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steel base and aluminum rings issues?

 
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  #8  
Old 07-24-2011, 01:45 PM
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Re: steel base and aluminum rings issues?

I have to agree with phorwrath as if it were a major issue then it would be an issue even for using different types of steel. Different alloys could see as much change (or at least close) in expansion in steel types as there is between steel and aluminum. There may be something to using all the same manufacturers materials such as Nightforce suggests.
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  #9  
Old 07-24-2011, 03:04 PM
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Re: steel base and aluminum rings issues?

Quote:
Originally Posted by rcdinaz View Post
There may be something to using all the same manufacturers' materials such as Nightforce suggests.
One thing for certain. If we use all Nightforce materials - Nightforce makes a lot of money...
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  #10  
Old 07-24-2011, 03:47 PM
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Re: steel base and aluminum rings issues?

From the engineering toolbox (Coefficients of Linear Expansion) come the following values:

Aluminum: 22(10-6 m/m K) or 12.3(10-6 in/in oF)
Steel 13.0 7.3
316 SS 16 8.9
410 SS 9.9 5.5
Cast Iron 10.4 5.9

As can be seen, there is a huge discrepancy between aluminum and everything else other than 316 Stainless steel.The main reason that aluminum sight bases exist are either 1) its cheap and even cheaper to machine 2) it is an integral part of a forged or cast aluminum receiver, in which case it is probably the best solution. With aluminum rings, all the components are a close match, receiver, rings and scope.

If the scope bases mount on the rail directly over the mounting screws, that has no impact on whether a flat rail will deform due to stress that builds up due to mismatch of expansion coefficients. Believe it or not, this is a very common engineering problem which can cause havoc when temperature shifts occur. If you don't notice its effect on performance, good for you. That alone does not make it a good choice (mismatched materials).
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  #11  
Old 07-24-2011, 05:08 PM
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Re: steel base and aluminum rings issues?

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Originally Posted by westcliffe01 View Post
Where the rings attach to the scope base, they are much closer together than the rail overall length, so the total dimensional change is less. Consequently, even though the scope tube is aluminum, with the rings being close together less deflection will be taking place and at some point a change in impact would not be measurable.
My comments were with regard to this "quote" from your first Post. You state that where the rings attach to the scope base, they are much closer together than the rail overall length. Well the distance between the rings is the same on the scope base as it is on the scope tube. So the differential expansion or contraction will be identical over an identical distance between materials with different rates of thermal expansion and contraction. The length that the scope base rail protrudes past the scope rings in both directions is inconsequential. Same with any of the scope tube that extends outside of the scope rings.

Further, the differential expansion or contraction between the receiver and the one-piece scope rail is only effective across the distance of the fasteners - the scope base screws. Any extension of the rails past the scope base screws will simply result in those portions of the scope rail shifting position differentially with respect to the surface of the rifle's receiver. No differential movement forces can be generated where the two surfaces are no longer connected or restrained beyond the scope base screws.

So I may have misunderstood your communication. And I must still misunderstand.

These facts are true. The differential movement between two dissimilar metals is a function of their coefficients of expansion, the span or distance between fastening points, and the total change in temperature. If the temperature doesn't change - no differential movement occurs. If the distance between fastening points is very small, very little differential movement occurs. Further, differential movement can only create undesirable forces between the two dissimilar metals between points where the dissimilar metals are fastened or connected together. For example, the scope rail could be extended to the same length as the barrel forward of the receiver, and as long as it's not connected to the barrel at any location forward of the forward-most scope base screw on the rifle's receiver, there will be no flexing compression or tensile forces generated between the two dissimilar metals forward of the forward-most scope base screw.

The biggest reason I use aluminum bases and rings rather than steel, is to reduce weight. My hunting is almost all backpack hunting in the mountains of Alaska. The 7075 grade aluminum is more than adequate strength-wise, and weighs much less than any grade of steel.

One of my points earlier was that even if you place a steel scope rail on a steel receiver, and mount steel scope bases on that steel one-piece scope rail base, these are all still fastened to an aluminum scope tube. There's no way to keep everything steel unless you purchase and use steel scope tubes, which virtually nobody does. The fact that the scope rail and the scope tube is separated by about 0.75" will allow for some flexing of the scope rings - more so than between the rifle receiver and a scope rail that are mounted flush together. But there is still going to be some forces imposed due to the differing thermal coefficients of expansion between all of these steel components, and the aluminum scope tube. So we can spend a bunch of time worrying about a problem for which there is no solution, or only worry about it when it becomes a demonstrated problem with its effect on accuracy. And I've not seen any demonstrated negative effects on accuracy that are large enough to stand out amongst all the other accuracy impacting variables. Simply the fact that the temperature changes can have affects on accuracy due to barrel bore expansion and constriction, differing rates of powder combustion resulting in differing muzzle velocity, mirage in elevated temperatures compromising the ability to target the crosshairs, etc.

Until proven to be a significant factor, I'm not going to begin worrying about transitioning from steel to aluminum. If it was a terrible problem, scope manufacturers would be manufacturing 4 lb rifle scopes out of steel tubing, rather than aluminum, and then convincing us that if we continue to use aluminum scope tubes, we'll never obtain reliable, repetitive, consistent accuracy.

Another point of fact and consideration, which demonstrates the relative insignificance of dissimilar metals between receivers, scope rails, scope rings, and scope tubes:
Using a one-piece scope rail creates undesirable forces between the rifle receiver and the scope rail between the scope base fastening screws - and because the scope rail screws are separated by approximately 5 inches on a one-piece scope base, the differential expansion is 5 to 10 times greater than what would exist between the two base screws on a single, two-piece scope base. So there's differential movement generating potentially detrimental forces created over this 5-inch distance as temperatures change. A quick way to reduce these differential forces is to switch out and use a two-piece scope base setup. Then the scope base screws are only separated by 1/2 to 5/8". Do I use two-piece scope bases or recommend two piece scope bases? No way. A one-piece scope rail fastened to a solid one-piece steel receiver with four fasteners is a much more rigid setup. More resistant to lumps and bumps that could cause point of impact changes and loss of the rifle's zero. I only use one-piece bases, and don't worry about the differential expansion and contraction between my aluminum one-piece bases on steel receivers, because differential expansion is a minor issue compared to having the rifle lose its zero due to a light thump or bump while out hunting in the wilds of Alaska.

Further, my range of temperature shifts in Alaska are relatively minor. I worried about this subject once many years ago. After consideration of the above 'facts', I've never worried about it again for purposes of mounting scopes on my long range hunting rifles.

If anyone has an article which reports on experiments that have quantified the negative affects to accuracy consistency caused by the use of aluminum scope rails and aluminum rings on steel rifle receivers - please post the links. There's theoretical problems, and there are real problems. It's doesn't hurt to be aware of the theoretical ones. But it's not paying attention to, and avoiding, the real problems that will bite you in the butt.

Last edited by phorwath; 07-24-2011 at 06:25 PM.
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  #12  
Old 07-24-2011, 05:43 PM
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Re: steel base and aluminum rings issues?

So the comments on the ring positions will vary from scope to scope and rifle to rifle. Most of us try to get the rings as far apart as we can, while maintaining appropriate eye relief. Sometimes thats not an option. I have a savage 24 with a short aluminum scope rail which was machined from a standard one piece rail. This is not too common a gun and no-one seems to make scope mounts for it anymore. In this case, the rings are further apart than the fastening screws, since the break action receiver is so short and it has a very skinny 222 barrel emerging out the end.

The main difference between the 2 and one piece rails is lateral stability. The 2 piece bases are able to twist (rotate) or shift sideways more readily than a one piece rail, which has to bend to do the same thing. That is why the perception exists that a one piece rail is more rigid. Granted, my experience in this problem is related to a field where the thermal gradient is much larger and therefore the deflection of the components is greater. However, the goal in my job is to make the assembly stay together (attached) and not maintain sub MOA alignment.

Many people do not loctite the rail onto the receiver and therefore, whichever end is loosest will simply slide when severe expansion or contraction occurs. Trying to achieve this reliably in assembly is a near impossible problem, since the torque of the front and rear screws would have to be different and the smoothness of the rail surface and the front and back of the receiver would have to be consistent too. These conditions typically do not exist in mass produced parts like rifles and scope bases.

The impact of expansion and contraction on a scope tube with a diameter of 25.4 or 30mm is in the axial direction, and tubes of that kind can take those loads pretty well. Being round, they have essentially the same strength in bending in all directions. However, the scope base with its thin cross section has a weakness and it does not take too much to deflect it up or down. Side to side is not going to happen. This is the reason why integral scope bases (machined into the receiver) are always going to be superior and one can dispense with the rail altogether. All of the best receivers have them, or offer them as an option on a custom build.
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  #13  
Old 07-24-2011, 07:02 PM
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Re: steel base and aluminum rings issues?

westcliffe01, completely agree with what you said on the CoLE which is the problem I was thinking of. I may have assumed wrong but I thought high grade aluminum allows like the ones used in building rifles had similar figures to those of steel. I did go look at how badger builds their rails and I will say it looks like the steel they use has very low expansion properties so you may be correct. So I guess there may be potential there with cheap materials. The only anecdote I really have to go buy since I dont have a lab is that I live in AZ and have rifles that have been in 0 degree weather and +150 (in a car) and I have not seen this problem but if it is very small I could have easily written this off as another issue.

I guess using like mfg components is a safe play when building or upgrading.
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  #14  
Old 07-24-2011, 08:04 PM
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Re: steel base and aluminum rings issues?

rcdinaz, there are a very small number of materials with tailored expansion coefficients. These are the special steels used to mount glass on space vehicles and "high end" jets that need reliable glass to metal seals (blackbird, B1 etc). It is also used for very precise optical assemblies where the sun may strike one side of the assembly and not the other, yet still stay in alignment (in space the temperature difference is quite extreme). Typically, it is Invar and Kovar that are used, together with fused silica (very low expansion glass). I have used these materials in the lab, where we make very precise measurements from ambient to 1000C and the measurement error at 1000C is only about 0.002" from the expansion of the fused silica (at 1000C) and the invar support (at about 90C).

With rifles, the problem is not so exotic, since the action is made from either a low alloy steel, or typically 416 stainless steel. Now and again, you find someone using 17-4ph. If it is a custom action, get the scope rail machined into the receiver. If not, pick a rail with the most similar material to the action.

With aluminum, both A356 (popular cast material) as well as 6061 T6 have an expansion coefficient close to 25 (metric) so it is what it is....
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