I have a Stag Arms Model 15 Flat-top 5.56 cal...and need some guidance--  I am into light-weight / no frills...and have some Magpul flip-up (composite) front / rear sights.  
Couple questions--

a) the gas-block is a picatinny rail but is approx 1/2 inch lower than top rail (where rear sight mounts)..should I get a riser for the Magpul front sight that will allow front/rear alignment?...or replace the gas-block with another front sight that is taller to align w/rear?

b) Also am going with a (sic) cheaper scope (sub $250)...but one that I've looked at for my particular needs and really like it..and the wide recticle.  It is the Leupold 'Rifleman' 3-9x40 (sub 12 oz) w/1 inch tube...it doesn't have the 'clicker' adj...but rather the 'friction-type' adj.  I don't plan to remove the scope that often.  I plan to go with either the Leupold 1-pc mount...or some other QR-type mount that either will push it forward sufficiently. I have the P-223 2-pc mount at the time just to try when I purchase a scope.  (I realize the mount will likely cost ~equal to the scope...but for what I want from the gun/scope, the combo should work fine.)

Feedback is appreciated...

If your wanting to use a Front Sight on the Gas Block, you'll HAVE TO purchase a Front Sight that's made for the Gas Block. These sights are made taller on purpose just for this type installation to be in alignment with Rear Sight. Also MUST GET the Gas Block Sight in Metal(aluminum or steel) as a plastic sight will melt down with the heat over time.

Nothing wrong with the Leupold Rifleman scope. You will be fine. One thing you might want to check about the Rifleman Scopes, the last I looked at one, it was parallaxed at 150yds and NOT the normal 100yds. This may of been just on the 2-7x32(40?) model only. So check before purchase, and knowing how close/far you'll use the Scope.

The Leupold rifleman scopes were what I started using on my 1st AR...It has far exceeded my expectations...and has served me well. Now I own all kinds of stuff (1 NF...4 MK 4's...2 Trijicon ACOGS...5 Aimpoints....3 EOTechs.....and several other MRD's / other)

I have the same rifle, as said you need a METAL gas block front sight,I'm ran a Samson flip up with a magpul rear before I pulled them to install my scope. I'm running the p-223 3x9 with the p-223 rings and love the set up. Nice and low. the only draw back for some would be since the p-223 rings slide over the rail you cant run back up sights, but thats not a problem for me

Thanks guys!  ALL were extremely helpful in pointing me in the right direction.  

Just one more question...(since I'm a newbie to ARs/scopes/etc)...can one explain the earlier comment-- '...the Rifleman scope may be parallaxed @ 150 Vs 100 yrds'??  Or, point me to a thread that explains what 'parallax' really means and can educate me??

I've done a ton of research, shopping for scopes, sights, reading blogs, comparisons, etc...etc but haven't mastered it all...nor the 'terms'.

thanks!
Misipy

My other choice was the Bushnell P223 scope (3x9-40) too...just comparing it Vs the Leupold Rifleman (&/or VX1) for features, price, etc, I just gravitated toward the Leupold.  I like it's clarity, recticle... and, is ultra light.

thanks again!

Sorry...I meant NIKON P223 above...not Bushell.

 I think the Nikon Scopes are HANDS DOWN the better scope over the Leupold Rifleman Scopes. However with the Leupold 'Golden Ring" that Rifleman Scopes have, Warranty can play into this scenario, so choice is yours.
 Don't know for sure if  Nikon P-223(3-9x40)has the BDC(bullet drop reticle), which is handy for longer shots,  but Scope's packaging should say.

When I mentioned about a Scope's parallax, this means where the Scope's Len's are focused(generally) in Yards to give a person a clear, no double crosshair image(when moving head side to side or up and down). Also there will be an Eye Piece adjustable ring with lock-ring OR there will be what is called a "Fast Focus Eye Piece"(European Eye Piece) that adjusts to your sight to get the "Crosshairs" in focus. NOT TO BE ADJUSTED TO THE TARGET.
The fixed or adjustable power of a Scope has nothing to do with parallax.
Say a scope is Factory parallaxed at 50yds. This means that it's adjusted(lens) for actually between somewhere at 37 to maybe 65 yds. Rough estimate. Better the Scope, generally finer the parallax adjustments. When looking beyond probably 75 yds the target image will start getting a little hazy, and you'll start getting a "Crosshair Movement" when moving your Head. This is your parallax getting out of adjustment. Same when looking at Target at say 10yds also.
100yd parallax is adjusted for somewhere between 85 and 120yds. Looking through this type parallax, the Target Image might be very blurry if trying to shoot at Target at 30yds. It'll be like not wearing glasses and being "far sighted" and trying to see up close. Looking beyond 175/200yds the parallax won't be noticed as much, but it will be there. This is your normal hunting type scope that's used on millions of Centerfire type Rifles, and is usually a 3-9x40.
Again a 150yd parallaxed Scope is just like above, but parallaxed out further in yardage.
This is just a rough idea of how I learned it, and ONLY if you get a Scope "WITHOUT" an Adjustable Objective(A/O) you'll know what I mean when you start using it.
The A/O, or a Scope with a Side Focus knob, helps elliminate these parallaxes to fine tune a Scope and get the Target Image sharp and clear from as close as 5yds out to Enfinity. Lots of times though, even the A/O or SF yardage marks can be off. I like using a Lazer Range Finder to help find the true yardage to tell me how far my Scope's are off with their A/O or SF adjustments and it helps me to better range a distance in my Mind, when I don't have the Range Finder and out on a hunt, to keep my Scope and Rifle.within it's Kill Range I have them set at. After that, you start getting into Holdovers, Wind Adjustments and so forth.
My computer crashed last week and I lost lots of info I use/need, but the Internet has lots of good info about Scope Parallax and the terms used in how a Scope is made up in parts.
Hope this helps.

Really a LOT of help...got me more interested now in checking out even more scopes !!  I'll do some more research on the topic too.  
Thanks!

Misipy