Anyone, i bought a Sekhmet digital gauge for my new M3. Box came with a metal o-ring with an inner rubber o-ring in it. No instructions were included in the box as to wether i use it or leave it off. I am installing the gauge on an FX angled bracket. Any help would be appreciated.

That sounds like a dowty washer. If there is a plastic disc in the bottom of the hole where the gauge installs then no don't use the dowty washer and do not over tighten the gauge. Just snug is all it takes to seal. If there is no other seal then use the dowty washer.
 
One thing, as long as they are accurate, they will be MUCH more sensitive to pressure. That is, rather than saying 200 bar, they will have at least one decimal point to add accuracy. Some even have a PSI read out. So that's even more accurate.

200 bar = 2,900 psi

200.7 bar = 2,910.15 (which you cannot see on a "normal" gauge !

MUCH more detail ! Makes it into a little more work for those guys (and girls) that like to mess with the pressures in their guns.

Been thinking about one or two myself.



Mike
 
I myself have gone backwards and decided to keep things simple and low profile and go old school on gages by using the 23mm Huma gages that go from 0-250. The gages on pcp gun’s play two simple roles- to show max fill pressure and to set your reg pressure, nothing more, so why a large 28mm hulking gage on the gun? I’ll never try to read pressures on my gun from 6’ away and I don’t need to know how much pressure it took to fire one pellet. When I top off a gun I typically use the fill tanks gage, anyway, as reference.


I just simply fill to 250 BAR, and as long as my reg gage recovers back to the reg set pressure within 3 seconds after a shot, then that’s all I need. Plus, the fact that I shoot tethered 90% of the time over a chronograph, theres no need for me to be watching gages-

The simple look of gages-

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Anyone, i bought a Sekhmet digital gauge for my new M3. Box came with a metal o-ring with an inner rubber o-ring in it. No instructions were included in the box as to wether i use it or leave it off. I am installing the gauge on an FX angled bracket. Any help would be appreciated.

Always use same sealing method as is used in the specific location you are replacing the gauge on. On the M3 Impact there are two different methods: 
1st reg - special brass washer and oring 
Bottle Pressure, 2nd reg, and angled manometer bracket - xring

The dowdy seal included with sekhmet gauge is for another application.

Use new orings each time to reduce your chances of leaks. Here's the xring seal (you can find smaller quantities elsewhere if desired): https://www.mcmaster.com/90025K438/
 
Edit: deleted my whole response that was not relevant to initial question which I just realized was about a Caiman.

I think digital gauges are just about mandatory for the regulator position, but for fill gauges don't see the same need for precision. That said, if your fill gauge is super hard to see, then a Sekhmet may help since the display is bright and easy to read from all angles. Keep in mind, the sekhmet gauge is 20mm tall and may stick out too far for you application.