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Discussion Starter · #1 ·
Hey all -

I just got my first XCR, and I am a total virgin to the XCR/AR-15 style platform.

At the moment, I can't afford another stight system, so I will be mounting a Leupold Vari-X III cope that I have lying around. Can I just use scope rings to attach it, or will I need a base to raise them up higher?

Any help would be appreciated. Eventually I would like to check out some other sight systems and install BUIS, but the scope will have to do for now.

- Jim
 

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You'll need picatinny mounts. For height consider low to medium, with low probably being the better option if you dont intend on using BUIS.

The fastest way to make the XCR "feel wrong" is to mount your optics too high.
 

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Discussion Starter · #4 ·
Thanks for the advice all!!

So it sounds like I just need scope rings that attach directly to the picatinny rails, correct?
 

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That is correct.

You simply need Picatinny rail mounted scope rings.
 

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What size is your existing scope? Make sure you get rings that will be high enough for the objective bell of the scope to clear the top of the rifle. You want the scope low, but not so low that it touches the top rail. If it touches you may be putting stress on the scope tube when you torque down the scope rings.

I wanted to see what my scope would look like on the XCR, so I mounted my Badger medium rings on the rail and laid the scope in the rings. It didn't even come close to laying flush in the front ring. My scope is a Falcon with a 44mm objective. Depending on the scope and ring manufacturer, you may need medium or high rings, so check carefully before you buy.
 

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You can use Weaver rings too,not that you would probably want to,but Weaver rings will work on a Picatinny rail,but not the other way around.True 1913 spec Picatinny rails have a slightly wider recoil groove than Weaver bases,so a true Picatinny ring will not fit into a Weaver groove.Just make sure if you do use Weaver rings make sure when you place the rings on the rail you slide the apart from each other before you tighten them so they will not be able to move under recoil at all.
 

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You can use Weaver rings too,not that you would probably want to,but Weaver rings will work on a Picatinny rail,but not the other way around.True 1913 spec Picatinny rails have a slightly wider recoil groove than Weaver bases,so a true Picatinny ring will not fit into a Weaver groove.Just make sure if you do use Weaver rings make sure when you place the rings on the rail you slide the apart from each other before you tighten them so they will not be able to move under recoil at all.
Um, that's not quite right. You'd want to move them both as far forward as possible so neither would have any room to slide during recoil. If you had the rear ring slid toward the rear of the rifle, the recoil lug under the front ring would be absorbing all of the recoil by itself. On a XCR this probably isn't as much of an issue as it would be with a heavy-recoiling caliber, but it's good practice to always move everything that should absorb recoil towards the front of the gun.
 

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Ok First I don't think I'm wrong,but to understand this a little better and for any of it to matter let me interject this first
Picatinny/1913 - These rings and bases are similar to weaver style. The main difference is that they are based on specifications standardized by the U.S. Picatinny Arsenal in 1913. The main physical difference is in the size of the recoil lugs on the bottom of the rings and the corresponding larger size of the recoil slot in the top of the base. This size is .206”, much bigger than the .180” in the weaver style. Therefore, a Picatinny ring will not fit into a weaver base, but a weaver ring will fit into a Picatinny base. Some manufactures label their products as Picatinny/Weaver but are actually weaver style. Some Picatinny rings are tightened with an inch-pound torque wrench to ensure a return to zero when reinstalled. Our military M-4 carbines utilize Picatinny rails.
That is from the Opticsplanet website.....here.....
http://www.opticsplanet.net/how-to-install-bases-mounts-rings-riflescopes.html
In other words,not all Picatinny rings marketed as such are true 1913 spec rings.Leupold mark 4 rings are an example of this,they would be considered "picatinny rings" by most people,but they use a Weaver recoil lug dimension.
Anyway the part about me be wrong about ring placement.First your mounting rings on a semiautomatic firearm,why you say should that matter? Well you have two forces at work on the rings holding your optics,first is the initial backwards recoil,then you have the force or momentum the weight of the bolt,bolt carrier,and piston induce to the rifle in the opposite direction when they return to battery.Considering the stored energy the recoil spring has,it's a pretty good smack!.Remember,every action has an equal and opposite reaction.
Therefore putting your rings apart as far away from each other in their respective recoil lugs,keeps the rings from the possibility of moving in either direction.If you put them both forward as possible as you suggest,then both rings still have the possibility of sliding backward when the bolt,bolt carrier and piston assembly return home,maybe even more so because instead of sitting still on the initial backwards recoil impulse the scope now has the tendency to keep going rearward with the rifle.It might induce a little more stress on the front ring,but the rings should have enough holding power that it shouldn't be moving to begin with,after all look at European centerfire rifles,most just use a dovetail mounting rail without a recoil groove at all.
I've used cheapass $5.99 Leapers aluminum rings for several years,and have never come close to breaking a set.Even on my short barreled PTR-91k in 7.62x51,I've busted a scope on it but the never the rings.I can assure you that rifle while not brutal on your shoulder,will punish a scope.If you break those $175 Badger ones your in trouble.
 

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Ok First I don't think I'm wrong,but to understand this a little better and for any of it to matter let me interject this first
Picatinny/1913 - These rings and bases are similar to weaver style. The main difference is that they are based on specifications standardized by the U.S. Picatinny Arsenal in 1913. The main physical difference is in the size of the recoil lugs on the bottom of the rings and the corresponding larger size of the recoil slot in the top of the base. This size is .206”, much bigger than the .180” in the weaver style. Therefore, a Picatinny ring will not fit into a weaver base, but a weaver ring will fit into a Picatinny base. Some manufactures label their products as Picatinny/Weaver but are actually weaver style. Some Picatinny rings are tightened with an inch-pound torque wrench to ensure a return to zero when reinstalled. Our military M-4 carbines utilize Picatinny rails.
That is from the Opticsplanet website.....here.....
http://www.opticsplanet.net/how-to-install-bases-mounts-rings-riflescopes.html
In other words,not all Picatinny rings marketed as such are true 1913 spec rings.Leupold mark 4 rings are an example of this,they would be considered "picatinny rings" by most people,but they use a Weaver recoil lug dimension.
Anyway the part about me be wrong about ring placement.First your mounting rings on a semiautomatic firearm,why you say should that matter? Well you have two forces at work on the rings holding your optics,first is the initial backwards recoil,then you have the force or momentum the weight of the bolt,bolt carrier,and piston induce to the rifle in the opposite direction when they return to battery.Considering the stored energy the recoil spring has,it's a pretty good smack!.Remember,every action has an equal and opposite reaction.
Therefore putting your rings apart as far away from each other in their respective recoil lugs,keeps the rings from the possibility of moving in either direction.If you put them both forward as possible as you suggest,then both rings still have the possibility of sliding backward when the bolt,bolt carrier and piston assembly return home,maybe even more so because instead of sitting still on the initial backwards recoil impulse the scope now has the tendency to keep going rearward with the rifle.It might induce a little more stress on the front ring,but the rings should have enough holding power that it shouldn't be moving to begin with,after all look at European centerfire rifles,most just use a dovetail mounting rail without a recoil groove at all.
I've used cheapass .99 Leapers aluminum rings for several years,and have never come close to breaking a set.Even on my short barreled PTR-91k in 7.62x51,I've busted a scope on it but the never the rings.I can assure you that rifle while not brutal on your shoulder,will punish a scope.If you break those 5 Badger ones your in trouble.
The OpticsPlanet link you provided does not state anywhere to move the rings to opposite ends of their respective grooves when mounting. Please provide something other than your opinion about mounting rings or bases.

Here's Badger's mounting instructions:
http://www.badgerordnance.com/folders/downloads/3/Mounting rings and bases.pdf

And from that PDF we get these bits of wisdom:
First the base: ... snip ...
Now tighten all 4 screws, pushing the mount forward as you tighten. If you have an
adjustable torque wrench, the mounting screws should be 15 INCH-POUNDS.

... snip ...

Rings: ... snip ...
Once the rings are in the correct location, remove scope, push forward and tighten each
ring (the 1/2" nut) finger tight, using your 65 INCH-POUND torque wrench tighten each
nut.


According to you, the "equal and opposite reaction" theory would result in the scope rings moving forward and back the exact same amount due to the force of the rifle bolt returning to its forward position. I agree that semi-automatic weapons have a certain amount of forward momentum after the initial recoil, but if it were indeed equal to the initial recoil, why would you need to lean into the rifle when firing? Instead, you could stand perfectly straight and the rifle would "pull you back" from falling over backwards. In fact, there should also be no need for muzzle brakes as the initial muzzle flip should be accounted for by the "equal and opposite" force of the returning bolt. Fully automatic firearms should also be just as easy to fire, what with all that bolt cycling pushing the gun back towards the target.

Some scope base manufacturers have recoil lugs designed into the base that will keep the base from moving forward on a rifle's receiver under heavy recoil. They never build an "anti-recoil" lug into their designs.

Finally, with the rifle unloaded, hold the weapon at arm's length and release the bolt from its rearward locked position. The gun should wiggle slightly as the bolt returns to firing position. If you can honestly say that this wiggle is the same as the recoil felt when firing the gun, your statement about "equal and opposite" may hold water. I'd venture to say you could do this a thousand times with the heaviest scope mounted on the cheapest rings and not suffer any movement of the rings on the rail.
 

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Ok I should have left that statement out of the equation.I had a feeling you would overanalyze what I meant,yes I know...how are you supposed to know my intent.Well obviously you can't so I should have been more more precise.
You cannot however deny there is is a return force at work against the rings in the oppsite direction on a semiautomatic firearm.It may not be an equal reaction in terms of energy as the recoil impulse,however it would seem that the forces incurred by the reciprocating parts against the scope and rings would be significant since they(scope,rings) are already in motion in the opposite direction.In other words the impact of the bolt closing hard has to snap them somewhat to try to go in the opposite direction while they are still traveling rearward.
The instructions for Badger you posted are,I believe intended for bolt action rifles.Since they have no reciprocating parts to impact in the oppsite direction of recoil,both rings in the forward position makes sense.Either way if Badger rings are true Picatinny spec,then they should fit the Picatinny base with little or no play anyway and the point is moot.You can think what you want about my "opinion",I stand by my method for semiautomatic firearms,because it seems the most logical to me.
 

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You are both right... and both wrong (technically speaking).

The equal and opposite reaction that occurs when a shot is fired IS THE RECOIL. The recoil is the equal and opposite force of the bullet (and gas) leaving the muzzle. Now, because the bullet undergoes deformation and energy is lost to heat, we must look at the conservation of momentum to calculate the forces. Momentum = mass x velocity. The initial velocity of the system (bullet and gun) is zero: so net momentum is zero. However, after firing the bullet is traveling with velocity (v) and mass (m) in one direction (let's say positive); and the gun is traveling with a different velocity (v') and different mass (m') in the opposite direction (let's say negative). Since the overall momentum of the system is zero, than the momentum of the bullet (m*v*1) plus the momentum of the gun (m'*v'*-1) should also equal zero. In fact, mv=m'v'. m is very, very small and m' is huge (relative to m). We can see that the velocities are going to be inversely proportional to the masses, and therefore m (bullet) has a high velocity and m' (gun) has a small velocity.

Yet the energy that you feel as the gun recoils into your shoulder is expressed as KE (kinetic energy) = (1/2)*m'*v'^2. Now, the truth is that you not all of the recoil is passed all the way to you. Much of it is absorbed by the recoil spring. The recoil spring (being a spring) converts KE to potential energy. The amount of potential energy (PE) it can convert depends on the spring constant (k) and the distance the spring is displaced (x). The distance the spring moves is probably limited by the overall travel of the bolt carrier or slide. Therefore, the spring cannot convert all the KE into PE. The remaining KE (that is not converted by the spring) is what you feel. This is the main reason why shooting identical bullets from a semi-auto versus a bolt-action will have different felt recoil. The bolt-action rifle has no spring to absorb KE.

So the amount of energy that you feel when the gun recoils is equal to the gun's initial kinetic energy minus the amount of energy absorbed and stored within the recoil spring. The recoil spring then converts that potential energy back to KE as it recoils back to it's original length. So, the amount of force that you feel when the gun moves forward after recoil is approximately equal to the amount of force absorbed by the spring (there are however, more forces we could examine, such as the force exerted by the recoil spring rearward as it is both compressed and released... but not here). If you need anecdotal evidence of this, try replacing the recoil spring in your handgun with a stronger recoil spring. You may notice less rearward impulse from recoil, but you should also notice that the handgun rotates farther forward when the slide returns to battery.

Finally, we must recall that the rearward energy of the gun as it recoils is also dissipated where it contacts your body. Your body is soft (some of us softer than others), and as such creates an inelastic collision (between gun and flesh). This does not conserve energy (as some of it is utilized to deform tissue or whatever). Therefore, even more of the initial KE is lost before the bolt/slide recoils to battery.

So, the felt recoil will always be less then the force/energy of the bolt/slide returning to battery. However, both forces DO exist, and therefore, both of you are correct.
 

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Agreed, but just to clarify one thing: The recoil spring is not (in a gas-operated rifle) absorbing the recoil if the fired round. It is absorbing the recoil of the bolt after the bolt is pushed out of battery by the force of expanding gas upon the piston (in the case of the XCR). The bolt remains locked until the bullet passes the gas block about 9 inches from the chamber. Until the bullet passes this point, any and all recoil is being delivered to the entire weapon as a locked unit.

In practice, this makes little difference to the shooter, but the physics are interesting. Science is fun!
 

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That is true, but the force with which the bolt moves rearward is a result of the gas expanding forward. So either way, the the bolt (and recoil spring) are absorbing that energy. If all that gas travelled directly out the front of the barrel, the energy would be felt directly on the gun. However, the energy removed from the gas (when it is used to move the bolt backward) decreased the magnitude of the equal and opposite force of the gun moving rearward. That energy is then stored in the recoil spring until it recoils.

It really just depends on what you define as the system. I mean I also neglected to include the KE from the firing pin and hammer as they move forward.

Another thing to think about is the difference felt when firing the last round and the slide/bolt locks to the rear.
 

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... It really just depends on what you define as the system. ...
Precisely, and you excluded the "inelastic" tissues of the shooter's body in your previous post, which may have been a bit hasty. A portion of the energy that is "dissipated" in the body of the shooter results in tissue damage, which accumulates. That particular portion of the energy will become quite noticeable, given sufficent use. That having been said, I'm doubtful that, for any given discharge, it constitutes an otherwise significant portion of the energy communicated to the shooter's body.

Isn't "recoil spring" a bit of a misnomer, in this case? In a recoil operated firearm, the spring is compressed by the recoiling barrel and bolt. The energy that compresses the spring in a gas operated action is not from the recoil of the weapon.
 

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How about everyone buys a Larue one-piece mount then it wont matter. ;D
 

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I am in Dallas right now and wanted to go visit them but it is a 3+ hour drive. Doesn't look like I will make it this trip.
 
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