Thank you for following up about this!

The bolt has to weigh a lot more for those other calibers in a direct blowback firearm. In 9mm the bolt weighs about 1.7lbs. For 45acp and 10mm the bolt would need to be 2.3 and 2.8lbs respectively. It can be done but would require a lot of work to change calibers.

Edit: Source: https://10mmautocombat.wordpress.com/blowback-bolt-calculations/

For full transparency, here is the comment in question that got me banned.

FTN4 is the best printed suppressor out there. There are options for 22lr, 556, 308, 9mm, and 45. They need reinforcement, either an aluminum tube or a carbon fiber sleeve. With suppressors tax dropping next year, someone may come up with a better design eventually. However, the FTN series of suppressors is quite robust and can handle FA rates of fire, especially when it's made of PA6-CF or even better, PPA-CF.

You could also print out a UBAR3 lower. It can take a standard buffer tube and it's held in place with a u-bolt so it's pretty strong. You can print it with or without an integrated pistol grip. The integrated grip is definitely stronger but not everyone will like it. https://guncadindex.com/detail/UBAR3-X:1

I don't know if it exists. I haven't seen it anywhere online before.

The baffles are physically thicker in the FTN series than in a traditional metal suppressor so the entire thing has to be bigger. I did find this https://guncadindex.com/detail/SuppressorPack:2 and it looks like it uses FTN baffle geometry but sealing it up in a pneumatic tube. It has can be built 5-8 inches long depending on your needs. It also appears to be bored for 223 as well. IDK how well it performs. I'll be interested in trying both of them once I don't have to pay the tax to make them.

This is the only broadly usable forum in my opinion. Discord is fragmented and hard to navigate. This is the most like reddit in terms of use, search, and function.

I literally just had a comment nuked on r/fosscadtoo because I put a link to this forum in my comment. I will keep pushing people to join us over here so we don't have to deal with the bs with Reddit.

Nobody has made a 3rd burst FRT. While it is theoretically possible, it would be difficult to design and build something reliable. Heck it might be more difficult to make it for an FRT since the FRT is resetting the trigger every time whereas the FA doesn't. You might be better off learning to use the trigger and get consistent short bursts. Even the 3rd FA burst for the M16a2 was pretty bad. Eugene Stoner said that 3rd burst was the worst option. You would be better using a 5rd burst instead. That gives you time to react to the muzzle climb and pull back on target. Usually a 3rd burst will string vertically and by the time you've figured out the recoil, you have to pull the trigger again. Shooting 5 rounds with FA or FRT should be pretty easy, especially with a heavy spring and lower bolt velocity.

I don't think that actually exists yet. You could get a bufferless lower but then you would need to buy/design a magazine adapter. Alternatively you could always use this and then just screw in a cap instead of a buffer tube. https://guncadindex.com/detail/HT-D3D-AR45-10-V1:3

Hitchhiker is one of the cheapest 22lr builds but the hardware from MAF, including the barrel, is still $80.