.177 cal 12 FPE Discovery build - input appreciated

[AmBraCol]

Hey guys, I've been contemplating a 12 FPE Discovery build for a while.  No hunting around here so it's merely paper and steel shooting. Building up something different and unique is always fun so am starting off with "a clean sheet of paper". The main idea is to build an accurate 12 FPE class .177 rifle built on the Discovery type platform - with a few twists.

Standard Discovery valve - upgraded screws for up to 3,000 PSI fill
Standard Discovery fill port
Challenger transfer port - can be reamed out if too restrictive
Cothran tube - or standard Discovery - the Cothran does not come with an air pressure gauge hole
If using the Discovery tube, a Marauder pistol gauge block and gauge would be used
Maximus .177 caliber barrel
Discovery breech
Marauder Pistol trigger group
Double barrel bands with six screw mod
1399 stock
MellonAir rear velocity adjuster
Challenger hammer/stroke adjuster

My idea is that with a higher fill pressure and a properly tuned mainspring, it should give a very decent shot count. I'm a singleshot guy so the lack of repeater function means only that there's less complexity to mess with.  Do any of the Crosman bolts come with an extended probe? Or am I looking at having to buy a custom built probe?

Thoughts? Ideas on improving the above components?

[dmeguy]

If you are already ordering a Mellon power adjuster, I'd get one of his extended probes. His prices are hard to beat.

[AmBraCol]

If you are already ordering a Mellon power adjuster, I'd get one of his extended probes. His prices are hard to beat.

Thanks, I've got the adjuster in my parts box, but am contemplating putting in an order, still sifting through different projects trying to finalize what gets ordered where. 

[toddbrat]

Interested to see how this turns out. I've dropped my Maximus down to about 12fpe, and love it. I really need to buy a chronograph to see exactly what it is doing.

[Jeff Marshall]

My .177 Discovery with a HDD de-bouncer and a cut down striker spring yields
50 good shots at 12 foot pounds with a 1600 psi fill.

It requires 37 easy strokes of my Crosman hand pump to fill.

The same tune on a .22 Discovery yields 15 foot pounds.

[AmBraCol]

If you are already ordering a Mellon power adjuster, I'd get one of his extended probes. His prices are hard to beat.

And a good call on that.  I'd forgotten that I've got a flow through MellonAir .177 bolt probe in the parts locker.  Sold the .22 version as it wouldn't work with my left hand breech, but the .177 version's been sitting there waiting for something appropriate.  Checked it in a spare Crosman breech and it lines up perfectly, although won't know 100% for sure until such time as I get my hands on a .177 barrel and fit it up and check pellet fit. Hmmm... looks like it's a go!

[Gipper]

I am currently shooting something similar to this with 1 main exception...I converted it to a bottle.

Stock Valve
Eliminated pressure gauge
Barrel shortened to 19.5 inches
Prod trigger group
AR 15 butt stock with RVA

My bottle is regulated to 850 psi and fills to 3000 psi.  Currently shoots 8.4 AA's at 850 fps for a little over 13fpe and close to a 100 shots.  Your goal is very doable and enjoyable to shoot at this power level.



I no longer have the shroud or 1/4" riser on this just a home rolled LDC.

Brian

[AmBraCol]

I no longer have the shroud or 1/4" riser on this just a home rolled LDC.

Brian

That's an interesting build as well.  Are you interested in selling the shroud and riser?   

[Gipper]

I no longer have the shroud or 1/4" riser on this just a home rolled LDC.

Brian

That's an interesting build as well.  Are you interested in selling the shroud and riser?   

Both items have already found new places to reside! The shroud was just something I made.  The riser came from Baker airguns and included the necessary screws and cocking pi  and is a nice piece. Waa $28 plus shipping if I recall. I loved the look of it but had terrible poi issues with it. Everytime I picked the thing up it hit somewhere else. Just not enough breech support. I hate the breech screw position under the bolt, makes it a pain to remove.the.riser and barrel and the forward mounted screw just provides better support IMHO. I hear about shifting poi issues with discos but never with 2240s.

[Marc]

I have thought of something similar - does the Disco get quieter at lower power?  Don't really want to add a TKO.

[Jeff Marshall]

I have thought of something similar - does the Disco get quieter at lower power?  Don't really want to add a TKO.

A Hammer Debounce Device from AOA makes a Discovery MUCH quieter, and increases the shot count per fill by a significant amount.

http://www.airgunsofarizona.com/parts/crosman-custom-hammer-debounce-device-hdd/

[Marc]

I don't even own a Disco yet - are they hard to install?  And how is average Discovery accuracy?

[AmBraCol]

I don't even own a Disco yet - are they hard to install?  And how is average Discovery accuracy?

Nothing on a Disco is hard to install - unless you count pumping up from empty in some of the equations.  In this case their instructions call for emptying the gun.  On a Discovery this is not completely necessary IF YOU ARE FAMILIAR WITH THE GUN AND HOW IT FUNCTIONS AND FOLLOW ALL OTHER SAFETY PROCEDURES.  All you have to do is pull the breech, insert the HDD and reassemble.  The hardest part would be to cut down the rear plug of the breech if you want to continue to use it.  It keeps a lot of crud out of the breech so that's what I'd do if using the HDD.

On the other hand, one can build a SSG instead and get the same effect of debouncing the hammer.  It's a bit more involved, but in the long run I believe it has serious advantages over the AoA HDD device. 

[AmBraCol]

I no longer have the shroud or 1/4" riser on this just a home rolled LDC.

Brian

That's an interesting build as well.  Are you interested in selling the shroud and riser?   

Both items have already found new places to reside! The shroud was just something I made.  The riser came from Baker airguns and included the necessary screws and cocking pi  and is a nice piece. Waa $28 plus shipping if I recall. I loved the look of it but had terrible poi issues with it. Everytime I picked the thing up it hit somewhere else. Just not enough breech support. I hate the breech screw position under the bolt, makes it a pain to remove.the.riser and barrel and the forward mounted screw just provides better support IMHO. I hear about shifting poi issues with discos but never with 2240s.

Thanks, Brian.  Figured it was worth asking.    They are priced the same, still, just don't feel like springing for one.  To set it up properly would entail building a couple of proper barrel bands that would work with the increased barrel height, and I don't have the tooling to do it with right now so don't feel like the outlay is worth it either.    I DO appreciate the feedback on your experience.  It confirms my opinion on the barrel bands.

[Ribbonstone]

MAybe narrow the choices first?

(Although I really do like the 12 foot pound area for .177's.  I'm not too picky,and if I hit 11.7  or 12.2 foot pounds, don't really have a need to readjust them for the missing fraction.  That's pretty much where a FWB 124, HW 35, or a well pumped Benjamin 347 lived, and the squirrels haven't grown armor plate between then and now).

New tube or use a Disco tube.  I'm not real found of the 3K mods to the 2K tube, exp.not when not building for power. MAybe pick the heart of the rifle first?

Challenger port(s).  It's a lot easier to make a little hole bigger than make a big hole smaller.

For a 12 foot pound rifle, not sure it rerally makes a  difference which gauge block you use.

The double barrel bands (with screw mods) would be a plus if you DON"T intend a riser breech and a shroud.  Noise level may still be above the level you feel comforatable with.


This much I can tell you, from running a Disco .177 DOWN in speed (to about 12 foot pounds) and running a Challenger UP in speed (to about 12 foot pounds).  Even with a standard 2K tube (about 135cc's) running at 2K, there should be a solid 60-70 low velocity varaition shots per fill (something like 2000-1200psi).  Likely a lot more with more extensive mods/adjustments.

YEs it will be a bit quieter (with the same barrel length).  How much quieter is more a matter of tuning.


Lot of folks "sneer" at efficiency numbers, but when you have a single energy goal (12 foot pounds in this case), than consider that the higher the efficiecy at 12 foot pounds, the LESS air being squirted out at each shot, and less used does make for less noise produced.


The other thing about LDC's is that they come off.  Use them when you need backyard quiet, take them off when you don't.  Really does change the look of the rifle.


This one was built on a 2260 tube(HiPAc conversion).
 With LDC:



Without LDC (and a froint sight becasue it just looked naked without something up there).

[AmBraCol]

Hey, Ribbonstone.  I was hoping you'd chime in here.  What would you recommend for reaching this goal?  The reason for contemplating the 3,000 PSI fill is to provide greater amount of air on board.  I just don't have experience tuning in such a way as to make good use of that much air, yet anyway.  

One concern of mine in using a Discovery tube is the exposed gauge in a 1399 stock setup.  In a standard wood stock it's well protected, but would be sticking out there in the barebones setup I'm contemplating.  On the other hand, going with the Cothran tube would mean no gauge at all, so would have to monitor shot count in order to stay in the plateau of the curve, no just glancing at the gauge to see if you're "still there".

Any pointers in meeting the main goal of an efficient 12 FPE bare bones rifle are quite welcome.

[Jeff Marshall]

I don't even own a Disco yet - are they hard to install?  And how is average Discovery accuracy?

Nothing on a Disco is hard to install - unless you count pumping up from empty in some of the equations.  In this case their instructions call for emptying the gun.  On a Discovery this is not completely necessary IF YOU ARE FAMILIAR WITH THE GUN AND HOW IT FUNCTIONS AND FOLLOW ALL OTHER SAFETY PROCEDURES.  All you have to do is pull the breech, insert the HDD and reassemble.  The hardest part would be to cut down the rear plug of the breech if you want to continue to use it.  It keeps a lot of crud out of the breech so that's what I'd do if using the HDD.

On the other hand, one can build a SSG instead and get the same effect of debouncing the hammer.  It's a bit more involved, but in the long run I believe it has serious advantages over the AoA HDD device.

Also, when a HDD is installed, a bit need to be ground off the rear of the bolt to restore full bolt travel. It is a quick and easy job with a bench grinder.

[Ribbonstone]

No surprises here....I generally go for the simpler fix.

Which won't give you the highest efficiecny, but can land in a really good area (1.3-1.4 FPE/cuin) for a nice low velocity varation shot sting at 12 foot pounds with a bit of experimentaion.


Up to you if you like the plastic stock look or not (and without a fore end, it does handle a bit awakwardly...so figure something out for that if you go with the pistol trigger unit.

Simplest would be to order  a complete Challenger striker (with stroke adjustment), Challenger rear end cap (which adjusts) and a Challenger transfer port (which you'll carefully drill out in small steps), another barrel band (which you'll drill eventually)  And some tiny drill bits (Dremel sells both a Metric and an Inch small set, which is commonly availible at HomeDepot or Lowes...and will work in a regular electric drill).

If I had to gess where to start...in inches...would be 3.32nds(about 9.4").   If going metric, look for something in the 2.2-2.5MM size.  Which seems small, but  in the ball park for 11-12 foot pounds.

Lets make the machinest cringe...but if you want a very slightly larger hole, run the same drill (spinning) in and out a few times.  Likely will become .1" (and we don't care if it's not 100% round).

Form there, it's really a lot of testing to see how the rifle reacts.More stroke. less spring tension?  Less stroke/more spring tesnsion?

With two barrel bands, I tend to mount one as close to the breech as I can and deal with inletting the stock.  That breech, and it's dinky little front screw, isn't the most secure system in the world, so the rear mounted band serves mostly to fixate the breech (and makes that little screw much less important...just put it in "snug" rather than super tight).

Does require a stock (or fore end) inletting mod, but it's pretty simple as it isn't a complex shape.



After playing with 6 screw bands,I pretty much gaver it up.    Did make them, used them,but as the bands really only do two possible things, only the bottm 3 screw really got used.

So....made them,but doen't use them as 6 screw bands. Bottom 3 screw (to the tube) are tight,top 3 screws (to the barrel) are just barelly loose...enough for the barrel to just barely move in and out.

Next time up, didn't bother with the added screws to the barrel....worked just as well. The little bit of play in the top part of the band does no harm I can see. With the rear end of the barrel clamped down tight with the rear band, the little fraction of an inch of play in the front band won't exceed the elsatic limit of the barrel...even if you do hit it, will just spring back to where it started.

Just as an aside....that 1399 plastic stock has too much drop for my buildand is a little long (trigger reach...but that's OK, have the long arm/skinny old guy build).To make up for it with scope use, tend to add a cheek riser.

In my case, that would be some holes in the stock, some lengths of 1/4" Al. rod, a painted 5/8" length of shaped wooden dowel whaped to a kind of torpedo, and some glue.



Want MORE?
We all like MORE (kind of like the insurance commerical with the giant belt buckle).

Well...you have a Disco striker that'snot being used.  The sides of that striker serve no use (you'll notice the Challenger striker is flat sided). Top carries the cocking oin, so you can't change that.  Bottom is the sear notch, so you can't change that.  But with a bench grinder, can pretty much remove the sides.



Some posters don't feel that a 20-30% reduction in weight does much of anything....but they weren't trying for 12 foot pounds.   It does seem do something good at the low end.


The reduction in striker weight usually needs a bit more spring tesnion (and maybe a little more open transfer port).  Once you get it back up to 12 foot pounds (by increasing striker spring tensiuon),  and find 5 things: (1) a few more shots per fill at 12 foot pounds, (2) a bit of reduction in the PRESSURE range of the sweet spot (maybe from 2K to 1.2K down to 2K to 1.3 or 1.4K) (3) a little less air use per shot to reach 12 foot pounds, and  (4) a little less noise becasue of the lower air use.  Add in (5) a little less effort to refill it if you are a pump filler.

[c_m_shooter]

My discovery is running the upgraded valve screws, .080" transfer port and has a heavy maurader hammer spring in it at the moment.  From a 2500psi fill I'm getting 40 shots within 10 fps at 13.5 ft lbs.   I was running the same transfer port with a 1200 psi Altaros regulator and a crosman 1377 hammer spring for 70 shots at 12 ft lbs until the regulator quit regulating.  I am thinking of switching to open class field target next season and may open up the transfer port a bit and try the balanced valve in it for a little more power but trying for a flat string with no regulator.

[AmBraCol]

Wow, Ribbonstone! Thanks for the feedback.  Yeah, the skeleton stock isn't the best looking, but the crazy thing seems to work OK for me. I'm running one on the 1377 and it fits just fine.  My problem is that most stocks seem too "thick" or "high" for my face, making it hard to get behind the sights/scope.  This one works fine.  I expect it will be nose heavy, but that's not a bad thing for off hand shooting. What I've not gotten sorted in my mind yet is the whole gauge thing. Need to remove the stock from the 22 Discovery and refresh my mind on it.

Yes, that band up by the breech is a good thing.  Inletting my 22 Disco stock was no problem and worked great.  I'm thinking a bit about  if a 2400 barrel band with miniature "forearm" would work on this - especially if one were to affix a longer forearm to it.  Again, need to pull the stock and try mocking up a couple of options. 

I'll need to check out the drill bit sets you mention while up north.  Haven't found much of anything down here other than the typical 1/8, 1/16, etc. Come to think of it, not even any metric ones - which is odd given the territory we live in.

I've got the Challenger striker and adjuster, although I favor the Mellonair adjuster due to the ability to more easily swap springs when experimenting.  Actually, there's not too much that I need to pick up for this build, most of the guts are already in the midden heap, it's the tube/breech/barrel/trigger group that needs acquiring. 

Again, thanks for the input.  I greatly appreciate your experience and the info you share.