Problems with Witness Hunter and Red Dot Optic

[Reverendpdp]

Greetings gun enthusiasts. I have an issue that I’m hoping I can find a remedy for. I have a Tanfoglio witness Hunter, (6 inch), in 10mm. I had to slide milled by Patriot Defense to accommodate a Leupold DeltaPoint pro. They did an excellent job by the way.  The problem is that the screws that hold the optic in place have sheared off completely while shooting. This is the second time it has happened. The first time it was with the Leupold factory screws, and now tonight it happened with aftermarket screws that I picked up at the local Ace Hardware. I do not really know how to remedy this. the first time it happened I took it to a local gunsmith that was able to drill and tap out the seared screws that remained in the slide. He charged me $75 each!  Yikes.  But now since it’s happened twice, I’m looking for other options. I’m not sure if the metal the screws are made from is too soft, too brittle, etc.? Should I consider having the slide re-tapped with a larger screw? I’m not sure that the DeltaPoint can handle a wider screw. Looking for ideas? TIA

 

By the way, I’m using standard 10 mm ammo, 180 grain 1200 feet a second. Nothing really hot. 

[radny97]

Fellow witness hunter owner here. This is why the hunter frame from the factory has threaded holes in the frame (rather than the slide) for mounting an optic.

CZ pattern guns like Tanfos have slides with lower mass than many other guns due to the rails riding inside rather than outside the frame. This means you get the low bore axis, and lower reciprocating mass in the slide that make CZs such smooth shooters. It also means that the lighter slide reciprocates much faster. 
But when you add a slide mounted optic, then that fast moving slide can really slam hard back and forth and eventually shear and break your mount screws. It sounds to me like that’s what you have going on here. 

The surest solution is to ditch the slide mounted optic and go to a frame mounted optic. It’s bulkier and maybe doesn’t have the cool factor, but it’s actually more accurate, and the tanfo hunter is a big gun anyway, so I’m guessing this thing isn’t your edc and slightly bigger won’t matter too much. 

Alternatively you can try to get some heat treated screws that are stronger, if you can find them and assuming the heat treating doesn’t make them too brittle. You could also install a much heavier recoil spring to slow down the slide and tune the gun to not slam back and forth too hard. Doing this, however, will require a lot of experimentation and will make the gun ammo picky.
 

Final note. Mounting optics on the slides of 10mm guns is still something in its infancy. 10mm is a hot round and will bang up the mounts and optics more than other calibers. You may just be experiencing the growing pains of an area not many manufacturers have figured out yet. Optic ready 10mm guns have only been mainstream for about a year at this point. I’ve already heard anecdotes of other 10mm guns breaking their optic mount screws with frequency, so this isn’t an isolated problem related to a Tanfo. 
 

Good luck. 

Edited November 17, 2023 by radny97

[Reverendpdp]

Thanks for your reply. That makes a lot of sense.
I reached out to Patriot Defense who milled the slide and they also recommended that I go up in the recoil spring weight to slow down slide recoil. The recoil spring that I have on there now are the stock ones so I don’t really know which springs to get And in which weights. Recommendations would be appreciated.

Also, I seem to be having conflicting info concerning how much torque to use on the screws. Patriot Defense is telling me 10 foot pounds while Leupold is telling me 25 foot pounds. What am I missing here? 

[shred]

If your screws are shearing due to recoil, the recoil pins (the protruding pins on the slide that fit into holes on the optic) probably aren't doing anything.  Do they show any marks or wear?

 

If they're shearing due to overtorque, then there may not be marks there.  Do you check them periodically?

 

Unbreako makes about the strongest screws, but IIRC DPP factory screws have a precise-fit unthreaded section to help with alignment.

 

 

[shabodah]

There is no way a Hunter slide is moving as fast as a stock 2 or limited slide, so, your experience is the opposite of what would be expected from physics.

[Reverendpdp]

So you’re saying the problem with the sheared screws is not due to slide speed or blow-back?  Then…?

[shred]

The screws are shearing due to the slide impact forces at both ends of travel.  Years ago I stuck an airbag accelerometer onto a STI Edge .40 slide and saw 60G+ peaks.  The problem here seems there's nothing assisting those screws like pins or ledges to take some of the force off the screws.

 

[Joe4d]

what shred said,,, I mean you had the slide custom milled for a specific optic... Just sounds like bad machining to me. When folks first started doing this some of the shops asked you to buy a sight from them or send yours in with the slide so they could get an exact fit.  I imagine the milling is a bit large,,, Probably fine on a 9mm,,, but not so fine on a 10
The screws should just be holding the sight down, the sight should fit down fairly snug fore and aft and not allow any movement for and aft even without the screws.

[Reverendpdp]

There are pins on the slide just not very tall. That’s due to the nature of the slide itself. Not much milling room. And it does fit snugly.  Another potential issue is in the optic itself. The recess for the pins on the Deltapoint are rounded… so if the sight is slightly lifted it could slip due to the rounding.  Hope this makes sense.

Edited November 20, 2023 by Reverendpdp

[HesedTech]

5 hours ago, Reverendpdp said:

So you’re saying the problem with the sheared screws is not due to slide speed or blow-back?

The poster is correct about the mass of the slide, it is basically the same as 9mm.  However, 10mm should have a greater acceleration of the slide and therefore more recoil, we all feel it when shooting. Just compare the difference between 308 and 6.5 CM, same gun, similar sized cartridge, but big difference in recoil.

 

A full power 10MM does slam the slide harder than 9. You might try using the Sprinco or DPM recoil system. It does reduce/cushion the slide as it travels back. The forward movement will be about the same.

 

The frame mounted optic mount is a better solution but does limit holster options.

 

The other "fix" is the JB Weld solution. JB Weld is used a lot for seating optic mounts on rifles. Pretty simple, thoroughly clean mounting surface (free of oil and dirt) and put a thin layer down then mount the optic. The big issue is removing it. You will need to heat slide to remove the optic.

 

No matter what you do full power 10mm is harder on your dot than 9mm.

 

[Reverendpdp]

Ok so right now I’m leaning towards heavier springs, and stronger screws.  Anyone have direct links to these? Wolff springs only go up to 20# on the longslide. Would like to try 22.  Will look into Sprinco and DPM recoil system.
 

Also I looked at McMaster and Unbrako for screws but they have hundreds of options… AND they don’t have the exact same size, 

M4 x .7 x 15.8mm

Suggestions? 

Edited November 20, 2023 by Reverendpdp

[Reverendpdp]

Thinking also about the JB Weld option.  If I add JB Weld to the front part of optic where the recess for the slide pins are located, will that I ruin the optic when I remove it?  If have to heat the slide, I'm thinking that'll toast the internals on the optic?  No? 

[Ronemus]

On 11/20/2023 at 2:25 PM, Reverendpdp said:

Also I looked at McMaster and Unbrako for screws but they have hundreds of options… AND they don’t have the exact same size, 

McMaster will make custom springs to your specifications, but it is pricey; I had 5 made 20 years ago for ~$250.  They work very well, especially with a recoil buffer.  Plastic buffers don't last long, but copper is very durable if you can find one that fits.