Scope mounting???

[customcutter]

Not much experience with the AR platform, so I have a few questions. I'm building an 18" Liberty barrel for hunting mostly and if accuracy is there maybe some long range paper punching and metal bashing. I'm using a Cross Machine Tool billet upper with a Diamondhead VRS T handguard. I thought I would assemble everything and use a Leopould VX3 3.5X10X50 temporarily before buying a new scope.

Are cantilevered scope mounts stronger (better) than rings? Obviously the cantilevered would be mounted on the upper, but looks like a big fulcrum to me. But I'm wondering is there a disadvantage of using scope rings on the upper and handguard? Is there going to be flex between the upper and handguard?

[explorecaves]

There will be some flex between the upper and hand guard. How much depends on the interface and construction. If you are dead set on having your scope mounted that far forward where you would be mounting it to the hand guard, I would recommend looking into the COP upper from Aero Precision. http://aeroprecisionusa.com/upper-receviers.html

[NugginFutz]

I would personally never mount a scope across the hand guard / upper receiver. You're only asking for trouble and frustration. Doing so introduces the possibility of torquing the scope, and requires a near perfect alignment of the two sets of rails. There are many mounts which only attach at the upper receiver that are more than adequate for the task. If you need to forward mount your scope, Larue, ADM and many others make very solid cantilevered mounts.

As far as being stronger, it's a relative question. Both are strong enough, provided they are attached to the same rail. Most rings require the use of a riser, as the straight geometry of the AR's butt stock places the shooters eye higher above the bore than a bolt action, so you need to factor that into your decision, as well.

[Walter]

I used a Cross Machine Tool upper a recent build. I was not getting the accuracy that I wanted. I got a lapping tool and Squared the face of the upper. I am hoping to get better results. Also the barrel extention did not fit tight. This was with a 223 Lilja barrel.

[customcutter]

NF, thanks I definetly don't want to torque a scope. Just slipping the handguard onto the upper things look good but "eye balling" it doesn't mean much. I like the looks of the ADM.

Walter, that's disappointing to hear to say the least. I bought the CMT upper based on it being machined on a 5 axis mill and was hoping that would mean "tighter" tolerances. Did you measure the upper vs barrel extension to see where the loose fit came from?

Did you loctite the barrel extension to upper fit after lapping the shoulder? Have you had a chance to check accuracy since?

Edit:

I just measured mine the barrel extension tunnel and got .996-.997 in 4+ areas, with a cheap caliper.

[Walter]

I am not a big fan of the loctite the barrel extension, so I have not done that. I shot the gun with the scope in the same postion as before the lapping. It was off the paper. It snowed and I have gotten back to it. I was also hoping for better from CMT.

[montana]

Square the upper receiver extension using a lapping tool, lightly spread blue locite on the outside of your barrel and slip it into the upper receiver barrel extension making sure you don't get it on the barrel extension threads, torque the barrel nut down and it will be as tight as it needs to be. I have removed many blue loctite barrels from my uppers with out any problems.

[customcutter]

I guess I could mount the upper and handguard together and put them on my 24" surface plate and see if the top is flat. If everything is flat, I could try to figure some way to measure if there was any flex in the set-up.

[stanprophet]

I would personally never mount a scope across the hand guard / upper receiver. You're only asking for trouble and frustration. Doing so introduces the possibility of torquing the scope, and requires a near perfect alignment of the two sets of rails. There are many mounts which only attach at the upper receiver that are more than adequate for the task. If you need to forward mount your scope, Larue, ADM and many others make very solid cantilevered mounts.

As far as being stronger, it's a relative question. Both are strong enough, provided they are attached to the same rail. Most rings require the use of a riser, as the straight geometry of the AR's butt stock places the shooters eye higher above the bore than a bolt action, so you need to factor that into your decision, as well.

Like Nuggin said, its best to keep the ring off the hand guard. The cantilever allows the eye relief for the scope to be correct, and keeps the mounting off the hand guard. Personally I have run Larue and American Defense Machine. They are both very nice, I think for the money the ADM is a much better deal. I went with QD even though I had not planned on removing the scope. But when I pulled the rail off and switched to the MOE hand guard, the scope had to come off. So it comes in handy when doing work on the rifle to be able to pull it off, and put it back on without worrying about the zero. They are strong enough, if you could break either of the mounts some thing is seriously wrong, and the mount is the least of your worries. Plus the height is perfect for almost any AR platform.

[customcutter]

Thanks, hadn't thought of that aspect of return to zero. Definetly not possible if mounting to the handguard. Still curious to see if the rail is flat between the two when assembled though.

[tackdriver]

I had the m-308 20 moa mount and mounting it square to the rifle was a pain but once I got it right I was pleased. I have since purchased a Rock river 30mm cantilever an it seems like a very sturdy mount. I haven't had a chance to shoot but I will report back when I do.

[kamrr4437]

I would just look at getting an aadmount on the burris P.E.P.R. Havent had a problem out of either one on any rifles. I have the P.E.P.R. on a beowulf with a vortex and it works great. I think a solid mount would be better than rings. If the scope is sitting in the solid mount its perfectly straight. With rings i would think there could be some margin of error this is just my opinion.

[NugginFutz]

That brings up the prospect of long range shooting. Depending on your load, you will need at least 11 mil of elevation for a 1,000 yard shot, if zeroed at 100. Don't intend to shoot this distance? Stop reading here.

The Leupold you mention has 57 MOA of elevation, which works out to 28.5 MOA of elevation from a (non-existent) perfect, no drop 200 yard zero. 11.3 mil works out to 39 moa. Well greater than your 28.5, of which you'l likely have 22ish left, after zeroing your rifle. You mention using the Leupold until you get a new scope so, since you've read this far and are interested in stretching your Grendel's legs, consider the need for elevation when you shop for that new scope.

Also consider that you can buy an additional 10-20 moa of elevation by selecting a mount with a built in cant. The ADM has a 20MOA single piece for $150, Larue has one for about $250. There are a few others, but you get the idea. By putting a 20 moa mount on the rifle, your ultimate selection of scopes is increased.

Just some more to chew on.

[customcutter]

That brings up the prospect of long range shooting. Depending on your load, you will need at least 11 mil of elevation for a 1,000 yard shot, if zeroed at 100. Don't intend to shoot this distance? Stop reading here.

The Leupold you mention has 57 MOA of elevation, which works out to 28.5 MOA of elevation from a (non-existent) perfect, no drop 200 yard zero. 11.3 mil works out to 39 moa. Well greater than your 28.5, of which you'l likely have 22ish left, after zeroing your rifle. You mention using the Leupold until you get a new scope so, since you've read this far and are interested in stretching your Grendel's legs, consider the need for elevation when you shop for that new scope.

Also consider that you can buy an additional 10-20 moa of elevation by selecting a mount with a built in cant. The ADM has a 20MOA single piece for $150, Larue has one for about $250. There are a few others, but you get the idea. By putting a 20 moa mount on the rifle, your ultimate selection of scopes is increased.

Just some more to chew on.

NF, Thanks for the info that's what I need. Like I said I'll try the Leupold I know the elevation isn't there for it but if I find the accuracy I'll go for the Vortex PST 4X16X50FFP, if the accuracy isn't there I may decide on a 2.5X10X44 in PST SFP or HS-T. I'm a little concerned about the exposed turrets on the PST's though in hunting situations. Hopefully one day I'll get to make that trip out west and go elk hunting, so I could put it on my 7mag if needed.