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PostPosted: Sun Jan 26, 2020 3:57 pm 
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Joined: Thu May 07, 2015 8:30 pm
Posts: 1243
Location: Eastern Townships
Part 1

I’m starting this post because I want to put together all the references to this project. I’ve already posted pictures elsewhere, in different threads, so I will ''centralize’’ everything here. Hope you enjoy!

It all started as a question I was asking myself, is it true that break barrel loose power with a short barrel? Also, would a shorter barrel help with hold sensitivity? I know my Diana 24C, which has a 13 1/4’’ barrel, is very forgiving with ‘’shackyness’’ and very well balanced. My Benjamin Classic .22 is a great shooter, but a little hold-sensitive. I suspect this may be due, at least in part, to the long 18.5’’ barrel. I noticed that my Weihrauch HW85, with its 500 mm (19 11/16’’ approx.) barrel, shows the same behavior.

I came upon a used, badly bent Phantom .22 barrel that I got for free. There was good rifling in the unbent part, so I cut it to remove the bend, and also the muzzle part, which showed some bad machining. I was left with a 10.300’’ ‘’blank’’, which was finished to 10’’. The stock Benji barrel was pressed out, and the new barrel was machined to fit in the breech block, choked, crowned and threaded, and a new ‘’lead-in’’ was cut. Then the new barrel was pressed and re-pinned in place, and lapped. I can say also that it leaves very well defined and smooth rifling marks on the pellets.

I also wanted to make a cocking aid, since I was aware that the cocking effort could be higher, and it turned out to be an air stripper finally. I got the idea from a British airgun forum. It’s giving me 3 additional inches of leverage, and it shows, even with a weak spring! The body is made from chrome-moly steel alloy (4140) for rigidity, and the cone is made from 304 SS. Cone included angle is 60°, bore is 0.236’’ (6 mm), so roughly 0,010’’ each side of the pellet when it goes through. I’m in the middle of the build, so I haven’t tested if the stripper will play a big role in accuracy, but the pellets are not clipping!

I’ve tested how the stripper help for leverage with three springs : a ‘’weak’’ one, 0.099’’ wire size and 34 coils, 9 1/2’’ free length, the stock Benji 0.118’’ wire size, 33 coils and 9’’ free length, and a full power spring, 0.122’’ wire size, 36 coils and 10’’ free length (thanks to a friend who has his PAL and spare parts to lend!). Cocking effort is manageable with all three springs, but nonetheless rather hefty with that full power spring! I also tested the velocity with each spring of course (brought the chrony at my friend’s house), with the shorter 10’’ barrel, and a standard ‘’short’’ piston with no hole and a new seal. JSB Jumbo Express 14,35 grains pellets were used. Results are as follow : weak spring 425 fps, Stock Benji spring 698 fps, full power spring 724 fps. According to what I know, these numbers are in the ballpark for an un-modified, ‘’long’’ barrel rifle similar to mine. So at least on my particular gun, the short barrel doesn’t seem to have much negative effects on velocity.

After doing all this (and giving back the full power spring to my friend:) ), I wanted to see if I could up the power just a bit, in the 450 fps range, but also keep the sweet shooting characteristics of the weak spring. I could just shim the spring, but I wanted to test another idea I had : Is it feasible to add piston stroke, so more swept volume, and if so, how much while keeping the rifle safe to operate? So I modded an original piston by shaving the seal-retaining dovetail, and then shaving it another 0.050’’. With careful measurements I determined I could add 0.100 of stroke without major work (this is quite relative!), so I made a Teflon seal of a thickness (about 0.150’’) that made the total piston length 0.100’’ shorter. I had to grind about 0.050’’ off of the cocking arm ‘’shoe’’, and grind the piston cocking slot 0.050’’ toward the rear, so there would be no impact between the two parts when the rifle is shot.

The piston will be buttoned with bronze inserts, which are made but not installed yet, I still have to flat spot and drill the piston. After this is done, I’ll be able to test the rifle to see how much more power that additional stroke will give. So far, this is where I am into this project, will update soon.


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Barrels.JPG
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air_stripper.JPG
air_stripper.JPG [ 40.06 KiB | Viewed 339 times ]
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PostPosted: Sun Jan 26, 2020 4:27 pm 
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Joined: Thu May 07, 2015 8:30 pm
Posts: 1243
Location: Eastern Townships
Here's a preview of the ''part two'' of the build. In addition to the Teflon sealed and bronze-butonned piston, the action will be tuned with a new one-piece spring guide/retainer with Delrin spring guide rod sleeve, bronze and steel thrust washers, polished inside and out spring, and bronze and steel top hat. I still need to install the Delrin sleeve and finish the spring guide to correct length. More on this part later :) .


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Top_hat_parts.JPG
Top_hat_parts.JPG [ 75.22 KiB | Viewed 333 times ]
Top_hat.JPG
Top_hat.JPG [ 65.89 KiB | Viewed 333 times ]
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PostPosted: Sun Feb 02, 2020 6:17 pm 
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Joined: Thu May 07, 2015 8:30 pm
Posts: 1243
Location: Eastern Townships
Part 2

Well the fitted spring guide is finished and already installed, and I shot the rifle a couple times in the pellet catcher to feel the difference with the original spring guide. Difference is truly like night and day :) . Cocking feels much smoother, I no longer hear that binding sound as the spring compressed, and firing cycle is smoother too, no more spring vibes (twang), it actually reminds me of my TR77NPS (gas rammed). I haven't re-chronied the rifle yet, time is flying fast! I'll update as soon as I can 8) .

The spring guide metal core is made as a 1 piece part, instead of the original 3-pieces one, I used stressproof steel. Drilled 9.5 mm for the cross pin, and drilled and treaded for the 8 mm X 1.25 trigger group screw. There's a bronze ''seat'' pressed and epoxied at the base of the 1/4' guide rod, and a steel thrust washer goes in between this seat and the spring end (the steel washer adds 0.125'' of spring preload). A 3/4'' delrin rod was drilled 1/4'' through, cut to length and also pressed and epoxied in place.

After the epoxy cured, the guide was machined to size and final length. I'm working with closer tolerances than the original setup, so careful measurements were taken regarding the top hat and guide end distance, I kept it at a minimum to have the longest possible guide while still being able to cock the rifle :wink: . A longer guide can also prevent spring warp and breakage, since it always work on a staight axis. Add to this that the spring can swivel at both ends, so no binding occurs. Finally, the spring itself is polished inside and out, but the ends were kept rough, to grab on the thrust washers. I haven't added a plastic sleeve, or sheath, in the piston yet, but all vibes are gone :) .

First set of pics:


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SG_3.JPG
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SG_4.JPG [ 75.08 KiB | Viewed 175 times ]
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PostPosted: Sun Feb 02, 2020 6:27 pm 
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Joined: Thu May 07, 2015 8:30 pm
Posts: 1243
Location: Eastern Townships
Second set of pics:

The first picture SG_5 shows the parts assembled and ready to be installed.

In the second picture SG_6, you see the distance (1/8'') between the top hat and guide end when the piston is in the ''full-cock'' position, when you actually hear the click of the sear engaging in the piston's notch.

In the third picture SG_7, you see the distance (1/4'') between the top hat and guide end when the piston is in the ''ready to fire'' position.

Cheers!


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SG_6.JPG
SG_6.JPG [ 41.71 KiB | Viewed 174 times ]
SG_7.JPG
SG_7.JPG [ 41.29 KiB | Viewed 174 times ]
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