(old) discovery down tune
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(old) discovery down tune
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Topic: (old) discovery down tune (Read 2381 times))
Ribbonstone
GTA Senior Contributor
Posts: 12030
(old) discovery down tune
«
on:
April 11, 2014, 08:38:14 PM »
Maximum effort at an efficient Discovery. Realize this is an older Discovery, back when they came with striker springs that seemed to run best at about 1800-1900psi. It’s also well broken in, so runs smoothly with little friction.
#1. Put the original end cap (non adjustable) back on.
#2. Original spring (no pre-tension).
#3. Transfer port swapped for a .095” one (give or take .003” as it’s not quite round and being made of Delrin will get “squished” a slight bit when installed).
#4. While it was apart, cleaned and lightly lubed.
#5. New bolt probe o-ring. No detectable leaks anywhere (passed the “flour” test).
#6. Backed the over travel screw out a bit to be sure there was no sear drag.
#7. (This should not change the results). Was trying a new (experimental) LDC and swapping with a TKO for comparison.
#1. This is actually much better than expected, would not have predicted 49 shots inside of 4%.
#2. There is some room at the top. Could increase spring tension and get it running at closer to 2000psi. As it is now, seems senseless to fill it much past 1800psi or shoot lower than 1000psi.
#3. Call it a 17 foot pound average (and about a bar-cc/fpe of 10.2)
While chronographing, took time out every so often to align the chronograph with a target and try a 5 shot group at 20yards (actually, I moved the back stop rather than the chronograph).
#1. It needs a touch of left windage, but I like the 12 0-clock elevation.
#2. Still not a match gun. Nothing seems to let this barrel average better than this.
Overall, this is a whole lot more shots than I’d have predicted at 17 foot pounds.
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Louisiana
sr1sws
Sharp Shooter
Posts: 533
yes
Real Name: Steve
Re: (old) discovery down tune
«
Reply #1 on:
April 11, 2014, 10:27:01 PM »
Nice. My chrony showed up today. I'm planning on ordering some Challenger ports and a spring. Mostly I shoot paper, but occasionally a tree rat is in the wrong place at the wrong time. I would think 17fpe should be plenty for my purposes.
Thanks!
Steve
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Tampa, FL
.177 Crosman 147 ca. 1963 - original owner
.177 Remington Vantage NP - sold
.177 Benjamin Trail NP pistol - sold
.177 Benjamin Discovery + TKO, Hawke Sport HD 3-9x40mm
.177 Crosman 2300kt + TKO, Walther MRS Reflex
.177 Benjamin Marauder/SynRod Gen II - Regulated, WAR TSS, UTG SWAT 30mm 3-12x44
.177 QB79 + Ninja Pro SHP V2 Regulated Bottle, UTG SWAT 30mm 3-12x44, DIY Half-Shroud
Ribbonstone
GTA Senior Contributor
Posts: 12030
Re: (old) discovery down tune
«
Reply #2 on:
April 12, 2014, 12:34:23 AM »
There is a side benefit. Partly because it's using such small blips of air, is considerably quieter.
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Louisiana
grhouse
Guest
Re: (old) discovery down tune
«
Reply #3 on:
April 12, 2014, 01:30:41 PM »
Steve, you said, " I'm planning on ordering some Challenger ports and a spring." Will the Challenger port fit in the Discovery and what is the difference if you know. I've been thinking of making my transfer port hole larger but am unsure of what to use for the transfer port if I do this and do not have good access to a machine shop. I do have a small drill press if I could make one on it but I'm not sure what size to make it. Any suggestions? Gene
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Ribbonstone
GTA Senior Contributor
Posts: 12030
Re: (old) discovery down tune
«
Reply #4 on:
April 12, 2014, 03:01:32 PM »
Even though it wasn’t exactly a Discovery (tube was shorter) Bob (rsterne) did a good bit of work on port size in the Discovery.
http://www.gatewaytoairguns.org/GTA/index.php?topic=48203.msg468823#msg468823
If looking for more power, important bits here are that if the holes in the barrel or valve are .134”, it won’t really help to make the transfer port connecting those two holes any larger than .134”.
Challenger port has a very tiny hole (something like .05"), so it's the best choice (will fit a Discovery) for making ports SMALLER than the issue port. For larger than the issue port, measure the hole in the barrel/hole in the valve and make the port that size.
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Louisiana
only1harry
One of the original 3 GTA Musketeers (per Gene Curtis)
GTA Moderator
GTA Senior Contributor
Posts: 21632
ΜΟΛΩΝ ΛΑΒΕ! (come and get them)
Real Name: Harry
Re: (old) discovery down tune
«
Reply #5 on:
April 12, 2014, 05:34:48 PM »
Those are great groups and string. It's almost like it's regulated after shot #8.
I have been thinking about converting mine back to stock but I want to get 20fpe. Right now I only get 18 shots averaging 785-790fps or 25fpe with JSB 18.1gr Heavies and the power adjustor turned out all the way (lowest power setting), or 13-14 shots at 29-30fpe if I crank it up. I am thinking 25+ shots at 20fpe shouldn't be a problem with the stock spring and striker.
Harry
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Hudson Valley, NY
Pro-tuned guns:
AF Condor .25 #1, Condor .25 #2
Condor .22, Marauder .25, Disco .22 camo
RWS Diana 350 .22, RWS Diana 48 .22,
RWS 34K .20 Premium #17 of 30 custom built by Diana
RWS Diana 36 .177 (Gene & Bob tuned)
Factory stock & home tuned:
Benji Akela .22 bullpup
Gauntlet .22, Disco .22 #2,
Fortitude .22, Prod .22
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Hammerli 850 .22, Crosman 2240KT
Remington NPSS .22, Umarex M&P 45
Crosman 66, Crosman 760, Beeman P17
Crosman Air17, Crosman 38T
Several other Umarex & Crosman CO2 pistols
Ribbonstone
GTA Senior Contributor
Posts: 12030
Re: (old) discovery down tune
«
Reply #6 on:
April 12, 2014, 06:08:33 PM »
One of the reasons I’ve resisted regulators (except in the HPA/ QB tanker conversions) is probably because I pump fill.
If I can get 20 shots inside of 1% unregulated, and it takes 20 pumps to refill to that point, figure I’m “even” with a regulated gun that gets 100 shots at 1% but take 100 pumps to refill.
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Louisiana
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(old) discovery down tune