I have started building an SPR upper, as closely as possible to the original. (The PRI free float tube is too costly: Clark carbon fiber FF tube is what I will be using, since I currently have one collecting dust.) I may be using a VM Hy-Tech side charging upper and bolt: unknown at this writing…need more input on those. I am using a WOA SPR barrel. I need some thoughts and inputs on a BUIS.
I want to use the A.R.M.S. #38 SPR Rail. (It is for use with the PRI Carbon Fiber free float tube.) I like this rail, because it goes the full length of the FF tube, and thus allows the use of a flip up front BUIS; only need a low profile gas block. I have an email into A.R.M.S. asking if this mount can be used with a Clark or Briley carbon fiber FF tube. I will post the response, whenever that may be. I already posted a question about the #38, but since I like putting square pegs in round holes, I am still hopeful (but not much).
If the #38 cannot be used, I want to use ARMS SWAN #36 S-EX Super Extended rail. This will get the scope far enough forward for proper head placement and eye relief. The problems are
1. what gas block to use,
2. and front BUIS to use,
3. and will it need a spacer to fit a #40 A.R.M.S rear BUIS?
4. OR IF this will work...Will a standard FSB be aligned with a A.R.M.S. #36 S-EX OSR (it has a built-in #40 BUIS).
I want to have proper alignment between the #40 and the front sight. (PRI front BUIS is out; I don’t have a million dollars.)
It it was going to be a purely optics rifle, I wouldn't be having this problem. If BUIS's cost too much, that is what may happen.
For my SPR I used the YHM gas block flip up front sight. (WAY cheaper than the PRI)
The ARMS 40 by itself mounts to the upper's rail, not up on the #38 SPR rail. The SPR rail stops at the rear of the receiver just enough for the #40 and is contoured for it.
This is with the 40L. It is built to be at standard sight height , so the front doesn't need to have any spacers or such.
Pic of the rifle: home.comcast.net/~azrael2004/images/SPR/SPR1.jpg
Hope this is of some help.
You have a fine looking rifle.
I see how this works. It #40L is the thinner version of the #40, so that it is more streamlined than a standard #40.
I wonder if the built-in #40 on the A.R.M.S. #38 S-Ex Swan Sleeve is the same height when up. If all ARMS mounts and sights are built to be standard height, then I expect that the #38 is standard height.
I noted that your #38 SPR has cut out for the PRI tube. This may be a distractor for me since my Clark CF FF tube is straight its entire length.
The YHM gas block may be the way to go. I have read where these are made for either taper pins or set screws, with and without bayonet lug and sling swivel. Set screw without bayo lug, witout sling swivel may be the way to go, with a A.R.M.S. #38 S-Ex Swan Sleeve.
The Yankee Hill FS Gas block I used was this one....
This uses 4 screws to clamp it to the barrel. The sight will lock in the up position.
YHM also makes a model YHM-9394 without the lug.
My understanding of the #38 with the built in #40 is that it is at standard height when up.
I am considering using one for my Bushy.
The #38 I used was the ARMS SPR-PEQ-2-3 which has the cutout for the Gen 2 & 3 PRI tubes.
If you can still find the early ARMS SPR rail for the Gen 1 tubes it is flat bottomed all the way down.
At $320 for the tube and $189 for the ARMS rail it was hard to swallow, but I really liked the look and feel of it.
I too am looking at the Clark tube for my next build.
"If you can still find the early ARMS SPR rail for the Gen 1 tubes it is flat bottomed all the way down."
If one is for sale on the Internet, I cannot find it.
Looks like the 14" swan sleeve with embedded #40 rear sight and a standard FSB with N.M. post is my best bet for BUISs, and most economical.
Thank you for you thoughts.