Ar15 scope mounts question: 1 piece vs rings?

[sid]

soo im new to precision semi automatics, i am/was more a bolt action rifle guy and decided i need a precision semi automatic.
i build 2 rifles with parts i had laying around and bought a 18" and a 20" barrel and today started mounting my scopes.

i had 1 set of rings left over from a previous rifle, they are vortex precision matched rings and were high enough for the scope, so that rifle is finished, the second rifle, which had a little bit larger budget and better components received a temporary scope mount, its one of those ar15 1 piece offset mounts and it works, but its cheap chinesium and im looking to buy some good mounts.

here my question, should i get the classic 2 piece rings that you see on bolt actions, or a 1 piece offset mount?
whats the difference between them, and why would i need it? also, why are there different offset values?

 

[The DFC]

Generally the 1 piece mounts are to keep the scope mounted on the upper-without having anything clamped to the handguard. Different offsets are for different scopes-a 5-25 vs. an LPVO will have ocular lens in different spots to keep your eyebox where it needs to be and keep the optic mounted on the upper receiver.

 

[ColoradoShooter]

Also, the one piece mounts made specifically for AR’s will have the correct ring height (apprx. 1.5”) for proper alignment with your eye.

 

[sid]

ok, makes sense... thanks!

 

[Merle]

One piece scope mounts can also be had with a 20 moa cant as well.

 

[Ravenworks]

One piece scope mounts can also be had with a 20 moa cant as well.

This is always nice to have

 

[sid]

One piece scope mounts can also be had with a 20 moa cant as well.

that is a very good argument, thank you!

 

[MSTN]

A scope mount on an AR' has to do a minimum of two things:

1. Put the axis of the optic at the right height above the comb of the stock for the task at hand.

What is "right"?

Most AR' scope mounts hover around 1.5" on axis above the flat top upper receiver. Iron sights are 1.41".

Note that big block AR's, with more drop to the comb than their little brothers, really need less height, by about 1/8". This is a 1.26" high mount, below.

As an aside, clip-on night vision (CNVD) devices have an axis height of 1.535". That's why a number of scope mounts are this height or at least quite close.

BTW, a forward cant built into any scope mount is NOT a positive attribute when using a CNVD. If you need a forward cant, your scope does not have enough elevation built into it.

2. Provide proper eye relief for the scope. That's the reason for that forward offset thing.

So, to do both, the scope mount really needs to be a one piece affair.

A good, solid one-piece scope mount will also reduce the flex in the flimsy-a$$ aluminum upper receiver.

If you shoot an AR' from an erect position, the higher scope mount heights work better.

But, when you go prone, you'll want a lower mount.

It's easier to take a hard cheek weld standing than to crane your head up shooting prone. That's why the Noveske Shooting Team asked Spuhr to make them their original 1.26" scope mount years and years ago.

Of course, as soon as you make a rule, there will be an exception to the rule. if you're encumbered with a gas mask you might want a much higher axis optic mount:

 

[sid]

That's a nice write up, thanks MSTN !

One more question though, on bolt action rifles I was taught that the scope should be as close to the bore as possible, it seems like that rule does not count for semi autos.

 

[Downtown]

Well, we're used to a 1.5 inch offset already, the approximate height of the iron sights.

 

[The DFC]

Good stuff @MTSN -the gas mask height was the theory that went into the M16 carry handle height. It actually isn't a very good carry handle but when MOPP'd up nice to shoot with

One of your pics-Larue .260 rem w/Nightforce 42mm has a small part of the mount on the handguard rail and I was always taught to avoid this and the reason for the offset cantilever mounts existence?

 

[Downtown]

I've long wondered if a one piece mount takes out some of the concern on mounting on the hand guard, not unlike the old Swan Sleeve.

 

[MSTN]

The DFC - You are correct.

The clamp that secures the scope mount base to the upper receiver, on the left side of the gun (not shown), does not bear on the forend.

This system seems to have no negative effect. Here's my last group at 100 with Prime 130. I would have shot two more, but I was pretty sure I'd choke.

 

[MSTN]

Downtown - That old Swan sleeve on the Mk 12 Mod 0, stiffening up the receiver, is why the Mod 0's shoot so well.

 

[hlee]

That's a nice write up, thanks MSTN !

One more question though, on bolt action rifles I was taught that the scope should be as close to the bore as possible, it seems like that rule does not count for semi autos.

"Mount the scope as low over the bore as possible" is (was) a function of traditional wood stocks having a drop at comb height that ocomodates use of iron sights. This puts your eye much too low to get a good cheek weld with magnified optics. Without an adjustable cheek piece, the "get the scope low" mantra caught on. With modern rifles employing adjustable stocks, we can mount optics with large objectives high enough not to hit the barrel while getting a good cheek weld. Once you realize that you can mount a scope with a 56mm objective on an AR rifle- with a full length top Pic rail- without issue, the "get the scope as low to the barrel as possible" seems silly. Mount the scope in rings/mount that allow for objective clearance and for proper cheek weld and don't stress so much.

 

[The DFC]

The DFC - You are correct.

The clamp that secures the scope mount base to the upper receiver, on the left side of the gun (not shown), does not bear on the forend.

This system seems to have no negative effect. Here's my last group at 100 with Prime 130. I would have shot two more, but I was pretty sure I'd choke.

View attachment 7235725

Yeah-I've done that before and have never seen a negative effect either. It's the end of the world to some though

 

[hlee]

Yeah-I've done that before and have never seen a negative effect either. It's the end of the world to some though

Depends upon the rail and the upper. I have an Aero M4E1 enhanced upper/ rail combo that I tried that on. It was obvious that the front attachment point of the mount, though mostly on the receiver, was clamping onto the hand guard.

 

[The DFC]

Depends upon the rail and the upper. I have an Aero M4E1 enhanced upper/ rail combo that I tried that on. It was obvious that the front attachment point of the mount, though mostly on the receiver, was clamping onto the hand guard.

Accuracy wasn't good?

 

[hlee]

Accuracy wasn't good?

That I cannot confirm, as I moved the mount back before I shot the rifle. I was concerned that the rail may be pushing the mount one direction or another. Moving the mount back one lug so that it was completely on the receiver alleviated that concern. Maybe it was a non-issue, but why take the chance on a precision rifle?

My point was that overlapping the handguard slightly may or may not interfere with the proper mating of the mount to the receiver rail, depending upon the receiver and hand guard in question.

 

[The_Duke]

I have personally never liked the 1.5" standard. After shooting bolt guns with a nice comfortable cheek weld it feels akward to have the scope mounted so high when swapping to ARs. It may be a personal thing but after I switched to rings to lower the height I became more consistent