well... and he gets consistent POI shift and easy sub-MOA (average is probably around .6 or so) grouping out of a home-built gas gun in 6.5 CM with the can installed.
A long time ago, on an earlier snipershide post, this was discussed at length. There were machinist pictured posted on a do it yourself fix. too bad this post is lost to us, fuck you scout snipershide.
This is a known issue with several series of mounts for the sdn6. Some early mounts work perfect. Then there are the nightmare mounts. Then there are the you just got lucky series of mounts. pray for luck...
Modification as described above (quoted post) was the only way to get consistent performance and consistent point of impact shifts, can on/can off.
Every single mount either my friends or I purchased that didn't come with the can when bought new had to be modified.
We built a line mount (threaded stock-zero runout) to put the 51t mount on, to check for in line threads and can run out, internal and external.
We would have to do the above listed (quoted post) steps to assure tight can fit to the mount b4 doing run out tests. If neccessary, you could chase the 51t threads to get it as straight as possible. Once the externals were checked and modified as neccessary, we went to internals.
We then line bored the inside of the 51t all the way back to the threads, leaving no shoulder inside the 51t, opening up the bore hole to 1/2" true to the threads and mount. (308 mount) 3/8" or so on the 223 mounts.
Then we checked the thread length on the barrel to remove any possibility of muzzle crush or muzzle touch, a real issue on the pos mounts. Once we did all that and could lock the mount against the barrel shoulder, we had a working system that is consistent UNTIL you use it enough to cause wear and the looseness begins to return.
Then you are back to remodding or as in another post, hello rubber viton washer.
This worked for us.
Several people did the above then welded the corrected mount into the can and used it for a direct thread. This was consistency AS BEST YOU WILL GET with this can.
If you want precision sniper grade accuracy, DO NOT BUY AN AAC sdn6.....
If you want a working, full auto, beat it to hell ar can that will be here 10 years from now and don't mind first creating a working 51t mount, the sdn6 is your can.
I'm ok with mine on the blackout, a 6mm-223 40 degree Ackley mod, and a couple of ar15's. It's a working can but not a sniper can. Works on pigs very well.
Several of the other listed cans mentioned here are sniper accurate or competition grade, but you CAN get a lemon in any of them.... but you are guaranteed a lemon with the 51t mount over 50% of the time.
It is what it is....
Hope this was educational.
