QB Custom 30cal

[SeanMP]

I thought it was about time I posted up a thread on some of the custom stuff that goes through the shop. So I took the opportunity today to do a quick write up on one that's leaving for it's new owner today.

This particular one is a QB variant and I must admit its probably my favorite platform for full custom mods.

Specs:
30cal
25" Barrel with a  1:26 twist
Muzzle threaded 1/2-20
24" Tube
RVA with preload window
Probless bolt
.310" throat, .290" Transfer port
Peak velocity Mach 1 + with 50gr JSB or 140+fpe
14 shots at 100 FPE

Every piece polished out to 1200 grit and blued. All moving parts are tool steel or better, hardened and polished.
The stock is just a donor I used to setup and test fire the rifle.




Most of the

[SeanMP]

I noticed the colour of the bluing in those pics is not true to the original so I threw in another quick snap taken during assembly that shows the colour better

[rsterne]

Thing of beauty, Sean.... and the efficiency you are getting at 100 FPE is truly amazing....

Bob

[csdilligaf]

Sean,  that's nice work.  You do the blueing?  What is the .30 cal barrel if you dont mind me asking?  I know Lothar Walther says they have a new barrel that is actually for the .30 cal JSB pellet and not the .308 slugs.  Is this one of them?

[SeanMP]

No Sir this is the Sterne-Pero barrel

Bob and myself designed a rifled barrel specifically for the .300 JSB pellets. I believe it was the first and it certainly has proven itself as a real performer in independent testing.
Since then Bob designed a .300 rebated boattail for this barrel that is shooting MOA. The design for that mold is in the catalog at Accurate Molds

I own the mandrel but it permanently resides at TJ's for anyone to use.

Yes I do all my own bluing. I'm a little bit of a classicist and I prefer rust bluing.

[grumpy]

Your work never ceases to amaze me Sean. Gotta ask ya how does the QB valve compare to the custom Mrod valves you made. Meaning is more power available from one.

That many shots at 100 fpe is almost unheard of in such a small air rez. Again fantastic work sir !

[HappyHunter]

Thing of beauty, Sean....

Agreed...X2.

I was fortunate enough to see this gun first-hand the other day while visiting Sean and believe me, the pics don't do it justice. Between his craftsmanship/attention to detail and the deep, rich bluing it really is a thing of beauty.

Awesome work, dude!

[SeanMP]

Your work never ceases to amaze me Sean. Gotta ask ya how does the QB valve compare to the custom Mrod valves you made. Meaning is more power available from one.

That many shots at 100 fpe is almost unheard of in such a small air rez. Again fantastic work sir !

Well I'm not entirely sure I can answer that question fully. In theory both valves follow the same design philosophy so therefore they should be equally capable. There is one minor difference in that there is enough room in the QB body for a stem seal where there is not in the MRod body. That should only be a slight improvement in efficiency not power.
But as I understand the Mrod has fairly limited stroke capabilities. So preload becomes a must in the Mrod version whereas the QB variant has enough stroke to allow for a free flying hammer......that fact alone seems to be a key ingredient in huge power and efficiency.

[SeanMP]

Thing of beauty, Sean....

Agreed...X2.

I was fortunate enough to see this gun first-hand the other day while visiting Sean and believe me, the pics don't do it justice. Between his craftsmanship/attention to detail and the deep, rich bluing it really is a thing of beauty.

Awesome work, dude!

Howdy HappyH.

By attention to detail I'm guessing you mean @N@L

[rsterne]

How big was the reservoir on that gun, Sean?.... Pressure drop?....  The efficiency on that 14 shots at 100 FPE?.... C'mon, buddy, time to brag a little.... *grin*....

Bob

[SeanMP]

Hahaha...I don't want to tell tales outta school here Bob...I send you the raw data and you crunch the numbers

I do have the data here for a 90fpe avg string that is 15 shots. This is not from the gun pictured in this thread but the prototype gun that this gun is an exact copy of.
Reservoir Vol 7.19 cu in (118cc)
Start pressure 3050psi
End Pressure 1150 psi
Avg Efficiency 1.42fpe/cu in

A short 7 shot 120fpe string
Start 3100psi
End 1930 psi
Avg Efficiency over 7 shots 1.38fpe/cu in

I have recorded an instantaneous efficiency  of 1.78fpe/ cu in

[rsterne]

Mind boggling efficiency, pal.... simply OUTSTANDING.... you are to be congratulated.... Credit where it's due, for sure....

Bob

[Gremcat]

I never knew the QB could handle that wow.