Sako ring mount fail

[ID_Matt]

Anybody ever heard of this??? 3 out of 4 bolts to the scope bases sheared off, and are now stuck in the gun. Sako A7 300 win.

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[ramont]

Yes, it's not uncommon, especially with a 300 Win. Mag. What scope is that? The heavier and longer the scope is the more force the mounts will have to handle. Most people don't realize that a standard 3-9x40mm scope on a .308 Win will produce around 1500 lbs of force on the mounts with a standard 168gr cartridge. Obviously a 300 Win. Mag. will produce even more force.

The common solution to your problem is to have a gunsmith or a machinist drill out the broken screws and then drill and tap new holes using the next larger screw size. The larger screws will handle the force better and the larger holes will fix any damage that might occur while removing the broken screws.

 

[ID_Matt]

It’s a vortex viper hs-lr 4-16x44. I assumed it wasn’t a big deal but is just frustrating. I don’t expect those things out of a gun like that that. I wonder if putting a rail or better mounting system would help.


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[hodgeman]

How many inch/pounds were they torqued to when installed?

 

[gbflyer]

Screws over - torqued when installed. Not an undersized fastener issue. Tighter doesn't equate to stronger when it comes to fasteners. Something about entering the plastic region. Some engineer here could explain it. Anyway, very common. 25-28in/lbs is plenty. Drop of blue loctite on degreased threads for a heavy kicker.

I've taken those out with a very small drill bit and proper sized screw extractor. Best done set up in a mill, a drill press will work if you can clamp securely and are patient. Some heat is good, in case loctite was used.

 

[howl]

What is the brand of the bases? I have no help. I'm just curious.

 

[HiMtnHntr]

We're you shooting with a sled?

 

[micus]

damn man, that's crazy.

Could you get higher quality fasteners like grade 10 or 12 with a higher sheer strength?

 

[BrushyHillGuide]

How many inch/pounds were they torqued to when installed?

My question also.

Screws over - torqued when installed. Not an undersized fastener issue. Tighter doesn't equate to stronger when it comes to fasteners. Something about entering the plastic region. Some engineer here could explain it. Anyway, very common. 25-28in/lbs is plenty. Drop of blue loctite on degreased threads for a heavy kicker.

I've taken those out with a very small drill bit and proper sized screw extractor. Best done set up in a mill, a drill press will work if you can clamp securely and are patient. Some heat is good, in case loctite was used.

I’ll bet ^this^ is right - I have the same suspicion. No way that recoil alone caused that. I’ve shot a LOT of large calibers and mounted many many scopes with everything from junk to top of the line mounts and screws and NEVER heard of this except when screws were over-torqued. Always from guys not using torque wrenches to install screws.

Sux but easily repairable. Get a Wheeler Fat wrench and I’ll bet it NEVER happens again....as long as you use the correct torque value.


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[maninthemaze]

Maybe while it's at the gunsmith getting the screws removed or redrilled, he could install pins. That way, the hardened pins take the abuse from recoil instead of the screws.

Just my 2 cents and worth the price charged.

 

[RosinBag]

I have had one screw do that, but not three. It was on a 300 Jarrett. I had my gunsmith drill them out and retap them with the next size. I think they were originally 6x32 bolts and I think he drilled them out to 8's. Not a huge deal as mentioned above.

Also, mentioned above is the use of a "lead sled" type vise. Those don't allow the rifle to recoil as they are designed and the fasteners eat up that energy as the rifle isn't allowed to move rearward.

 

[gbflyer]

The fasteners carry some of the load, but most of it is carried by the interference of the two pieces of metal being held in tension (smooshed together) by the fasteners. I think they go to 8's on some custom receivers though for sure. Not sure about Sako, perhaps metric....no idea.