6x48 vs. 8x40 base screws

[6.5Swede]

Wedgy,
I appreciate you taking the time to upload the pictures and share some good information. Thanks for your help!

Steve

 

[just country]

morning, a thread pitch chart should answer ur question. the chart shows

different size fastners, with different thread pitches, in different

alloy's. (SP) a good machinist could answer ur questions.

gun smithy's have used these fastners pitches for mega years with

great success.lightbulbgun)

 

[6.5Swede]

morning, a thread pitch chart should answer ur question. the chart shows

different size fastners, with different thread pitches, in different

alloy's. (SP) a good machinist could answer ur questions.

gun smithy's have used these fastners pitches for mega years with

great success.lightbulbgun)

just country,

Thanks for your help!

Steve

 

[6.5Swede]

Wedgy,

Would you mind sharing some details on your pictures?

Thanks,
Steve

 

[Bravo 4]

I had my first incident with mount screws failing this year. They were 6-48 and both the rear screws broke. I let a buddy borrow my short barrel .308 during last deer season. He had dropped it and it landed on the scope (and forgot to tell me ). You couldn't tell until I shot it a couple months back, it went from 3/4" @ 100 to 8". I took the scope off to check everything and the two rear screw heads fell off. It is definitely a weak link. They will be upgraded!
My .338 RUM, .338 Edge, and .408 all have the larger screws.

 

[BallisticsGuy]

If I'm doing the math right then you'll get about 30% gain in resistance to shear provided all else is equal. Looking at my fastener reference charts that seems to be backed up.

 

[Wedgy]

Regarding the pictures it is a buddies(member here as well, I think he posted them) .338 that was sitting in a Lead Sled, no known "trauma" to the scope, just recoil. He concluded it was the jarring from the acceleration and sudden deceleration that sheared the screws. An object in motion tends to stay in motion. In his case the gun was stopped by the Lead Sled but the scope kept going.

A thread on Long Range Only was addressing the issue of heavy recoil then deceleration from big guns with big brakes. Apparently there is a very large recoil then counter action from the brake that happens so fast you only feel a net zero effect but the gun and it's parts are getting hammered. The air ram(??) pellet guns are notorious for destroying scope internals from their double direction recoil as well. An issue that arose was scope rings sliding backwards on the deceleration; they are mounted forward against the pic rail to brace for recoil but when the brake stops the gun the scope rings would slide back in the free space in the rain and ring cross bar. Even at max torque they moved. One of the guns was an Allen Magnum and they mounted two sets of rings, one front and rear seated forward, while the other set was seated to the rear of the pic rail. I put epoxy in the void on the rail/bar to bed it just like a recoil lug with wax or shoe polish so it will release.
I was getting some POI shift on my .338 Edge and decided to tear it down, make some changes and remount with four rings as well. I don't know if that was the problem, but now I can rule it out.
(photo from Long Range Only forum)

 

[Sully2]

If pure recoil IS what is actually breaking the screws ( not loose screws or anything such as that) then you need a SHEAR PIN in at least one hole and not a screw.

Drill out one screw hole to at least a #10 size and install a press fit pin with its proper sized tight tolerance hole in the rail. If it keeps breaking pins....you got a REAL problem!

 

[Rich Coyle]

I was getting some POI shift on my .338 Edge and decided to tear it down, make some changes and remount with four rings as well. I don't know if that was the problem, but now I can rule it out.
(photo from Long Range Only forum)

I did the same thing with a Freedom Arms .454 and T'SOB rings and mount. Never had a problem.