QB78 in 20 cal.

[Wallis]

Hey guys,

I am in the process of changing a qb78 in 22 to 20 caliber. So far everything is going good. I did some valve work on the gun. Right now Im waiting on my barrel blank to come in, I have order a walther match barrel 19 3/4 in length. if anyone has done this before me. what should I change/ check out. Once the barrel comes in I will start the milling process of the breech. Hopefully this will work out fine for me. if not it's only money :p

BTW great looking site.

If you plan on making a new bolt let me know, I have some 1/2 inch stainless steel that is soft enough to machine. I'll be glad to send you a chunk to use.

[Wallis]

I will let you know about the spare piece of stainless .

[Wallis]

Getting closer to doing my conversion. So far the barrel is in route to me as we speak. The valve is all ready to go, so is a scope I'm planing to use. I had to order another chrony, mine died. A mixture of water and live voltage going to the unit. Once i get everything together, I will post some pics of the progress. TTYL

A Mike Melick tuned 177cal. But he spoiled me with the 22cal I also bought from him. So I got me a 22cal barrel for it and done some machine work on the bolt and some on the barrel. Now Im good to go. I have two QB 78 22cal that I just love to shoot. Good luck on you project.

[Wallis]

got my barrel in today. I'm having a hard time figuring out which end is the muzzle. The barrel is choked, both ends are tight. Any ideas on finding out how. I tried the pellet test both ends area tight fit.

I have a Walther barrel blank and the end with the saw marks isn't the muzzle, check yours and see if it could be that easy. I am sending the bolt in the AM. Thanks, Mike

[Wallis]

Thanks mike, I e-mail the gentleman that I got the barrel from, right after I posted on here. He told it's the end the is flat.
Thank you for the bolt, I sure hope this conversion works like I want it to.  If not I can always try again,  at least I will have a spare 177 bolt if I ever need one.

One other thing I have a new chrony coming so I can see how my tunes are actually working.  

One more thing Mike..... What do you use for the breech sela on the qb guns ?

I've been using the 1/4 inch OD hard poly tube, the ID is .170. It's the same stuff they use for ice makers in the fridge, nothing fancy, cut it between 1/10 and 1/8 inch, to long and it will restrict the flow, to short and it will leak. At 9 cents a foot you can make a few screwup's and still not be out anything.

[IJL]

On the subject of breech seals I have a full PCP converted QB which will produce over 38fpe, my emergency breech seal was the outer sheath from 3 core mains cable. Please dont laugh, it hasn't failed yet!

IJL

[Wallis]

thanks guys. I'm going tomorrow to get someof the poly tubing.

[Wallis]

The conversion is on hold for a little while. My father  hasnt been able to find his small cutting tools for the lathe . So for now I have a barrel blank wihtout anything else.

[Wallis]

OK, the barrel is finished... the results as follows.....

610.2
588.3
605.3
606.2
608.1
603.1
604.1
598.9
599.8
601.2

average 602.5
spread 21.9
SD 5.862
max 610.2
Min. 588.3

This is with a standard valve, bolt drilled out, and hammer spring shimmed up. This weekend I'm going to change the valve to a modified valve and shim the spring up more. I will post the results later this weekend.

What did you use for testing pellet wise, sounds good. For that last bit of ooph try an extended probe. It will use more gas but.......

[Wallis]

The pellet I used was benji sherdian .20 cylindrical 14.3 gr. I was thinking about do that, but I only get 20 shots per fill now.