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SEMI-PRO SCOPE MOUNTING |
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jonoMT
Optics Master Extraordinaire Joined: November/13/2008 Location: Montana Status: Offline Points: 4853 |
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http://stonetip.com/diagrams/alignment_bars.pdf - should convince the skeptical that pointed bars don't work. See if you can guess which pair is misaligned by 2°. (This is a 3D-enabled PDF so you can actually click on the image and use the rotate tool to spin the bars around in any direction).
Now, if you want to see the flat bars (or don't see the difference with the pointed bars, check out http://stonetip.com/diagrams/alignment_bars_flat.pdf Edited by jonoMT - January/13/2010 at 23:52 |
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Reaction time is a factor...
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John Barsness
Optics Optimist Joined: January/27/2009 Status: Offline Points: 785 |
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Yep, the pointed bars can indeed indicate a "false alignment." I have generally checked them with a straight-edge as well, but all the models I've seen on the market also have flat bases at the other end, so can also be used by turning them around.
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John Barsness
Optics Optimist Joined: January/27/2009 Status: Offline Points: 785 |
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NV Hunter,
That's an interesting question. Let me answer it in several parts.
First, unless a rifle is really light in weight, muzzlebrakes shouldn't have any bad effect on mounts or scopes. It's only with the combination of a light rifle, hard-kicking cartridge and a brake that problems tend to be created. Or at least that has been my experience, and that of several custom gunsmiths I know.
Second, 8-40's don't really have much advantage over 6-48's. While 8-40 screws are greater in diameter, their coarser threads are also deeper than those on 6-48 screws, so the diameter of the screw at the bottom of the threads isn't much different. (I learned this from a riflesmith who is also an engineer.) Consequently, epoxyingbases has more effect than 8-40 screws.
Personally, I hate muzzlebrakes, but have shot a number of big, braked rifles out of professional necessity, chambered for cartridges from 30-378 to .416 Remington Magnum. Most of these have had scope bases mounted with 6-48 screws, and I've never had a problem.
Of course, one factor that's occurs more and more these days is shooters mounting bigger, heavier scopes. There's a big difference between the average 12-14 ounce 3-10x and something like my Nightforce Varminter 5.5-22x which weighs 38 ounces. But I've never put that scope on anything bigger than a .300 magnum, so don't know what the effects might be.
Probably the best solution for real hard kickers is scope bases integral with the action, like those on my .416 Rigby CZ 550. But I have also used standard 6-48 mounts on a .416 Remington Magnum that only weighed 8-1/2 pounds scopes, and never had a problem--though again, the heaviest scope I ever used on that rifle only weighed 12 ounces.
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jonoMT
Optics Master Extraordinaire Joined: November/13/2008 Location: Montana Status: Offline Points: 4853 |
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The question about larger screw sizes had me wondering what the shear strength of a 6-48 screw might be. I couldn't find a min. diameter for a 6-48 but since it's a considerably smaller thread pitch than a 6-32, which has a min. dia. of .1329" it would be safe to use that number:
Grade 8 bolt PSI: 150,000 screw radius: (.1329/2) = 0.06645 cross-section area: 0.06645 squared * pi = 0.01387 150,000 * 0.01387 = 2080.8 lbs. So you've got something like a ton of shear strength balanced against the recoil impulse, rifle and scope weight...and I won't pretend to know how that all interacts. |
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Reaction time is a factor...
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John Barsness
Optics Optimist Joined: January/27/2009 Status: Offline Points: 785 |
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I don't either, but do know that Melvin Forbes of New Ultra Light Arms only uses 6-48 screws on his rifles, which are among the lightest in the business, and the problems his customers encounter are with scopes, not the mounts. Melvin, by the way, is the "inventor" of the Talley Lightweight rings. He had another company make them for his rifles for many years, but when the other company went under Melvin approached Dave Talley. Dave said sure, he'd make 'em, as long as he could also make them for other rifle actions as well. Since the Talley LW's eliminate the base/ring connection that sometimes causes problems, they hold up quite well under heavy recoil, even though they're "only" made of aluminum.
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Jon A
Optics Journeyman Joined: March/14/2008 Location: Everett, WA Status: Offline Points: 670 |
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Since 6-48 and 8-40 are not widely used in other industries, the common references containing allowables for bolts and screws typically don't have these threads listed. But you are correct in that when the threads per inch is increased, the depth of the threads is reduced and the "stress area" is increased, so using numbers from more coarse threads yields conservative results. When you look at 6-32 and 8-32 vs. 6-40 and 8-36, etc, the stress area is increased fairly proportionally--not exactly, but pretty closely--with the #8's having roughly 50% more area. This varies a bit and depends upon the exact type of threads (MIL-S-7742, MIL-S-8879, etc) but you get the idea. So let's look at your comparison.
I think you must have been looking at 8-32 here, as .025" deep threads wouldn't make for much of a screw. The biggest stress area typically given for 6-32's in most texts is .0091in^2, with 8-32's coming in at .0140 (very close to your number). Now the next thing is we are mostly worried about a shear failure. 150 KSI is the Tensile strength of a typical Grade 8 screw. Shear strength is roughly 60% of that, about 90 KSI. This leaves you with .0091*90,000= 819 lbs for a 6-32 and 1260 lbs for a 8-32. With four on each base that's 3276 lbs for #6's and 5040 lbs for #8's. Again, that's just an example using 32 TPI fasteners so it's a bit conservative, the finer threads will be a bit stronger but the proportionality will be fairly constant. Whether you need that extra strength is up to you.
That gets complicated. It can all be calculated if you have all the variables, but getting those is the problem. For example, a big guy that doesn't give as much with recoil will be easier on the scope mounts than a small person who is pushed back by the recoil more. Using lead sleds or the like can be even easier. It's hard to quantify. Also, while recoil is certainly a worry, so is rough handling. Drop a rifle on a hard surface just right and you can put many times the amount of force on the scope mounts as even harsh recoil will. But naturally, for many cases it's an issue of a rapid change of momentum (impulse) where the heavier the scope, the larger the resulting force. So what works well for 12 oz scopes might not be optimum for "38 ouncers." Or in the case of a "scope down landing," it's the weight of the rifle that drives up force put on the mounts. Edit: Harder, easier, oops. Edited by Jon A - January/16/2010 at 15:14 |
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jonoMT
Optics Master Extraordinaire Joined: November/13/2008 Location: Montana Status: Offline Points: 4853 |
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That was some good info, JonA. FYI, the screw diameter I was using was for a 6-32 - the smallest I could find listed, which was .1329". (http://www.fastenal.com/content/product_specifications/SHCS.ALLOY.BO.pdf). Other references showed it as .131" and .1312". You were right that I was using the tensile strength figure (150,000 psi). I forgot the 60% rule. So I should have multiplied 0.01387 * 90,000 which would yield 1248 lbs. for shear strength.
Whatever the number may be, it seems that with the proper grade screws (i.e not the ones that came with the Trashco rings I bought ONCE) in combination with a base that has a recoil lug, there's probably plenty of strength in a reputable mount. |
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Reaction time is a factor...
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kokopelli
Optics GrassHopper Joined: January/12/2010 Location: Montana Status: Offline Points: 5 |
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This is where bedding bases and lapping rings, like cream, come to the top. The better the contact, the stronger the installation will be when you need it most. Do it slowly and thoroughly and you will be happy with the results.
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Jon A
Optics Journeyman Joined: March/14/2008 Location: Everett, WA Status: Offline Points: 670 |
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I see, you were using the minimum body, or shank diameter. That's usually what you would use for a nicely designed shear joint where fastener threads were not used in bearing. In this case though, the shear failure will happen in the threads so you must use the minor thread diameter to calculate the area. It makes a pretty significant difference. |
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jonoMT
Optics Master Extraordinaire Joined: November/13/2008 Location: Montana Status: Offline Points: 4853 |
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I thought that the min. was the thread...shows how little I know. |
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Reaction time is a factor...
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55spartan
Optics Apprentice Joined: March/07/2010 Location: Indiana Status: Offline Points: 54 |
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very interesting
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RifleDude
MODERATOR EVIL OPPRESSOR Joined: October/13/2006 Location: Texas Status: Offline Points: 16337 |
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However, keep in mind that on a typical 60-deg UN thread, as is used with most fasteners, when the fastener is under load, the first 2-3 threads underneath the bolt head are bearing the majority of the load, something like 70%, because the fastener stretches under load. Beyond that, there is a rapid decrease in load bearing for each successive thread, such that increasing thread length at some point doesn't provide an increase in fastener connection strength. This is the reason why ramp style threads such as the "Spiralock" thread design was created, to spread the load over a greater portion of the thread.
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Ted
Money can't buy happiness... but it's much more comfortable to cry in a Porsche than on a bicycle. |
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Sparky
Optics Master Extraordinaire Joined: July/15/2007 Location: SD Status: Offline Points: 4569 |
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I learned something new.
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Jon A
Optics Journeyman Joined: March/14/2008 Location: Everett, WA Status: Offline Points: 670 |
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While there's some truth to that Ted, it's a consideration for tension failure of a screw. The mode of failure we've been talking about above is shear--though in this case as well, the number of threads won't matter much.
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hometheaterman
Optics Apprentice Dolphin Overton Joined: February/10/2010 Location: NC Status: Offline Points: 84 |
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Great article in the first post.
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TD-Max
Optics GrassHopper Joined: March/13/2007 Location: United States Status: Offline Points: 32 |
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My own personal favorite is the Sako Opti-lock system. No frail screws or tippy bases to deal with and they also have the plastic centering rings to relieve tension. One thing they do lack is the ability to swap inserts though.
I'l be mounting one soon and will re-read the article (only part of it sunk in so far) before and probably during my install to be certain that I understand and verify the install.
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oyeme
Optics GrassHopper Joined: February/26/2007 Status: Offline Points: 9 |
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A buddy and I have been debating whether it is important to center the scope "travel" for windage and elevation before mounting a previously used scope on a different rifle. He claims that it makes no differance because the amount of reticle adjustment is compensated for when you have to make the appropriate adjustments to sight in the rifle. In other words, even if the scope was previously adjusted almost all the way to the left of its total available windage "travel"; it would not matter when you put it on a new rifle since by adjusting the reticle for the new rifle you have accomplished what you would otherwise have done if the reticle was centered. I disagree. Who is correct ?
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Bill E.
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Jon A
Optics Journeyman Joined: March/14/2008 Location: Everett, WA Status: Offline Points: 670 |
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If using mounts that are adjustable, such as the common Leupold style with adjustable windage, or Burris rings with the inserts, it is a good idea to center the travel in the scope and do your first rough adjustments when mounting with the mounts.
If using non-adjustable mounts, it will make no difference. The scope will end up where it ends up regardless of where the reticle was before you mounted it. Don't waste your time. |
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Kickboxer
MODERATOR Moderator Joined: February/13/2008 Status: Offline Points: 23679 |
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Unless you want to know how much adjustment range you have left...
If you don't need to know that, it is a waste of time.
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Opinion,untempered by fact,is ignorance.
There are some who do not fear death... for they are more afraid of not really living |
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MadMax
Optics GrassHopper Joined: July/09/2010 Status: Offline Points: 18 |
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Great post - alot good info
We always seem to have issues with scopes sliding in the rings, even with rifles with factory muzzlebrakes, I have an M25 mild 7.62 nato - I can't get cheapo bushnell to stay put - I have some QRW rings on order, cheapo Weavers are not doing it.
I had a Win 70 classic rear bridge so badly off I wound up adding a stack of brass shim stock under the base till after hunting season was over, now to show how bad my luck is, Winchester rifle div sold to Browning at that point - about 9 mos later Browning assembled a new Win rifle for me. I am thinking it was an 1/8 + too low the rear bridge, must of been a Friday afternoon built rifle LOL
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