357 is not going to fare well long term with the current design. Everything should be good to go for 38 special, even using +P loads.

I have a couple of ideas to reinforce the action, but printing the hammer and breech in CF nylon may be all it takes. I will get around to it eventually.

As expected, the optional steel safety plates do not reinforce the action. I have tried them on the 327 with no change in results.

38 special is going well. The armorer is testing it and hasn't really had any problems. 327 magnum has proven to be a handful. I don't regret choosing the 327 fed chamber, but likely we will recommend people go with the 32 h&r magnum chamber in stead. 32 h&r still used magnum powders, so you can really utilize the rifle length barrels. It is a very capable round, and the slight shorter chamber will play nicer with 32 long, 32 short, and 32 auto. I really like shooting the 32 longs out of mine more than anything. No problems other than the hotter 327 loads hurting the brass. The gun holds fine but the brass is wrecked with the hotter 327 loads.

So, you seem like you are in the right track.

Use a 1/4" drill bit as a mandrel to hold the beginner shells for sanding.

For intermediate (or any upcoming load that uses small primers) you use a T25 Torx bit as the mandrel. I recommend a new one with crisp edges. You can also use the torx bit after smoothing the cases to ream out the primer pocket to size.

The beginner loads use a 209 primer. The 209 primers protrude out the back of the case because of the incorporated rim/flange. This makes up for the difference in headspace.

The headspace gauge is .2mm thicker than the shells (or beginner rim+209 flange thickness). This build with 43r Ammo is very forgiving with headspace. If you accidently used a beginner shell to headspace, you can just file some off of the breech end of the barrel, and file the extractor groove a little deeper. If you mess up and have too much headspace, you can sand some off of the front of the receiver to make up for it.

Baby barret is one of my favorite builds I've done. I recommend opting for a full size barrel. It's also one of my most accurate full DIY builds.

CCI standard velocity is subsonic as well.

Barrel length also plays a large role in how quiet a gun can be. My baby barret, m4gery, and sr722 all have barrels longer than 21" and are very tolerable with standard velocity ammo without using a can.

On the other hand, my 22lr builds with 1.5-4" barrels are some of the loudest pistols I've fired.

CCI quiet "semi auto" ammo helps a ton too, even with the shorter barrels. Aguilla sniper subsonic is really nice just harder to find and some guns don't cycle it well.

Probably the SY22. I've never built one, but many have with success. The NULL is supposed to be an improvement over the NAG, but I have yet to try one of those either.

It Is known to be an unfinished model. Although it contains some very intuitive design features, there are too many flaws to be worked out as it is.

I'm not saying that it's impossible to get one working, but it's not going to be a fun time, and will require overhauling many of the models.