These cans 51T by chance? I know that SDN6 is.
I have 2 SDN6's and 2 556SD's. All four use 51T mounts. I use 'em mostly on SBR's now and have two M4's that have 'em blind pinned on. These are NOT precision cans. They're hard use cans for M4's and such mostly. Good for SBR's too. The cans are nice and well made, but the mount is just shit and they never would listen to anyone and that's why they went out of business (rather sold out to Remington). Those mounts will do everything from fit tight, to wiggle a little, wiggle a lot or even wobble on the end that'll just guarantee endcap strike. I used to fix 'em using a honing stone, a good eye and six hours. That'll make 'em lock up tight if you know what you're doing. A lathe would be better.
But I found the best option is to buy ten mounts per rifle/can and select the one(s) that you wanna keep, the ones that fit tight. Out of ten mounts, you should find one or two that fits tight (but may hit shit luck and have to send 'em all back and do it over). Ten per usually do it for me. This helps if the can has wiggle to it. If it fits tight it'll simply cause less disturbance/problems. Sell the ones you currently have on here, they may work for somebody. It's what I did.
Being a "dated design" doesn't mean much, at least not always. The guts haven't changed that much with respect to baffle designs. New cans are still made like that. I have a book from the 60's that shows some of the modern designs still in use today. The SDN6 is STILL one of the better hard use .300BLK cans out there. The 556SD is barely 6", if that, off the end of the mount and works "proportionately" on an M4. Works well on SBR's too. The brake mount is best and actually works as a brake, the Breakout 2.0 is next works okay, but stay away from the 3 prong flash hider --they sound like a tuning fork when fired since all 3 tines are identical. These AAC cans in general work well with SBR's due to them not being "precise" enough in one area or another for use on longer rifles. Though some folks still make 'em work and a tight mount is necessary.
TBAC is a precision can. I've got 3 of them, an older 30CB9, a 6.5 Ultra 9 and a .30 Ultra 5. With the Ultra, it was a full 4dB's quieter than the 30CB9 so there was a big change there. They just came out with a new .338 can but the Ultra will be around for a while yet as far as I know. The Ultra is not only quiet, it's also light and it uses a thread over brake, so it's similar to a thread on but better. I love my TBAC's. Best CS in the industry, they're always on here helping folks, answering questions, etc. Excellent warranty though I haven't had to deal with that. I bought the 30CB9 literally the month before the Ultra came out, was a bit sore about it, mentioned it to Zak and he immediately squared me away on that 6.5 Ultra 9 for a killer deal. I got the Ultra 5 a bit later based on that deal (and Ultra's performance) at regular price. It's awesome on a .300BLK SBR with subs and works well to keep the noise down with full size rifles while not being long. The 9" is where the performance is at. Light, quiet, etc. POI shift is repeatable and groups can actually tighten up, depending.
Elite Iron makes a can similar to TBAC but out of SS. They use a thread over brake and they make the brakes for Harris rifles. They made my .50 HTI brake such that it needs no shims to go on TDC. The can is likewise indexed TDC so POI shift is not only repeatable, but it's vertical too. These cans weigh more, he uses 316SS, but there's enough cut out it's lighter than you think. I use their .50 and have their .338 can (don't have the rifle yet). Built like tanks and they work, prices are good if that's an issue too. Especially for what you get. TBAC is a lot lighter though, and may be quieter, I have no way of comparing like cans side by side. It's just an option though for another precision can that uses a thread over brake.
There are others but these I have more experience with and consider to be more precision oriented with their thread over brake mounts.
