I have searched and getting conflicting information. On a 20" barrel the gas port is 12" from the receiver (rifle length gas system) correct? Why does the gas block extend past a 13" rail? I am getting ready to do a build and do not have the barrel to measure it yet. I ordered a low profile gas block to fit under the rail BUT if it’s going to extend past a 13" Seekins rail I will want something that looks better.
Will the gas block be under the hand guard?
The front of the gas block will be close to flush with the front of the rail if it’s a “standard” 1" gas block such as the KAC, Badger, Vltor or the Noveske variants. I guess it depends on how the hand guard mounts and where the manufacturer takes measurements from. A longer gas block will obviously affect the measurements.
I have a 13" CMR on my rifle length gas system and you can’t see the gas block. You should be good. You may be looking at a 12" rail, where you would see half of the gas block. 
Sorry for the crappy pic, tapatalk won’t upload in a high resolution format.
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I googled 20" barrel with Seekins 13" and noticed on several images you could see the block. AR15 Complete Rifles - AR15.COM
There are a couple examples I found. So so low profile gas blocks are shorter/longer than others? I guess I will know when I get it built and if it sticks out try to find one shorter. I remember the YHM I put on my AR10 being short. Hmmm
Thanks for the pic. What kind of gas block do you have?
It’s a BCM block
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I apologize, I have a CMR 14" which actually measures out to 13.7". You’ll need something a little longer than 13" to completely cover a BCM block. Hope that helps
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