I just ordered my tax stamp.  I'm going with a 11.5 barrel.  Which one I like one of the faxons  but it has a .650 gas port. What do I need to know about the sizes of the gas port?  I know I need to get the appropriate gas block size.

Don't worry about the gas port size.  Pay attention to the  gas block size.  I'm guessing you are looking at the 11.5" Gunner barrel and it has a 0.625 gas block diameter.

Gas block are usually 0.750 or 0.625.

depending on your hand guard choice, you might care about the size of the gas block itself, to make sure it fits under it (assuming you are going to have the hand guard covering it)

personally id go with something better on a sbr. Nothing wrong with faxon barrels , I guess. however for just a few dollars more you can buy a BCM 11.5 stripped or even a DD barrel. especially if going with a suppressor later on as you dont want a barrel slightly out of spec and destroy that suppressor. over gas issues can be fixed with an adjustable gas block and under gassed can be reamed out. but you wouldn't have that issue with a DD, BCM or spikes barrel

I built mine with a Faxon Gunner 11.5", I like it and is shoots well suppressed and unsuppressed, granted itis fairly new so only a few hundred thru it so far.

I honestly am suprised that the first detractor mentioned about that barrel wasn't that it has a midlength gas system.

as for a few dollars more, I paid 100 less than a BCM, I am usually a BCM whore, and just about half of what a DD barrel would have cost me. Are they better, most definitely, but so far it seems to be a decent barrel for the money.

You do have to bear in mind that if you go with the Faxon Gunner that unless Brownells has the Carbine gassed one(their exclusive if I recall) you are stuck with a Midlength which might be finicky with function. Mine works well with a H2 buffer and also works like a champ with a VLTOR A5 system using a A5H2 buffer, shot Tula suppressed, Sico Hybrid ejection about 4-5, and unsuppressed, ejection about 3. It was a bit gassy with American Eagle, but not bad, more smell than feeling gas on my face.

As for the Gas, as stated before .625 gas block, but because it is a 11.5" mid it has I think a .090" port, they suggest a adjustable gas block, but I can attest it seems to run just fine without one. I chose not to use one, due to the rail I am using not playing well with midlength gas systems, have to remove material from the gas block to clear the built in front sight in the rail, and I am not one to remove material that may cause the product to not work properly

Edit: Gas port on the Gunner is .110"

I went with a Ballistic Advatage 11.5 inch barrel with carbine gas from Toms tactical.

It’s oos now, and I got it on sale for $100.

Faxon is good stuff. I have built several Faxon uppers this year for my customers and I get good feedback on them. Doing a 10.5" 300BLK right now, with the balance of Noveske parts.