Debating whether to get a ARMS #38 or #40. Have reads lots of olds posts and lots of good information.  Also, plan on getting an Eotech if that changes anything. Hanguards are Oly Firsh. Any comments, or pics appreciated.  

Considering your choice of optics, I would just get the #40. If you mount the EoTech on top of the rail on the #38, it will sit too high to see the irons. If you just put the EoTech on the flat top itself, backed up with the #40, you will be able to co-witness just fine.

The #38 will put an eotech too high to co-witnes. The #50 SIR and the #40 will work and it gets the eotech out where you can get faster response time between targets. The #40 will also co-witness if the eotech is put on the receiver.
Good shootin, Jack

I agree with newarguy and 3rdtk, the #40 is your best choice for the EOTech.  mount your EOTech on your upper receiver and the #40 will co-witness throught the EOTech optic window with the front sight.

You've just heard from three highly respected and experienced members.

There's not much more that I can add other than +1. [:)]

I agree with jack however that the #50 SIR is probably your best bet. I am a fan of forward mounting dot optics and agree that it is faster, clears your FOV, makes 2 eye open shooting easier etc. The #50 SIR will give you a rail out over the handgaurds at the correct height to mount an EoTech.

You could also use a FF RAS and do the same thing... or a RAS II for that matter. Some people say you will be aqble to do it with the Oly FIRSH, but I still dont trust those FF tubes that dont lock into place for optics.

I forgot to answer the question. A lot of folks use the #38's in dif. lengths, as seen on this forum, but not with an eotech. Aim Points, sniper optic's, etc., that don't already have a too high built in base like the eotech does.
Good shootin, Jack

Thanks guys for all the info. Im building my 1st AR and want to set it up right. Thanks again.