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View Full Version : Can someone explain AR scope "risers" to me?


Blue Ridge Reef
10-16-2011, 8:57 AM
Here is my Calguns built rifle, pardon the dark photo:
http://i53.tinypic.com/29olpts.jpg

Looking at pictures of similar rigs using the Eotech I have, I keep seeing the same optic mount. http://www.laruetactical.com/larue-tactical-eotech-qd-mount-lt-110

For a finished product like this (rifle and photo by Pacrimguru):
http://i51.tinypic.com/11jral4.jpg

It seems counter-intuitive to deliberately make the red dot farther away from the barrel, but there has to be logic behind it. I can shoot with my flip ups where the optic is currently mounted, raising the optic doesn't seem like it would help matters. And to be honest, I pretty much *never* use my BUS on this rifle. But if this mount would improve my shooting in any way or provide practicality I am not seeing, I will consider it.
And a secondary question, any BUS you'd suggest? I am not thrilled with the MBUIS. I have used Troy and K.A., and like those better -but if I'm going to invest in high end ones, I want to do my homework on all options. I *love* the level front sight in Pacrimguru's picture, but I PMed him about it and that one is part of the gas block on that rifle. Not sure it would be feasible to remove the Vltor lo-pro to replace with that. Anyway -thoughts, opinions, etc. appreciated.

dieselpower
10-16-2011, 9:45 AM
The RDS is used with both eyes open. Some have adapted their mind to ignore the body of the RDS and they only "see" the floating dot or in the case of an EOTech the reticule. The center weight of your hold on the firearm with the distance from your eye makes the best spot to mount an RDS nearly over the delta ring. You will see many Aimpoint 2 and 3 series mounted with cantilevers positioning the RDS over the delta. Many people mount the RDS at the last spot on the rail just before the delta. What you don't want is the RDS at your nose where the body of the optic is now causing one eyes sight to be blocked. Place your hand in front of one eye... you can "look" pass it with the other eye, but your vision is blurred.

The "riser" places the RDS above the BUIS center at a 1/3 co-witness. This allows you to have a wide viewing field while the dot is in the center. It really doesn't matter much on flip up BUIS systems, but it does with Fixed BUIS systems.

Its all about cheek welds (where your cheek is positioned on the stock while aiming) in relation to your aiming or sight picture.

http://i655.photobucket.com/albums/uu272/Wiringguy/cowitnessdiagrams-1.jpg

Hope this helps.

starsnuffer
10-16-2011, 9:51 AM
I've always felt that a better way to accomplish this was to use shorter BUIS, like the Troy Micro flip sights, but to each their own.

-W

dieselpower
10-16-2011, 10:30 AM
I've always felt that a better way to accomplish this was to use shorter BUIS, like the Troy Micro flip sights, but to each their own.

-W

Sorry, you don't understand the trajectory of the projectile if you think that. The micro flip ups are made for HIGHER rails, NOT to decrease the "height above bore" of the BUIS or to allow a 1/3 co-witness.

While its possible to work out your own dope for a custom BUIS system, there is little benefit to it. You would also need to manufacture a different stock for your cheek weld. You would also need to make the rear sight higher than Troy has since you would need a level that is constant for a 224 trajectory which places the muzzle end higher allowing for the arc. With a level BUIS system closer to the bore you get no long range since the projectile drops from view of the BUIS system that lacks a elevation adjustment.

Look at my drawing here at the top. Think about a BUIS system closer to the bore, what does that do to the trajectory as seen through them? What about the angle of the bore in relation to the height of the two (front and rear sights)?

http://i655.photobucket.com/albums/uu272/Wiringguy/50yardzerords.jpg

Blue Ridge Reef
10-16-2011, 10:40 AM
Thanks for your help, Diesel, though I'm still a tad unclear. With a 1/3, it looks as if the iron sights or RDS one would not align -the same question I had about the Troy micro. If the rear sight is lower, why wouldn't you shoot higher? And with the VIS, there's no delta ring (that I see). I'm assuming proper positioning is just forward of the magazine well, though. Do you think I should move my optic forward a couple of notches?
I'm sure I'm overlooking something very basic! Thanks in advance for your patience.

starsnuffer
10-16-2011, 1:27 PM
Thanks for correcting me, Diesel. I see what you're saying. I run the micros on my MR556 and I forgot that the rail height is different then a regular AR.

-W

negolien
10-16-2011, 1:28 PM
You're good. Risers are to get up on the big gas block sights I think but I' am not real sure. I 100% co-witness with my XPS just like 1st picture and my xps is about same spot. Shoots fine no issues.

dieselpower
10-16-2011, 3:09 PM
Thanks for your help, Diesel, though I'm still a tad unclear. With a 1/3, it looks as if the iron sights or RDS one would not align -the same question I had about the Troy micro. If the rear sight is lower, why wouldn't you shoot higher? You would shoot lower, the angle of the barrel is lower. And with the VIS, there's no delta ring (that I see). I'm assuming proper positioning is just forward of the magazine well, though. Do you think I should move my optic forward a couple of notches? Thats up to you. can you see pass the body with both eyes open?
I'm sure I'm overlooking something very basic! Thanks in advance for your patience.

You are correct. With a 1/3 co-witness the Iron sights line up in the lower 1/3 of the optic, the optics dot will be seen as normal since it moves to where your eye is in the glass. The reason to use a 1/3 co is when you have a normal and proper cheek weld the BUIS is low and out of the way. It only takes a millisecond to flip them up and reposition your cheek to use them. Many people say they want a 100% co-witness in case the RDS fails, then the BUIS is right there, which is fine. On flip ups you have to move and manually flip them up so you are breaking your cheek weld anyway. Its all about the cheek weld and whats comfortable to YOU and only YOU. Some people want the same cheek weld no matter what aiming device they are using. Some can swap to different ones without issue.


I see no reason to use a 1/3 co on flip ups. I ran my C-more without the spacer for a 100% co-witness for a long time. I have now placed the spacer back in to go with a 1/3 lower Co-witness. In my normal and customary cheek weld I look over the Fixed A2 BUIS into the RDS and the dot is 100% in the center. I have a flip down forward tower that stays out of the way until I need it. I flip it up, lower my cheek weld and use the BUIS as my POA (point of aim).

I also fancy the HK style front post over the mil-spec A2.

Blue Ridge Reef
10-16-2011, 5:44 PM
Thank you! I completely understand now. It's dark out, but I'll check my field of view on my rifle in the morning. I seem to shoot well with it, but it wouldn't surprise me at all to make a minor adjustment like that and not believe I hadn't done it sooner!

Odd how many Eotech/LaRue's I've run across on ARF.com with flip ups. Perhaps some of that is cool factor, but those things aren't cheap! Thanks for saving me the bucks.

dieselpower
10-16-2011, 7:01 PM
Thank you! I completely understand now. It's dark out, but I'll check my field of view on my rifle in the morning. I seem to shoot well with it, but it wouldn't surprise me at all to make a minor adjustment like that and not believe I hadn't done it sooner!

Odd how many Eotech/LaRue's I've run across on ARF.com with flip ups. Perhaps some of that is cool factor, but those things aren't cheap! Thanks for saving me the bucks.

actually gear up and walk around as if you were using the firearm at night. Snap it up as fast as you can. Practice NOT focusing on the RDS and staying focused on your target. practice clearing your house at night, clearing your backyard, check your gear and see what its like using it without the aid of daylight.

try not to scare the neighbors

shafferds
10-16-2011, 8:15 PM
Thanks for the informative post Diesel