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Posted: 2/7/2002 4:15:52 PM EDT
I was discussing scope mounts with someone and they came up with the notion that an aluminum scope mount (the one in question is made of 7075 aluminum by Mounting Solutions; my choice at the moment) "vibrates", thereby detracting from the accuracy of the rifle...
His recommendation for a scope mount was the Leupold Dual Dovetail, which I don't want to use.

I thought about his comment on the vibrations of the aluminum and decided to post it here for comments.

My theorizing debunked his theory for the following reasons:
1. The Leupold Tactical scope I plan to use is made of aluminum.
2. The Remington PSS I just purchased has an aluminum bedding block in its stock.
3. The Harris bipod I'll be using is made of aluminum.
4. With all of the aluminum parts and accessories available, this theory of his didn't make any sense.
5. If aluminum and steel were incompatible for rifle accessories, we'd be forced to use either/or.

I see no merit in the comment he made in relation to actually affecting the overall accuracy of the rifle and its components.
Does anyone have any information on this?
Link Posted: 2/7/2002 5:28:36 PM EDT
Go to Varmint Al's web site. He has a great section on scope mounts. This guy is really sharp.
Link Posted: 2/7/2002 5:32:19 PM EDT
[Last Edit: 2/7/2002 5:39:09 PM EDT by Boomholzer]
I am unfamiliar with the mount in question. The aluminum material alone should not be in question.
The mount vibrates? Well yes, the whole rig will vibrate after a shot.
The target has been acquired, the trigger pulled and the bullet has been sent on its way.........
After the almost instantanious pressure charge of the cartidge sends vibrations down the barrel, through the receiver, and up to the scope mount, the mount will experience vibration. The mount will also return to zero after the event. Furthermore, such minute vibration is way below the human factor of error (in terms of accuracy).

I am assuming that this person is of the opinion that the mount will not re-zero itself after a shot(s). The event of a shot (in any modern sporting caliber) is not enough to compromise the aluminum mount.

Is this person speaking in full-auto terms? If then; again, the human error would bury the off-zero vibrations of the aluminum-mounted optics.

I find no merit to that argument.
Link Posted: 2/7/2002 7:21:48 PM EDT
bybon--
Been there before. Excellent site and I found what I was looking for. It appears that the concern is not 'vibration', it's 'thermal expansion', but, not much of a concern.
I'm sticking with my mounting decisions based on what I read here.
The link to VarmintAl's shooting page is:
varmintal.com/ashot.htm

For those of you that don't go to the link, here's the text:
ONE OR TWO PIECE SCOPE BASE.... The question of one piece or two piece scope base is a tricky question. There are
a number of other factors that need to be considered to optimize the situation. Let's assume that that both bases fit the action
reasonably well. First, the action is usually 4140 steel or equivalent with a Coefficient of Thermal Expansion (CTE) of 6.2E-6
in/(in-°F). One exception I know of is the Stolle Panda action which is aluminum. Aluminum 6061-T6 has a CTE of 13.1E-6 or
more than twice that of steel. The difference in thermal expansion is the major problem with a one piece aluminum base on a
steel action. When the base is screwed onto the action at say 72 °F and assuming it fits well, it is stress free in the axial
direction. If the assembly is then heated to 120 °F (which is possible in full sun), the differential in expansion between the base
and action for a 5" span is:

dL=L * da * dT,

where dL is the length difference (in), L is the span length (5 in), da is the difference in CTE's between steel and aluminum
(6.9E-6 in/(in-°F)), and dT is (48 °F) the temperature change.

Running the numbers, the 5" section of a free base would lengthen about 0.00166" more than the action would. The
cross-section area of the B-Square base on my Savage 12BVSS is approximately 0.12 sq-in and force required to compress
the 5" section of the base 0.00166" is,

Force = K * dL = 398 lb

Where the spring constant, K = Area * Elastic-modulus / Length

Area = .12 sq-in

Elastic-Modulus = 10E6 lb/sq-in

L = 5 in

K = 240,000 lb/in and the force required to shorten the base 0.00166" is 398 lb. This is a lot of force! The stress in the
aluminum base is 3,300 psi which is well below the yield stress.


Link Posted: 2/7/2002 7:22:27 PM EDT
--------continued:
ASSUMING THE ACTION IS RIGID.... This assumes that the much larger steel action is rigid and is a reasonable
assumption because the action is much larger than the base. Taking into account the flexibility of the action involves a statically
indeterminate problem. This would require very sophisticated Finite Element Analysis including the geometry of each action and
one piece base to determine the deformations of both. Now assume that your rifle scope is made of aluminum. If you use a two
piece base, then a 0.00166" displacement goes directly into the scope tube. If the section area of the scope tube is half that of
the base, then the force would be half or 199 lb. But in either event you are going to shorten the center of the scope. Using an
aluminum one piece base, resists some of the load and lowers the load transmitted to the scope. I think you would change the
point of impact more by stressing the scope than stressing the action, all else being equal.


But, if you have a scope with a steel tube, and use a two piece base, then the mounted scope goes through temperature swings,
matching the action's length change, without being stressed. This is the best of all worlds. It is also similar with an aluminum
Stolle Panda action and an aluminum scope.

SUMMARY.... So, in summary, I would suggest:

Steel action, aluminum scope: use a one piece aluminum scope base.

Steel action, steel scope: use a two piece steel or aluminum scope bases.

DO THE BASES SLIP UNDER THIS LOAD?.... The screws and static friction between the base and action transfer the
shear load from the base to the action. If you assume two ea. #6-48 screws on each end of the base, and torque them to about
24 in-lb, then the clamping preload the two screws is about 1750 lb on each end. Using a coefficient of static friction of 0.3,
reasonable for the dry surfaces here, it would require a force greater than 500 pounds before the junction between the base and
action could slide in relation to each other. This is also one of the reasons to tighten the screws well on your scope base. If there
is only one screw in one end of the base, then there could be some sliding and that could lower the load to a maximum of 250 lb
or so. I don't think the screws affect the conclusions I made above.
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Works for me!
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