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Author Topic: Discovery 0.22 Tuning  (Read 11677 times)
scottinharwood
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« on: September 18, 2008, 09:10:22 AM »

Discovery 0.22 Mods and Tunes

All:

I went inside the Discovery 0.22 to see if I could tune it in much the same way I was able to tune my BAM B51.

I found the discovery cleaning and polishing process of the trigger and hammer assemblies to be very similar and straight forward.

I polished the hammer with first 600-grit then 1000-grit oiled wet-dry sandpaper by chucking the hammer into my wood lathe (protecting the chucked surface with tape) and spinning it.

I polished the inside of the hammer guide tube and the hammer bolt pin slot with both the 600 and 1000 grit oiled sandpapers above.  I put a light coat of Moly Paste in the hammer chamber then wiped it out and I put a small amount of white lithium grease on the bolt pin and guides.

I polished the trigger assembly pieces and added white lithium grease to the portions that came in contact.

I added a 1/8” plastic disk behind the spring and also added an adjustment screw so I could tighten the hammer spring.

I added a transfer port o-ring to the breech side of the transfer port to insure a transfer port seal.

I cleaned the breech block and barrel and used white lithium grease on the bolt assembly.

After putting it all together I fired it at about 2/3rds full (into the green range) and found my velocities were down to about 720 fps, but pretty constant.  I then filled to a full 2000# and began turning in the adjustment screw to tighten the hammer spring.  7-turns later, I am firing at 850fps.  I will have to run a chrony string from full to, say, 1000# to see how the shot string looks later on.

That’s it for now, I will advise on the chrony results and any resulting changes I make later on.

Oh, yeah, after the tune, without turning up the spring pressure, I definitely hear a hammer bounce now that I did not hear before.
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
melloroadman
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« Reply #1 on: September 18, 2008, 12:54:10 PM »

Scott what spring did you use for the hammer ? Marvin
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scottinharwood
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« Reply #2 on: September 18, 2008, 01:18:24 PM »

Hey Marvin:

I kept the original spring.  Everything runs smoothly at the moment.  I was tempted to shorten the spring a little, but one step at a time with analysis steps in between.

I have yet to do a full pressure shot string analysis.  

As with the B51, I figure that the hammer cylynder and hammer need to be dry and not lubricated; but there seems to be plenty of slop so vacumn is probably not a problem, just lube friction.  If I will not try a dry hammer and cylynder until after I run a shot string.

I must say, mechanically, I kind of think the B51 is superior - go figure.

Scott
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
melloroadman
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« Reply #3 on: September 18, 2008, 01:33:43 PM »

N/T
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scottinharwood
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« Reply #4 on: September 18, 2008, 01:53:03 PM »

Yes I did, I ran a string from a 2100# fill down to about 1000# as shown in the following.



* Benjamin Discovery 022 pre-tune.pdf.jpg (49.37 KB, 960x720 - viewed 714 times.)
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
melloroadman
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« Reply #5 on: September 18, 2008, 02:49:57 PM »

It looks like you almost locked it up . At about shot 19 when things leveled off what was your PSI . And again at shot 35 what was you PSI ? Marvin
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scottinharwood
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« Reply #6 on: September 18, 2008, 11:58:45 PM »

Marvin:

The starting fill was at 2100# by the Hill Pump guage and that put the gun slightly into the yellow fill pressure area of the gun guage, above the green area.

Shooting down to 2000# brought the gun guage pressure to about the top of the green area; that woud be shot #9/10.  Shot #12 is where I hit the first black line in the Green area of the gun pressure quage.

Now, on the chart, the shots highlighted in green area on the chart start at shot 20 and that pressure was 1700#.

From this I concluded that my optimal fill without mod would be 1700# (Hill Pump Guage) and I would get about 20 good shots.  The max spread in the Green shot area of the chart equates to about a 0.4 inch change in drop position of the pellet at 35 yards.  By drop position I mean if you shot strait out (bore site) at the high velocity, then from the same position at the low velocity, I calculated the second shot would be about 0.4 inches lower - basically, at 35 yards, you should not expect any better than 0.5 inch groups in the optimal pressure area of the un-tuned gun - more like 1-inch groups when you factor in hold, breezes, scope adjustment, and on and on ;.)  So far the best I can do is 1.5 inch groups at 35 yards from a sand bag rest.

Scott
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
melloroadman
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« Reply #7 on: September 19, 2008, 09:33:48 AM »

Scott were those reading after the lower link mod? Also with my .177 at 50 yards I use to shoot 5 shoots and then turn the adjuster in 1 turn and shoot 5 more and keep a 3/4 inch group with a fill of 1800 PSI  with a 10.5 CPH . Now what I have tried is the same fill and pellet but I screw the adjuster in 2 turns after it touches the spring and shoot 10 shoots and keep the same group size . From my last chrony readings taken that changed my FPS from 860 to 880 . I think if I change it to a .22 next year since winter is on us and go to a heaver pellet I would have  better results. You might want to try some different pellets and power adjuster setting if you are going to go for longer distances.Also we have to remember we are working with about 135 cc of air  Marvin
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scottinharwood
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« Reply #8 on: September 19, 2008, 10:32:32 AM »

Marvin:

Yes, I put the lower link mod in before I did the pre-tune shot string.

Good point on the air, see the next post, I have the test results for the post-tune condition (including the lower link mod)

Scott
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
scottinharwood
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« Reply #9 on: September 19, 2008, 10:42:38 AM »

All:

Below are the shot-string test results I did after I got home from work today.  Bottom line is I can fill to a pressure higher than the 1700 psi ideal pressure of the pre-tune Discovery with the lower link mod.  I now can fill to 1900 and get a good 25-shot string averaging 844 fps.  My idea of a good shot string is >= 830 fps, fairly consistent shot-to-shot velocities and velocity changes.

All testing was done with 0.22 Crosman Premier Heavies (14.3gn) pellets.  I use a Beta Chrony with the gun muzzle about 4.75 inches from the first sensor.

As I stated earlier, I added tension to the hammer spring by placing a 1/8 inch plastic disk behind the spring and adding a power adjustment screw and turning it in 7-full turns after I reach the point where the screw contacts the spring.

I like this tune at this point.

I think now that I am pretty sure that there is a crown problem with this gun, at 11 yards (my basement range length), the group size is about 1-inch without consistent orientation of shots on the target.  Anyone have a sure-fire (make that fool-proof [I am the fool here] ) way of re-doing the crown?  I also want to do my BAM B51 crown too.

Thanks

Scott



* Discovery Post Tune Test Results - smaller.jpg (95.91 KB, 868x739 - viewed 379 times.)
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
melloroadman
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« Reply #10 on: September 19, 2008, 11:25:02 AM »

Scott is the muzzle end touching the fill tube ? If so loosen the breach and re tighten it and see if that will move it away . Also I know some people have moved the barrel support to the end of the barrel towards the muzzle as well with good results . I have not had any problems with accuracy so I have not done this. You might after trying these two things do a search on re crowning or give Timmy 1959 a PM I know he has re crowned several barrels .Marvin
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scottinharwood
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« Reply #11 on: September 19, 2008, 11:44:32 AM »

Thanks Marvin, I will try moving the support forward about 1/2 way.

Scott
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
scottinharwood
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« Reply #12 on: September 20, 2008, 07:07:56 AM »

All:

Earlier in this post you know that I took a stock Discovery and moded and tuned it without buying any parts, just using the tools, screw and o-rings I had available; see the pictures below if you do not believe me - the power set screw is a hoot!  Like I said, stuff just laying around in the shop.

The final velocity results came out very close to where I wanted them; I wanted about 850fps for about a 20-shot string.  What I ended up with was an average of 844fps over a 25-shot string, but my shot group accuracy was all over the place.

My final tweak follows Melloroadman's (Marvin's) advice wherein I moved the barrel support band up to the front of the barrel.  My 5-shot accuracy at 11 yards is now less than 1/2 inch - cannot get more precise than that since 0.22 CPH's are not wad-cutters and tend to tearout the paper.

I now get 25-shots at 844fps average accuracy with my low velocity at 830fps and my high velocity at 855fps.  

Previously, my sweet area for a shot string started at a 1700# fill, shooting down to about 1000#.  Now, I can fill to 1900# and shoot down to about 1200# and get a good consistent 25-shots at about 22.6fpe; thats good for field target and hunting small game out to 50-yards (12fpe remaining energy).

All in all, this is a successful series of mods to make, and anyone can do them to improve the power and shot to shot consistancy of their Benjamin Discovery's.

Thanks Marvin, you were a great help.

Scott


* Discovery Tune - Final Configuration.jpg (42.37 KB, 500x375 - viewed 401 times.)

* Discovery Power Adjust Screw.jpg (90.66 KB, 500x375 - viewed 717 times.)

* Discovery Tune - Bolt Probe Ground Down.jpg (84.57 KB, 500x375 - viewed 604 times.)

* Discovery Tune - Various Tools and Stuff.jpg (46.64 KB, 500x375 - viewed 456 times.)
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
melloroadman
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« Reply #13 on: September 20, 2008, 08:28:06 AM »

I am glad to hear that it all worked out for you . Now if it helps or not let me say that paper plates with those 3/4 inch colored self adhesive  dots on them make great targets and they are thick enough that they do not tear out like paper . Marvin
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scottinharwood
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« Reply #14 on: September 20, 2008, 09:35:06 AM »

Sounds real good, lord knows I have lots of small orange target dots - I keep using the big ones first.

Also, at Home Depot, they had a Disney-theme'd paint one time, and I loaded up on about 300 Mickey Mouse head silhouette paper paint chips - a good target to vent frustrations on!
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AA S400Xtra 0.177, BAM B51 0.177, Crosman Discovery 0.22, IZH M-46, RWS-54 0.177, Crosman 1377C, Crosman Storm XT

Air Gun Tuner\'s Motto, \"At least do no harm to the existing airgun accuracy and power!\"
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