Power Mods for the Discovery Air Rifle? Let's Create a Sticky!

[Lostcoast]

My results were good after following Bob's advice  but I am averaging 832 fps with JSB 18.13 jumbo exact heavy for right at 28 fpe over a 20 shot string. I have a stock spring with a power adjuster, stainless steel fluted hammer and stainless steel hammer pin. Where I made the most gains was the transfer port. The stock port that comes with the BNM multi shot breech is already large enough in diameter to increase the airflow. I fill to 2300 psi (I have the upgraded valve screws) and shoot down to 1500 psi.

Hey Phil,
That's looking good indeed - and simple! Such a great way to start.. Do you know that the diameter of your transfer port is?

I'm learning to document better. ran this string a couple weeks ago. just for reference the port that comes with the BNM breech is .180 I was considering drilling the valve but I think I will just leave the Disco as it is for now. I primarily use it for nutters. They hate it lol.   

Average: 841.86
StdDev: 10.65
Min: 821
Max: 858
Spread: 37
True MV: 851.97
it's a bit over 4% but my POI holds for the 20 shots. it seems to build slowly then drop off quickly. I don't know the reason for that.
Phil

That looks like really good performance to me, and with a stock spring - wow. And with a .180 port - much larger than what most mods I have read talk about, which recommend .161. This is one of the areas where there seems to be a lot of disagreement. I've been having some folks tell me not to go above .156 in the transfer and barrel port or the shot count and string consistency will suffer a great deal, but that doesn't seem to have happened with your Discovery.

I'm forgetting - do you have a high flow gauge port on your rifle?

[Lmgunner13]

My results were good after following Bob's advice 
I'm forgetting - do you have a high flow gauge port on your rifle?

No I don't and I clipped 2 coils off the stock spring when I installed the Discosrus power adjuster.

[rsterne]

Using a 0.180" transfer port with a stock valve exhaust port and stock barrel port really shouldn't make much difference, compared to a 5/32" transfer port, which is already larger than the exhaust port.... In either case, the choke point then becomes the barrel port, which is the same in both .177 cal and .22 cal.... at only 0.134".... Here is a chart showing velocity and shot count in a Disco with a half size reservoir (from a 2260).... The valve and barrel ports are stock, only the transfer port size is changed.... Shot 1 was at a 2000 psi fill.... Note that the 4% ES shot strings with the larger ports started BELOW 2000 psi (ie the first 3-4 shots were more than 4% below the peak velocity)....



This data should apply 100% to a Disco, except the shot count would double, as the Disco reservoir is twice as large as the 2260 reservoir I tested with.... Note that as you increase the TP diameter, the shot string gets shorter and "peakier" (the ES gets worse)....

Bob

[Lostcoast]

Thanks for that great chart, Bob. Someone is recommending that I try a transfer sleeve of .150 and a transfer and barrel port of .151- .156. Looks like it will be good for velocity but pretty darn 'peaky'.

I'v decided that I am not quite ready to dive deep into valve modification work, so I am going to keep the power mods simpler for the time being:

- install a wide bore gauge port
- install .150 transfer port sleeve
- ream the transfer port and barrel port to .151?
- use stronger hammer spring (I will be trying the "43" and the "45" Hill Airgun springs;
- try higher fill pressures of 2400 - 2700 psi

How does this look?

[rsterne]

Certainly nothing wrong with simply matching all the ports to the size of the valve exhaust port, which is about 0.150".... I think for simplicity I would just go 5/32" (0.156")  for everything, and then round off the front bottom of the exhaust port where it meets the valve throat.... Yes on the PRod gauge port, to get more air into the valve.... To get the flow through the throat to equal the 5/32" porting, you will need to either use an MRod poppet (and bush the valve stem hole).... or grind down the Disco poppet shaft to 1/8" in the exhaust port area....

You should be able to get the performance you want without going above 2400 psi.... You may need a bit stronger spring to use that pressure.... You certainly will need a much stronger spring to use 2700 psi, along with stronger valve screws.... and I don't think you will like how hard it gets to cock at those pressures.... I see no reason to do that in a .22 cal Disco.... Shooting the 18 gr. JSBs in the 900s is really all you need....

Bob

[jrn156]

Just picked up my first PCP, a Discovery in .22 the info here is awesome thank all of you who contribute. I work a lot on regular firearms but airguns have opened up a new ( for me) area. But then again maybe not that new my first firearm related love was a Daisey 880 that I wore out.

[Hoosier Daddy]

Posting for future reference.
Disco mods should be a lateral "read across" to the Maximus.. no?

[Lostcoast]

For those of us who are not interested in or able to do valve modifications ourselves, Bill at Magnum Airpower just released a Magnum Valve for Discovery and 22XX conversion airguns. Prices range from $30-$53, depending on the configuration that one chooses. Bill says the valve is capable of delivering 60-70 FPE in the Discovery. Here's the link to the valve: https://onlinestore.magnumairpower.com/products/magnum-valve-discovery

My goal has not been maximum power but rather power in the range of 30-35 FPE for at least 20 shots. Tim Hill of Hill Airguns suggested to me a few months ago that I do modifications to the valve but leave the transfer port alone and even use a smaller than stock transfer port sleeve. His claim was that the valve mods will increase power while the smaller transfer port sleeve will help maintain a more consistent shot string. Since he is a respected expert I took his advice. I bought a modified valve from Magnum Airpower and then experimented with port sleeves of various diameters. I ended up settling on a .062 diameter Euro flavor Maximus port sleeve that I enlarged to approximately .088. Using the Magnum Airpower Magnum Power Spring and a fill pressure of 2350 psi I am getting a high of 904 fps and a low of 868 fps over 20 shots using the JSB 18.13 pellet. 20 shots within 3.8% of ES and an average FPE of 31.8. No doubt I could fiddle more for even better results but this is very satisfactory to me at present, especially because the accuracy is excellent.

[USAFANG6799]

AWESOME results Kevin!
Naturally the Complete Valve with Marauder Stem and Bybass Cap is out of stock  Hope he gets some soon.

Matt

[KnifeMaker]

Sticky-Sticky-Sticky!!! Please

Knife

[K.O.]

I have been looking  to see it the stock barrel bands can be opened up to .5" for use if you use the Baker Big bore Breech and a .5" barrel..?

Was saving to Build a short shot count 40+ disco for the BBT 30g .22...  got sidetracked by the Maximus... It's 26" barrel should be a direct fit tho...

[Lmgunner13]

For those of us who are not interested in or able to do valve modifications ourselves, Bill at Magnum Airpower just released a Magnum Valve for Discovery and 22XX conversion airguns. Prices range from $30-$53, depending on the configuration that one chooses. Bill says the valve is capable of delivering 60-70 FPE in the Discovery. Here's the link to the valve: https://onlinestore.magnumairpower.com/products/magnum-valve-discovery

My goal has not been maximum power but rather power in the range of 30-35 FPE for at least 20 shots. Tim Hill of Hill Airguns suggested to me a few months ago that I do modifications to the valve but leave the transfer port alone and even use a smaller than stock transfer port sleeve. His claim was that the valve mods will increase power while the smaller transfer port sleeve will help maintain a more consistent shot string. Since he is a respected expert I took his advice. I bought a modified valve from Magnum Airpower and then experimented with port sleeves of various diameters. I ended up settling on a .062 diameter Euro flavor Maximus port sleeve that I enlarged to approximately .088. Using the Magnum Airpower Magnum Power Spring and a fill pressure of 2350 psi I am getting a high of 904 fps and a low of 868 fps over 20 shots using the JSB 18.13 pellet. 20 shots within 3.8% of ES and an average FPE of 31.8. No doubt I could fiddle more for even better results but this is very satisfactory to me at present, especially because the accuracy is excellent.

I have my disco at 28fpe and 4% for 25 shots. Fill from 2200 down to 1600 psi. at this point I think I'm done with the disco as all I use it for are nutters out to 40 yards.  for close range pesting I have the P-Rod set at 22 fpe for 24 shots and long range and larger game get hit with the Hammer! which is my AT44S 10 QE  .25 that is now tuned to a stunning 40 fpe for 18 shots with baracuda 31.02gr pellets. Every tool has its purpose.

[Lostcoast]

For those of us who are not interested in or able to do valve modifications ourselves, Bill at Magnum Airpower just released a Magnum Valve for Discovery and 22XX conversion airguns. Prices range from $30-$53, depending on the configuration that one chooses. Bill says the valve is capable of delivering 60-70 FPE in the Discovery. Here's the link to the valve: https://onlinestore.magnumairpower.com/products/magnum-valve-discovery

My goal has not been maximum power but rather power in the range of 30-35 FPE for at least 20 shots. Tim Hill of Hill Airguns suggested to me a few months ago that I do modifications to the valve but leave the transfer port alone and even use a smaller than stock transfer port sleeve. His claim was that the valve mods will increase power while the smaller transfer port sleeve will help maintain a more consistent shot string. Since he is a respected expert I took his advice. I bought a modified valve from Magnum Airpower and then experimented with port sleeves of various diameters. I ended up settling on a .062 diameter Euro flavor Maximus port sleeve that I enlarged to approximately .088. Using the Magnum Airpower Magnum Power Spring and a fill pressure of 2350 psi I am getting a high of 904 fps and a low of 868 fps over 20 shots using the JSB 18.13 pellet. 20 shots within 3.8% of ES and an average FPE of 31.8. No doubt I could fiddle more for even better results but this is very satisfactory to me at present, especially because the accuracy is excellent.

I have my disco at 28fpe and 4% for 25 shots. Fill from 2200 down to 1600 psi. at this point I think I'm done with the disco as all I use it for are nutters out to 40 yards.  for close range pesting I have the P-Rod set at 22 fpe for 24 shots and long range and larger game get hit with the Hammer! which is my AT44S 10 QE  .25 that is now tuned to a stunning 40 fpe for 18 shots with baracuda 31.02gr pellets. Every tool has its purpose.

Yea those Hatsans are nice..

I did some experimenting with other transfer port sleeves for greater power and with the stock .141 port sleeve I get 14 shots within 3% of ES and an average of 39 FPE using the JSB 18.13 pellets. I love how versatile my modified valve Disco is now through the simple changing of the transfer port sleeve - its like having multiple guns in one.

[Magnum Airpower]

 The Magnum Valve opens a lot easier at high pressure so it doesn't require anywhere near as much striker spring tension. The new Magnum .045 spring is only 1.5" long and compresses a lot smoother and easier than the older ones. The combination of the two makes for a very powerful, efficient and easy-cocking Discovery or custom build. I have a 90 cu in tank on my Disco .22 with a 26" Maximus barrel. It produces 905 FPS with 33.95 grain pellets which is about 62 FPE. I get about 100 consistent shots per fill. It's extremely accurate with JSB Monster 25.39 grain (1000+ FPS) and JSB Beast 33.95.

[Rob M]

a pic of the 90 cu in disco would be cool!

[Hoosier Daddy]

It produces 905 FPS with 33.95 grain pellets which is about 62 FPE. I get about 100 consistent shots per fill. It's extremely accurate with JSB Monster 25.39 grain (1000+ FPS) and JSB Beast 33.95.

Get the *(&^ outta here!!!
Seriously?
WOW

[dmeguy]

Bill offers some pretty awesome custom parts and has fair pricing on Crosman factory parts. https://www.magnumairpower.com/

[Hoosier Daddy]

Bill offers some pretty awesome custom parts and has fair pricing on Crosman factory parts. https://www.magnumairpower.com/

That would make me a "repeat customer" I bought one of his PA's for my 1740.
He is one "Good Joe"!

Thanks!

[dmeguy]

I ordered one of his Magnum Valves and swapped out the top half of the frankengun in my avatar. At 1900 psi, it is launching 21.14 grain baracudas  at 945 fps with a 26" barrel. That's well over 40 fpe and it didn't take a super heavy hammer spring to get there.


I really think that is where his valve will shine, is in a regulated gun. And the price is very reasonable compared to others out there. It's perfect for those of us that don't have the tools to modify a stock valve.

[TF89]

I know this thread is old and yet I have a question about boring out the Discovery valve.  I'm talking about the front half only. 

EDIT
I'm responding to my own post.  After measuring and looking at the valve I see it is not the o-ring section, it is the cutout below it that is the narrow part.  I drilled the front half of the valve to 9/32 (.2812).  This give enough of a lip for the poppet spring to fit and leaves about a .10 wall in the narrow part of the valve.  I think I will stick with that and just quit while I'm ahead. 

When I didn't have the valve in front of me I had illusions of grandeur and hollowing out the valve even more than I already had done and now I don't think it would be such a good idea. 


Here is what I was originally thinking.

"I'm thinking of drilling out the front of the valve even more and wondering how thick or rather how much wall needs to be left before danger of the valve wall collapsing?   I'm more concerned about the o-ring area and not wanting to get that too thin.  I will be stopping about 1/4" from the bottom with a tapered reamer to leave material for poppet spring and still have a smooth sloping transition.  The only reason for drilling out the hole is increase plenum space behind the de-pinger. "

By the way, this sticky is the absolute bomb for information. 

Thank you,
Dave