AR pistol buffer tube length

Hi all. Need some help trying to solve a problem. I have a Freedom Ordnance FX9 PCC that came with a KAK blade and buffer tube. The tube has threads that are about 1 3/8 in length. I have since bought an SBA3 and SOB brace that came with an aftermarket tube and the threads are only about 1 inch in length. I’m told this is standard tube thread length and not the 1 3/8? Does anyone know if this is true? Possibly a standard tube won’t work on the FX9. Definitely the rear of the receiver from end to buffer pin is longer on FX9 than AR but everything I read on internet says it takes a standard tube. The tubes I have are too short on threads when add castle nut and retainer plate.

sarduy said:

The length of the threads doesn’t matter much as long as they reach the buffer detent

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That’s my issue they don’t. The distance from receiver end to detent is longer on the FX9. I think I just need to find an adjustable tube with longer threads. Sent a message to Freedom Ordnance as they now have a model that comes with SBA3 so gotta be something that works

Update: it does fit on my FX9 but not by much. My struggles were because by time it’s over buffer detent the retainer is almost tight to receiver making it impossible for a novice like me to keep the pin spring in. I was saved though by the internet as saw how you can tap the pin spring hole with a 4-40 tap and put a set screw in to keep spring in place so easy to thread tube with retainer. Did that today and problem solved