Crosman "Big Bore" 760 BYOB Gun (Bring your own bottle)

[Tack Driver 10]

The 760 is the Rodney Dangerfield of airguns.
It just don't get no respect!
Some of the reputation is rightfully deserved.
The time has come to change this perception.

Going to convert a late 70s version into a bottle rocket.
Acquired ten 760s in a lot from EBay for $15 a copy shipped before the maddness.
All are the 77-81 version 5 with the brass bolt.

Here's the game plan:
Add a drop block to attach an HPA bottle.
22 cal big bore barrel. (big is a relative term here)

Recently modded a 13xx gun in a similar manner and it made over 30 FPE.
Here it is mocked up:

[Insanity]

Tuning into this one. I think I passed on that lot I was looking for old 760 at the time.

[Back_Roads]

 I like the barrel band/ support built into the drop block 

[Rob M]

following

[Tack Driver 10]

Got quite a bit done despite the cold weather.
Been in the mid teens for a high temp with sub zero coming next week.
I think I'll take a vacation to Alaska to warm up....LOL

The valve is similar to the 13xx gun.
The stem is bored up close to the TP at .187".
Pinned with 2 8-32 screws counterbored into the valve.

Turned the barrel at the breech end to 5/16".
Cut off the nipple on the tiny breech and counterbored it 1/4".
Will get a Delrin TP.

Machined the receiver halves where the probe slides.
Opened it to .234" for 22 cal.

Just have a few small items left and the gun should be ready to go.

[Rob M]

very cool , nice to have the old school cast parts and not have to ream out the flimsy plastic rear barrel Tp they make now.

[Blutroop]

Following

[Tack Driver 10]

Made the probe.
Since the original is 17 cal turned it down and threaded it 8-32.
The 22 cal probe screws on.

The hammer is 12 grams made of Delrin with a metal core and poly bumper.

Finished the remaining minor parts and assembled the gun.
Pumped it to 1200 psi, has a very slow leak at the gauge.
No problem dry fired it inside this morning, 90 dB on the sound meter.

[Tack Driver 10]

Was hoping to chrony the gun today but a major catastrophe befell me.
I shot the gun and the bolt would no longer cock the gun.

Took the gun apart and discovered the valve stem has been beheaded.
Funny thing is the gun still sealed with the stem disconnected from the head.
When I drilled the holes to pin the valve I nicked the stem.
Probably pulled it a bit and shooting it finished it off.
Made a shiny new valve stem, same diameter but taller with more insertion.
That should take care of it.

[Tack Driver 10]

The gun held 1200 psi rock solid overnight.
Has a sealed 90 gram CO2 bottle installed so running off the plenum.
Makes it easy to pump and fix any leaks etc.

Got to chrony the gun this morning, 03 degrees (-16 wind chill), between snow storms.
Pumped the gun to 1600 psi, 22 cal 24" barrel, 12 gram hammer/2300 spring.

14.3 CPHP
806 FPS 20.62 FPE
818 FPS 21.25 FPE
820 FPS 21.35 FPE
820 FPS 21.35 FPE

Pumped the gun to 2000 psi
775 FPS 19.07 FPE

Pretty "respectable" considering the light 12 gram hammer.
With a stock 30 gram hammer and an SSG the gun should really perk up.
The 13xx gun made 926 FPS 27.22 FPE with a heavier hammer so this gun should be close to that.

[Back_Roads]

 
 Right up there with Maximus power, Interested to see shot counts...

[Tack Driver 10]

Put a stock 30 gram 760 hammer and 2100 spring in the gun.

2000 psi

14.3 CPHP
1004 FPS 32.00 FPE

25.4 JSB Monster
812 FPS 37.18 FPE

33.95 JSB Beast
700 FPS 36.93 FPE

Without an SSG uses a lot of air but had to try it.
Now I'm thinking a 17 cal Plastic Fantastic 760 that breaks the sound barrier.

[Rob M]

awesome, probably the only 37 fpe 760 on the planet.

[Tack Driver 10]

Weather cooperated today, a balmy 26 degrees....LOL

Removed the 2100 hammer spring and used a 2400 spring with a .360" spacer.
The 2400 spring is lighter and shorter so should detune the gun some.
Ran 2 test shots yesterday at 1800 psi, 884 and 888 FPS.

Used an empty 90 gm CO2 bottle pumped to 1800 psi.
Going to run a shot string until power falls off.

14.3 CPHP
01. 797 FPS 20.17 FPE--1800 psi
02. 882 FPS 24.70 FPE
03. 888 FPS
04. 890 FPS
05. 890 FPS 25.15 FPE
06. 893 FPS--------------1600 psi
07. 891 FPS
08. 897 FPS
09. 899 FPS
10. 898 FPS 25.60 FPE
11. 892 FPS--------------1400 psi
12. 888 FPS
13. 889 FPS
14. 885 FPS
15. 878 FPS 24.47 FPE
16. 870 FPS
17. 865 FPS--------------1200 psi
18. 858 FPS
19. 850 FPS
20. 835 FPS 22.13 FPE--1000 psi

First shot was low for some reason but then settled down.
Might not have closed the bolt fully???

Has a good amount of hammer bounce without an SSG.
I think a 9 ci bottle regulated at around 1250 psi might be a good fit.
Should also bump up the shot count substantially.
Guesstimate 60+ shots from 3K to 1250 psi.

[Tack Driver 10]

Made an SSG and test hammer.
The SSG uses a light 2300 spring and the hammer weighs 19 grams.

Gone is the 3 shot burst sound replaced with one tight punchy note.
Last night pumped the gun to 1800 psi and dry fired it to 1000 psi.
Got 26 shots vs 20 without the SSG.

Made 3 test shots this morning at 1600 psi
14.3 CPHP average 870 FPS 24.03 FPE

[Rob M]

nice.. wonder if you could mod that reciever to take the 5 shot stripper clips.. I converted a m417 to co2 a while back and the stripper clip was a nice feature for once.

[Tack Driver 10]

Would be difficult but have two M4s and a black/gold 66 that have the clip receiver.
Both are plastic fantastic guns.
How did you mod your M417 for CO2?

[Rob M]

machined a drop down on the main tube and pinned the valve..( i used a 9oz tank and an asa adapter ) Ive done quite a few pcp and co2 conversions over the years ( probably dozens )  the m417 was a way better gun on co2 , the stripper clip with some practice was pretty fast for follow up shots. Problem with the m417 is remounting the forearm , no matter the config , You need serious clearance internally on that big forearm.

[Tack Driver 10]

Made a new block and swapped the 90 gram CO2 tank for a 9ci regulated bottle.
Set the reg at 1400 psi.

I like the form factor of the CO2 bottle on this gun so.....
Might put the CO2 block and tank on a 760 plastic fantastic.
Won't that be fun.

14.3 CPHP, SSG with 2300 spring and 19 gram hammer.
Ran 5 shots over the crony

915 FPS 26.58 FPE
918 FPS
916 FPS
914 FPS
915 FPS

Should cut the reg down some and settle in at around 800+ FPS
I'll have to put a scope on it and see how it shoots first.

[Tack Driver 10]

Put a scope on the gun and after sighting it in benched at 20 yards with a 15 mph cross wind.
Tried different pellets:
CPHP, hated them
H&N Barracuda Hunter Extreme, nope extremely inaccurate
RWS Superpoint Extra, I've had better
H&N Barracuda Match, getting warmer
H&N Field Target Trophy, 10 shots dime size, you get the Trophy

Not going to win any FTT competitions but did OK fresh out of mod, trigger needs improvement.
The BMs were 5.52 and the FTTs were 5.55 so I think the fatter pellet did good in the loose Crosman barrel.
Might have to get some BMs in triple nickel and stretch it out to 30 yards.
I would feel comfortable pesting with this gun as is though.

No USPS mail boxes were harmed in this experiment, this one was recycled.