My Hammerli 850 AirMagnum re-do (long)

[fivestar45]

A while ago I started to get involved with a .22 cal 850. As I only shoot using co2 power, my aim was to make it as efficient, and powerful as I could using co2 only. So as usual I nickeled, and dimed my way through the project which had about a two year punchline.
I was hindered a bit along the way by a bad OEM barrel and a bad replacement barrel that Umarex U.S.A. shipped to me. So to start things this is what the second barrel looked like after disection.

You will notice the dent in the ID at the back of the front sight screw hole.
After that cut I faced it on my lathe, and re-crowned it. Now that was one problem down.

Next up was getting rid of the roll pin that holds the barrel to the receiver. I did that by indexing TDC of the barrel in the mill and milling three small flats to match up with three 8/36 grub screws that I drilled and tapped in the top of the receiver.


Also drilled and tapped for the front sight, and closed off the top port hole.
Barrel work done.


I was never crazy about the OEM sights so I re-worked a Williams WGRS 54 Peep to fit the 850's scope rails.

And added the fiber optic front sight from a QB78 deluxe that I had in my parts bin.

Now on to another personal peeve, the bolt handle. I just didn'nt like the big OEM handle. That one was easy. I turned and polished a St/St shaft and planted a Brass Ball on it. Done.


I started out bulk filling the Factory 2x12g adapter from the valve end

After a while I got bored with removing it to refill it and decided to make a new cap that was long enough to show through the fore stock and added a Forster fitting with check valve. All Good, now it can be
filled, or topped off without removing it from the gun.




Now onto the power plant (valve). Since I only intend to use co2 for power I initially figured that I'd follow Raold's lead. I eliminated the screen under the piercing pin, and drilled three supplemental feed holes around the perimeter of the pin.
The next area I wanted to improve was the stem and seat. The OEM parts do a good job of sealing but I thought that they required a lot of striker spring to make things work especially in warmer weather. I felt the key here was a smaller diameter seat and a harder seal material than the OEM O ring. So off to the lathe to turn up a new brass valve top with a raised seat that finished up a few thou over 9/32" dia. and opened up the bore a bit to 1/4". Now the stem. I used a pc. of drill rod and turned a Red Turcite head.

This combo wouldn't give me a dependable seal at co2 pressures. So on to try another stem, this time White Delrin. My results in the past with this material have yielded a reliable co2 seal.


Bingo! A reliable seal. However, when chrony testing this valve using a stock striker and spring I'd get 50-60fps drops from the first shot (about 720fps) and each shot there after.
So I decided to try two modifications which I would usually do one at a time in order to evaluate their impact. Mod #1 I reduced the striker weight from 59g to 53g I did this by opening the spring bore and turning a Delrin insert to bring the I.D. back to size. And lastly,#2 a lighter than OEM spring, a Crosman part that I had on hand. The results...better than I'd hoped for. Now remember, this is co2 power and a short barrel about 74deg'sF.

After this I energized myself to scratch build a larger 1" OD cot tube from 2024 Aluminum.


After some Duracoat.
The Finished Product

I hope this experiment helps some of you folks. Even though it was a bit time consuming it shows that with a little re-doing more power can be gotten from these guns when running on co2 without the need for a heavy striker spring,and striker or shorter shot count. Most likely this valve combination will give similar results when running the 850 on bottled air.
Have Fun
Anthony

[Busta Cap]

Wow!  Interested in an 850. That is my next gun.   Very nice job!!