Are FRACUs authorized for garrison? If not, where is the regulation?

Ho boy, this again. I think I spent a good half-day the last time this question came up trying to run down a definitive answer. There's a good argument for it, but it comes down to local policy - check with your leadership.

There is nothing out there that says you can or can't (aside from local policy). You CAN wear them in garrison if you're in 8th Army as their SOP* specifically allows it; 10th Mountain used to allow it, but not since 2020; ARCENT does* allow it; etc.

There was an Army-wide exception to policy for the OEF-CP FR ACU from before the OCP mandatory wear date, but there is no indication that a "FR" policy was extended to the next version of the uniform.

So, some do and some don't. Your leadership is going to have to tell you.
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* https://8tharmy.korea.army.mil/site/assets/doc/resource/8A-Blue-Book.pdf
* https://www.usarcent.army.mil/Portals/1/USARCENT%20Standards%20Book%20%2001%20FEB%2023.pdf

The only thing I see according to DA-Pam 670.1 chapter 4-1 c. - The fire resistant (FR) combat uniform in the OEF camouflage pattern (OEF-CP) may be worn until the wear out date of the UCP ACU.

And in AR 670-1 chapter 4-3 b. (1) the combat uniform is prescribed for year-round wear unless otherwise directed by the commander.

I see your confusion, maybe COL Randall C. or MSG (Join to see) can clear it up for you. CPT (Join to see) maybe you are on top of this topic.

For some reason, folks (Commanders) don't like their Soldiers wearing the FRACUs in garrison. Most Commanders will make it a local policy. But I've never found anything in DA PAM 670-1 nor AR 670-1 that gives specifics as to where they can and cannot be worn.

I don't know of a policy or regulation that forbids it but I am not a fan of wearing them. Tankers and crews are issued FR overalls. They are used when needed. I feel these should be treated the same way. To me this is something that looks unprofessional. I wouldn't say not to wear them but I have about 8 sets of them and I have only worn them when I was deployed.

I think your question is answered, has anyone else in your Battalion war them? As for asking the CSM, you have researched, so you would have a question for him. What is the local policy.

The FRACU is OCIE. That is not governed by AR 670-1. I agree with everyone else here--check the policy for your battalion and brigade or Fort Campbell garrison for their guidance.

OK, I am going to say this and please take it as I mean it. Just because you can do something, (as in there is no regulation that says you can't, or a part of the regulation says that you can, maybe) does not mean that you should do it.
Here is what I mean: Obviously, since you are writing in, you are concerned with the Command Sergeant Major's reaction. Either one of these two is true: 1) You were wearing them and he did not like or said you could not, or 2) Someone else told you to go change, and you want to tell them that you can in fact wear them. Your reluctance to ask the CSM shows that Two is the most likely. Some NCO said that they were not authorized.
There are times when you can win a battle and lose a war. So, I think, all things being considered, I might hold those 8 uniforms until I was going to be someplace that actually had a chance of catching fire. If you office is in danger of bursting into flames, then I might think twice about it, but to just be able to tell, most likely your platoon sergeant that he is a moron just ain't worth it.