Just confirming things here.
On rimfire semi autos with detachable mags, I can have all the evil features I want.

I'm using a standard lower, and I'm going to epoxy a 7" wooden 1" dowel into the buffer tube. That will make the upper "Permanently" 22lr semi auto or single shot center fire in the case the upper was switched out for a "backup"

Can I use a 5.56 barrel and use a cut gas tube to hold the handguard retainer in place but not be attached to the upper along with a conversion bolt?
That would make it a single shot 5.56 if I change the bolt, or a semi-auto .22

Pull the bolt carrier assembly.

Install a CMMG 22LR bolt.

Buy some 10 round rimfire magazines.

They are fun, accurate and reliable.

Enjoy!

You're presumably going with a conversion bolt, no? What about getting a dedicated 22LR barrel and bolt for a dedicated upper?

Seems playing with the gas tube length would achieve the single shot function if a standard BCG was installed, but that could be a hassle because it gets kinda technical, IMO. The authorities that would be interested in your rifle may not be highly knowledgeable on such things. Just a thought.

I'm interested in other' opinions of this and the dowel to plug the recoil tube. Why not just go without a spring & buffer? I'm sure 'they' could try to claim intent but shit, that would be going on a hunt for a violation.

I figure that a dedicated lower to go along with a dedicated upper would be fine. Having a second lower with just a single upper could be a problem...

Another option with the CMMG conversion or dedicated CMMG style 22LR bolt is using the Redi-Mag magwell adapter so that 15-22 mags can be used...

I have a 5.56 m4 style barrel that I got used for really really cheap. I want to eventually build a dedicated, but it's gonna cost some money for the barrel.

Obviously I didn't comprehend the part in your OP where you said you were gonna us a conversion bolt. My bad.

That said, I don't know how the state would view an epoxied dowel and/or inoperable gas block/tube? I get it and see your intent (or lack thereof)...

Yes, it will, approximately doubling the cost of the rifle, maybe more.

I installed a CMMG bolt and my rifle is as accurate as any plinker needs to be and way more accurate than the 10-22 I used to have.

Why do you want to epoxy a dowel into the stock extension.  It's not necessary.

Once the rifle is configured with a rimfire bolt carrier, you are done (meaning you can no longer be classified as an assault weapon).

Now, if you chose to put a center fire bolt back in, you need to do it in a manner which does not create an unregistered assault weapon.  An easy way to do this is, don't do it.

My reasoning is this, since it's chambered for 5.56 and is marked 5.56 on the barrel, it could be argued that it is a 5.56 rifle. If I put the dowel in the stock, it won't be able to do semi auto except with the .22 conversion in. That means if it were successfully argued that it was a 5.56 rifle, it would only be considered a single shot 5.56 as you can't chamber a round without popping the receivers open and doing it manually.

I paid less than 50$ for this barrel and it came with a front sight post. I would like to get an M4 .22lr barrel with a fsp in the future, but they're going for 200$ now, I can get a really nice 5.56 barrel for that price.

Once it is equipped with a rimfire bolt, the rifle is incapable of even chambering a center firing round much less firing it.