Finally got my Rainstorm II tuned up for squirrel season

[randy_68]

First off, a big thank you to Knife, Manny, Robert Sterne and others for all the info on tuning, porting etc.. I learned a lot and it was a fun and frustrating tuning adventure that worked out the way I wanted it to.

Good thing I bought a Yong Heng cause I sure used a lot of air and pellets.
This gun was doing fine with the 25 gr JSB but I wanted to shoot the 34's in it.



It’s been a battle to get this gun back to 60 fpe after I installed a reg. From the factory with 2 coils cut off spring I managed a string from 865-898-865 in 15 shots.  I wanted at least  two mags or 18 shots at a minimum of 60 fpe and regulated so I bought a Huma reg and started tuning.

 With the Huma .25 plenum I could only get the 34 gr JSB to 810. Not near good enough. Next I opened up all the ports to .190 and I gained 30 fps up to the 840's. Getting better, but not quite there. 
 
Next, I made a longer plenum and adjusted the reg to 145 and was able to get the speed up to 865. No particular formula to make the plenum . I had a piece of thick walled aluminum pipe and just chucked it up in my Atlas MKII and bored inside, turned the outside and finished the ends. That's just where it ended up, about 50% bigger.

      I tried a couple combinations of hammer and valve springs but never could get the 34's up to speed. It was shooting the 25's at 915 or so for 20 shots using a C730 hammer spring. After pondering a while I decided to open up the ports to .201 and smooth out the flow.

  Now I was in the 870's with the weaker C730 spring but there isn’t much room for adjustment with it so I put the factory spring less 2 coils back in. After adding about a 1/16" shim and adjusting flush I am getting 20 shots 883-899-883 with the 34's and that is on a 3100 fill.

 I Iried filling higher but my first shots were several fps lower. Hopefully it will settle down and tighten up a bit but I am very happy I finally got it where I wanted it.

Not sure why when I fill it higher than 3100 I get lower fps for about 5 or 6 shots , 865-880 range. It acts like an unregulated gun and keeps climbing then levels out so maybe I have a slight internal leak into the reg.
Here are some pics of the reg and the plenums

[Nomadic Pirate]

Well, 20 shots @ 60 FPE is pretty nice indeed.

[Wayne52]

Job well done on your RSII there Randy !!!

[KnifeMaker]

Very impressive Randy! and thank you for the mention.


I'm playing with mine as a bullet shooter at the moment. very accurate, but had to remove the Reg. Turns out that I need a much longer plenum.


At 65 fpe now, and very good groupes at 76 yards.    It is a real pleasure to shoot.


Knife

[Habanero69er]

Nice Randy. Rifle is shooting strong & with that beautiful stock, looks great.   

[Bob Pratl]

Randy, thanks for all of the information on your modifications. I will be duplicating them for my RS II .25 when I find time.
Your larger plenum is 2.300" and Michael is suggesting a longer one so what length do you guys suggest?

[KnifeMaker]

As Bob has found, a plenum volume of 1cc per fpe is desirable. The .25 Plenum is 18 cc's and the .30 factorm Huma Plenum is 30cc. as per Hubb of Huma.


So twice as long as as the .390 plenum would be about right.


The .30 Plenum is 2.159" long as measured with a dial caliper. 


Knife.

[Bob Pratl]

Michael, so a 4 inch plenum may be needed to get close to 60 FPE. Has anyone tried one at that length? I just received a 6061 aluminum tube 1.25 OD x 1.00 ID so I will fabricate one and give it a try next week.

[randy_68]

I don't know about going to a 4" plenum, but I'm getting 60fpe with my 2.3" one. I think my biggest gains came from porting and smoothing the corners for better flow.

Correction--my plenum is 2.030 long. Not 2.30.

[Bob Pratl]

I don't know about going to a 4" plenum, but I'm getting 60fpe with my 2.3" one. I think my biggest gains came from porting and smoothing the corners for better flow.

4 inches does seem a little too long and wouldn't that have a tendency to make it louder and lower the shot count.

[KnifeMaker]

I am getting 64 fpe with the 2.1" plenum, but need stiff hammer spring. Longer plenum would require less spring, so may very well be a wash with the shot count. If this is the case, it should be quieter rather than louder.


Mr. Restern has done a lot of testing in this area. Perhaps he will chime in here. [size=78%]  [/size]

[randy_68]

I'm pretty sure my gun is a little quieter now too, but more powerful. For sure it is no louder.