Adjustable check rests: what sorts of specs should I look for?

[Pond, James Pond]

I have a Sig M400. It wears Magpul furniture, including a fairly simply extendable stock.

I am looking at getting an adjustable cheek rest to go with the scope I am planning to buy now that I realise that I need higher scope rings to avoid the front sight post interfering with my sight picture at low mag settings.

On Midway I see things like "GLR16" and "GL-shock" spec cheek rests.

Which would fit my Magpul?

I just want a cheek rest that I can easily raise or drop according to whether I am running Irons, RDS or scope.

[marine6680]

I wouldn't go higher than the typical mounting height for optics on an AR... Which is higher than typical scope rings on a bolt, but not hugely tall.

Yes the front sight will show up in the lower area of your field of view, on lower settings, but it isn't too distracting once you get used to it... Which took me a couple minutes.


I find that I do not need a check rest using a typical hight cantilever AR scope mount. If you do need a little height, then half an inch is about all if I had to guess.


Magpul makes risers that fit on some of thier stocks. They are fixed, not adjustable, and come in different heights, 1/4 up to 3/4 inch I believe.

[Pond, James Pond]

I wouldn't go higher than the typical mounting height for optics on an AR... Which is higher than typical scope rings on a bolt, but not hugely tall.

I'm not sure what the normal height for an AR is, but given how prominent the front sight post is, I think I'd go for the highest I can get for a reasonable price. At the moment that looks like Leupold High Q-R rings that raise the scope tube 25mm off the rail.

That still lets the front sight stick its head passed the rim of the scope by 1cm. Admittedly it will be some way ahead of the objective, but not much: I am a little concerned that I'll spend a fair amount of cash on a scope and find that I can't make full use of the BDC reticle I am going for. Perhaps a duplex would not be as affected...

On the plus side I don't think I'd be using more than the first 3 beads for .223 being a flat-shooting cartridge.

It would mean a cheek-rest though, as I think my line of sight would be about 2 cm below the scope's centre-line. Because I don't want to leave myself with only the scope or the sights, I want the cheek-rest to be adjustable, so that I can switch between the two with relative ease. If an AR type is not possible with my Magpul stock, then I'll get one of those soft, padded strap-on versions.

[marine6680]

Height depends on the scope tube diameter, but average is 1.5 inches.

Here is a link to Burris' site, for their AR mount, it gives dimentions.

http://www.burrisoptics.com/arpepr.html


The front post is pretty much a non issue, at low zoom, you are most likely not shooting at long range and can shoot normally. The post will not be in the way of the scope reticle. The post will only be in the lower part of your vision.

At long ranges, when you use higher magnification, the post blurs out and disappears, so it won't affect hold overs.

I have the Burris mount and a 1-4x scope. My other AR with a red dot is mounted at the same height. I don't find I need a riser for either rifle. I can transition between irons and the red dot easy, and I can see/use the scope on the other rifle easily. Even my fiance has no issues.


The 223 is pretty flat shooting. When setting up irons or a red dot, I do a 50 yard sight in. I can then hold normal point of aim and only be a couple inches high or low out to a bit over 200 yards.

I believe it works for 50m zero as well.

[Fishbed77]

I am looking at getting an adjustable cheek rest to go with the scope I am planning to buy now that I realise that I need higher scope rings to avoid the front sight post interfering with my sight picture at low mag settings.

Completely unnecessary for a carbine like the M400.

At anything but the very lowest magnifications, the FSB will be invisible. At lower magnifications, it will be ghosted and non-obtrusive.

At no magnification and using a red dot sight, depending on your set-up, the dot will either be super-imposed on the front sight (absolute co-witness) or above the front sight (1/3 co-witness). In either circumstance, the front sight will not be obtrusive at all when using the RDS properly (both eyes open and focused on the target). This is the set-up the US military has used for years with the Aimpoint M68 CCO.

[Pond, James Pond]

At no magnification and using a red dot sight, depending on your set-up, the dot will either be super-imposed on the front sight (absolute co-witness) or above the front sight (1/3 co-witness). In either circumstance, the front sight will not be obtrusive at all when using the RDS properly (both eyes open and focused on the target). This is the set-up the US military has used for years with the Aimpoint M68 CCO.

That is the case at the moment: with my RDS, I have what you've just taught me: 1/3 co-witness.

However, that is my RDS, not the scope I am looking at buying.

If you guys say the FSB will not interfere, I'm inclined to trust your collective experience.

To make my stance clear: my ideal situation would be to fit my scope, as low as possible so that I can keep my existing cheek-weld height, ie that of my irons. It would keep the whole set-up leaner and meaner (IMO) and avoid having to buy yet another bit in an adjustable cheek-rest.
However, I don't want to do that at the expense of my scope's perfromance because of the FSB intereference.

So, do you still feel that low rings, putting the scope centre-line at the same height as my rear peep hole sight, thus keeping my existing cheek weld height would not put my scope in conflict with the front sight?
If so, then I'll be very happy, but I just have no means of making that judgement myself, with my experience base.

[marine6680]

That would be too low.


Here are my rifles... My rifle has a red dot mounted for lower 1/3 co-witness. The fiance's rifle has a 1-4x scope, mounted at the same height as the red dot is at lower 1/3. I will show two versions, before free float and after.

Before free float, if the scope was on 1x magnification, the sight picture was similar to a red dot lower 1/3 co-witness. When on 4x, the front post ghosted and blurred away to being unnoticeable.

There is at most a centimeter difference in height of the sighting plane between the irons and the scope/red dot. I treat the scope and red dot as the primary sight plane, and the irons are secondary. I just squish my face town a little and I can use the irons.

[pilpens]

Here is a list of scope mounts and heights:
http://www.maxicon.com/guns/optics/o...ece_mounts.htm

[Pond, James Pond]

@marine6680:
Uncanny: if your first two ARs had a FDE lower and receiver, they'd both look like mine as it is and mine as I want it!! The furniture is exactly the same.

From your description it sounds like your cheek-weld with that stock gives you a line of sight straight through the optic and you need to squeeze down for the irons.

For me it is the opposite. My cheek-weld puts me right in line with the irons and I'd have to raise my cheek a bit (also about 1 cm) to be looking right down the optic. Not an issue with the RDS but would be with the scope.

This means that I would need some kind of additional cheek-rest, but I may look at the soft strap-on rest that can be removed/replaced easily if I choose to go back to irons/RDS.

Time for some measurements.

@pilpens
Thanks for the link. Useful as a guide, although I probably going for a pair of single Q-R rings. I did see a short section on those and that has been handy. Partly for cost, partly for the flexibility of positioning.

[Pond, James Pond]

Just taken the measurements, using the RDS as a model, a tape-measure precariously balanced on the rail and a foot as a rest. (only the best technique in the PJP household! )

Firstly, I think I may have made a mistake about how my cheek-weld's performance: I tried another technique and managed to get a cheek-weld that put me right in the middle of the RDS, not the lower third as is needed to look through the irons. That implies my cheek-weld could be similar to marine6680's.

Secondly, I can now see that if I get a set of 1" rings such as the Leupold High PRW rings, I think the FSB would occupy the whole bottom half of the sight picture at 1x.

That may be a bit of a gamble. I think a 1.25 to 1.5 inch ring height would be better for my sight picture but leaves the problem of cheek position.

The choice is clear as mud, eh?

[marine6680]

My scope mount is 1.5 inches to the center of the Rings.


I was going to suggest you try adjusting your check weld to see if you could get a sight picture without a riser.


Here is a couple pics to help visualize co-witness.



Lower 1/3 real world pic.

[Pond, James Pond]

I have to keep going back to the rifle to try and reproduce the effects you've shown.

To achieve what you showed in that picture I've tried to mimic the sight picture by cutting a 20mm circle into a piece of card.

The problem with this is that I can't be sure I am looking through the circle at perfect right angles. As you know, with a scope that is clear as your get the black shrouding effect if you are not square on.

Still, from what I can see, if I really squash my cheek down firmly, with the mock card objective set at the 1" ring height (also the bottom edge of the RDS), I get a similar view to your picture.

If I "raise" the scope with 1.25" rings as well as loosening my cheek weld the FSB drops far lower, almost off the edge.

Was I looking perfectly horizontally through the card both times? I don't know.
Can I be sure of my conclusions? I don't know!!

[marine6680]

I believe you are over thinking and complicating the issue.

Remember... co-witness is a red dot thing. I mention it because the mounting height to achieve absolute or lower 1/3, is consistent for the most part between rifles. And mount height between a scope and red dot are similar.

The vortex sparc red dot comes with an adjustable mount... The instructions show that a 36mm mount height gives an absolute co-witness. A 40mm mount height gives lower 1/3.


If you buy a scope with a 30mm tube, you will want rings that put the scope center-line around 40-42mm above the top of the rail to properly avoid the front post from interfering. As we mentioned, it will be visible on low magnification, but occupy the lower area of your field of view.


Since you have a red dot currently, if you have it mounted in a true lower 1/3, if you feel like you can get a proper check weld, then you will be fine when you switch to a scope with a similar mounting height.

[Pond, James Pond]

Since you have a red dot currently, if you have it mounted in a true lower 1/3, if you feel like you can get a proper check weld, then you will be fine when you switch to a scope with a similar mounting height.

Wow... so it really was that simple!!

I guess I was making it a bit more complicated. Well, my likely scope is a 1" tube, but following your example, I should be able to get 1" high rings and I'll only have lower third FSB interference...

I may still only buy the scope for now and then offer it up to the rail, holding it at different heights to see what is acceptable...

[marine6680]

If you measure from the center of the scope tube, or the middle of the mount were the lower and upper rings join. The height will be 1.5 inches... Or about 40mm.

[Pond, James Pond]

I saw a shop selling Warne's rings. The 1" tube high mount rings were actually only 15mm, so about 3/5s of an inch tall.

No good!!

I may have to look to Midway!

[zach_]

I wound up using the Millet mount. The center of the ring winds up 1 9/16" above th rail. I get a green blur from my green sight post at low power. The blur is at the top on the image though. Take your finger and run it up the front sight post. Mine image starts to distort at the top. Everybody else says the post comes into the bottom of the field of view???

[Pond, James Pond]

There's an 8 week (if not longer) wait on the Nikon Monarch 3 1-4x20, so that is on hold for now. I've asked an online shop if, instead, they can do me a deal on the Leupold VX2 1-4x20. It retails at £80 more than the Nikon and that is a bit steep. For the same money I can get a Nikon Monarch Gold 1.5-6x42!

If they can lower the Leupold price I may well go for that. Rings may have to wait!

[marine6680]

Hopefully not too long.

[tobnpr]

If you need to raise the comb height for cheekweld, consider the Magpul PRS- it's a great piece of kit if you have the coin. Also allows for adusting LOP.