Just got an order from McMaster today and I have something that should be useful to add.
Part #92855A313 m3 screws in 12mm length for Bar 10 scope rail. You might want to note that any m3X12 screw wtih a .5mm pitch will work, the ones I got were simply the cheapest and are silver as opposed to black. These work perfectly and screw in really smoothly. I got these because my stock ones were only 8mm and were to short.
Also, I got some piston o rings for my BAR 10, but I don't know if they you should put them up since they don't give perfect compression. They actually completely fail the compression test with my finger over the nozzle, however, I am getting about 225 feet of range out of my stock BAR 10 with them, so IDK. I'll try stretching them soon and see if they seal any better.
Part #9263K247 Metric Viton(r) Fluoroelastomer O-ring, 2.5 Mm Width, 17 Mm Inner diameter.
I'll probably be ordering again soon and I'll try again to get the right size. I think the only thing that matters is outer diameter granted that your o ring is not uber thin. The ones above have a 22mm diameter, I expect that what is needed is a 22.5 or 23mm diameter, so if anyone wants to give it a try, that's what I'd reccomend. If anyone who has a perfectly sealing o ring for their vsr/bar 10 and digital calipers could take a measurement of their o rings outer diameter that would be great.
Edit: Here is a combination that could possibly work: 2.4mm thickness with a 17.8mm inner diameter. OD=22.6mm.
or another possibly option same as above with an 18.3mm ID for a total of 23.1mm OD.
Note to self: test a few sizes like this!
Edit#2: update I am now getting 100% compression with these o rings. Here is how.
1. I removed the o ring from the piston
2. I stretched it around the outside of the cylinder\
3. I heated the o ring a bit with the flame of a lighter
4. I cleaned the cylinder and o ring with isopropyl alcohol
5. I gave the o ring a light coat of 10 weight silicon shock oil and reinstalled it.
This takes less than 5 minutes, and where before I had terribly compression, I now am able to plug the nozzle and cause the piston to hang. I'll be trying som other sizes eventually, but with this method it won't be a big priority.