Here is my 53 with an AAC M4-2000.
Also fits the 33K
How is accuracy for the 53 with the can installed? It is my understanding that there are two issues with barrels of less than 10" for use with a can. The first is the pressure and to a lesser extent the temperature of the gasses leaving the barrel. My understanding is the pressure decrees behind the bullet the further down the barrel the bullet goes, after the first couple of inches. So with a barrel less than nine inches the powder is still burning and is still at a very high pressure. So this very high pressure gas hits the first baffle with more force and temperature the shorter the barrel is. As with most materials, the metals tends to soften with an increase of temperature. So with many shots in rapid succession the materials of the can, especially the first baffle, the temperature goes up and wears more rapidly per shot at higher temperatures. That is why AAC tends to use Inconel (which handles high heat better than other metals even stainless steel) for the blast baffle (the first baffle the propellant gas hits) in their rifle caliber cans. But there is a limits as to temperature and pressure any material can stand. There is also a factor of the materials in the powder. If there are "fillers" in the powder to increase volume but don't burn up and have some mass, these fillers can act as the media (the sand) in a sand blasting machine. These particles can hit the baffle at a very high speed and wear the internals of the can. The shorter the barrel, the faster and harder these particles hit the internals of the can.
There is also the issue of stabilization. The shorter the barrel, the less time the bullet interacts with the rifling of the barrel. That is part of why the most highly accurate firearms tend to have longer barrels. The bullet and barrel have more time to interact with a longer barrel, stabilizing the of the bullet like a gyroscope. Again there is also an element of temperature that effects this interaction. The barrel tends to heat up from the friction between the bullet and the surface of the barrel. The burning of the powder has less of an effect on the barrel temperature as the fiction between the bullet and the inside of the barrel. A longer heavier barrel has more mass to absorb the friction heat that a shorter lighter profile barrel. Also the warmer the bullet is, the softer it becomes. The softer the bullet, the harder it is for the rifling to impart the spin which stabilizes the bullet that makes a bullet more accurate and stable.
It is my understanding that because of these factors a 10" barrel seems to be the cutoff point most manufacturers are comfortable with their cans being used with. I would think with a shorter barrel/higher pressure a manufacturer can tell the barrel length their product was used with. Without that 10" barrel length the stabilization of the bullet will be more limited. If the bullet isn't stabilized baffle strikes will be more likely. I think the 33K would be a good length for the tighter baffle diameter if a 5.56 can. I would be very concerned with baffle clearance/stabilization of use of a 5.56 can on a barrel of less than nine inches. Barrels wear over time. As the rifling wears stabilization won't be as good as when it was new.
I bought a 7.62 can for two reasons. First there is no reasonably priced commercially available subsonic 5.56X45 that will cycle a semi auto/full auto firearm reliably. There was a company that did make pressed tungsten powder subsonic 5.56X45 that would cycle the action. It was a little over $2.50 per round. I did a quick search and could not find it. The rifle rounds that I could find that would cycle the action were 7.62 (300 BLK OUT and 7.62X39) diameter. So it would make sense to have a can for that diameter round.
Second, If I was going to occasionally mount the can on a 5.56 barrel shorter than 10" I would want extra clearance for stabilization of a short barrel. So if the 5.56 bullet wasn't perfectly stable from the short barrel, there still wouldn't be a baffle strike as the 7.62 can would give extra clearance. I have a couple of 53 ARs and a 7.5" AR upper so I don't worry about baffle strikes with a 7.62 can. I don't know as I would want to put a 5.56 can on guns with that short of a barrel. I also have a couple of 53Ks. With a 5.1" barrel and the extreme muzzle pressure of that barrel and with the very limited stabilization, I have no plans to mount my 7.62 can on a 53K. These are just things I do and some of the reasons why. There have been a couple of threads here over the years where a 5.56 can was mounted on a 53 and the owner complained of accuracy problems. In both cases, as I remember, the accuracy issues were because of baffle strikes. YMMV.
Scott