XS60C PCP Repeater project

[stalwart]

Set the length of the bolt so it barely pushes the bullet clear of the front of the port, and retracts just enough to clear the port. You don't want extra plenum there.

It will then clear the mag nicely when cocking.

[RHytonen]

Set the length of the bolt so it barely pushes the bullet clear of the front of the port, and retracts just enough to clear the port. You don't want extra plenum there.

It will then clear the mag nicely when cocking.

I was hoping I could assume that - thanks!

I realize I'm in danger of hijacking your thread, so I just truncated this post,
and am relocating the extended discussion to the "FDPCP windowsill cannon" thread where it belongs.

Please do look at it when you have a chance.
ALL comments (especially yours and Bob's, as its heart is a Grizzly) are gratefully appreciated.

[RHytonen]

Eric-
Did I see somewhere back, that you cut off the front of the receivers (starting with the "Bobcat")
I assume that was to get rid of the threaded portion for some reason.

-Do you still do that on the Grizzly?

(Save me some - I'll buy them off you, to use to make the many pieced barrel bands I'll be needing!)

[QVTom]

The N-forcer has an M6 fill port. Aren't there "probe" style fillers available that fit these? If so, where would I find one? I like the idea of needing no wrenches to top these off.

Automation Die & Mold, the N-forcer folks, sell a fill adapter as an accessory.  I made my own and have a few extras, you can have one if you wish.  I think they are 1/4 npt.

You can see the fitting in this picture.


Tom

[stalwart]

Thanks, man!

 

[stalwart]

Eric-
Did I see somewhere back, that you cut off the front of the receivers (starting with the "Bobcat")
I assume that was to get rid of the threaded portion for some reason.

-Do you still do that on the Grizzly?

(Save me some - I'll buy them off you, to use to make the many pieced barrel bands I'll be needing!)

LOL... I'm saving them... I'll start a box for you.

The chopping is just for lightening and is done only to 3K guns.

Thanks for the detailing here! I am following your thread, pondering right along with you.

[stalwart]

Have I mentioned yet, today, how much I love GTA?

USPS is bringing me something I have been trying to find for a very long time... the only thing left between me and building the 60 I have always wanted.

The generosity of the members here is not news to many, but I'm running into some of the most unselfish people I have ever met.

Prepare for a pleasant surprise, and some gratitude from me that borders on mushy.

[RHytonen]

Have I mentioned yet, today, how much I love GTA?

The generosity of the members here is not news to many, but I'm running into some of the most unselfish people I have ever met.

and you should include yourself in that

[stalwart]

Rod, given that I'm getting badly needed support from guys so far above my pay grade they will never need a d**n thing from me... means that paying it forward is my only option.

I do it gratefully. I hope that shows...

[rsterne]

I got a chance today to make a limited travel spring guide for the Grizzly.... It looks like this....



The 7/32" OD spring guide is threaded both ends to 10-32.... On the front, the single nut is loctited and peened in place and turned down to 0.36" OD (same as the spring) to be a loose fit in the 3/8" hole in the hammer, and tapered to match the bottom of the drilled hole.... The space between that and the rear nuts is the length of the spring plus RVA screw, minus just over 1/2", which results in 7 lbs. of preload in the spring as installed in the photo above.... The two nuts at the back are installed against the shoulder on the rod and locked together.... When they hit the back of the adjusting screw, they stop the guide in its tracks, leaving the hammer to fly forward on its own to open the valve.... I had to drill the hole in the stock deeper to miss the nuts when the gun is cocked.... Moving the RVA screw forward reduces the space between the front of the guide and the hammer, which results in less distance for the hammer to coast before hitting the valve, and also results in compressing the spring further when the gun is cocked, storing more energy in the spring.... Therefore the RVA still works as intended....

The original plan was to use a 3" long, 11 lb/in. spring, with 5 lbs. preload and 15 lbs. force when cocked, but to get that to work I had to run it right up until it touched the hammer.... I ended up with a 13 lb/in. spring, with 7 lbs. of preload.... I think I needed more force because the guide is no longer part of the hammer weight, reducing the weight available to open the valve from 102 gr. to 84 gr.... The end of the guide misses the hammer by 0.12", meaning that the 1" cocking stroke compresses the spring an additional 0.88", giving about 18 lbs. force when cocked.... This is when adjusted for 971 fps average (94 FPE) with the 44.8 gr. JSB, the same as I had previously.... Although the maximum cocking force is only 3 lbs. less than with the previous short, stiff spring (2.5" long with a 24 lb/in. rate), the gun is much nicer to cock, with no tendency to not complete the cocking stroke.... I think this is because the force on the bolt handle is much more constant (7-18 lbs.) instead of increasing so rapidly as you pull the bolt back (0-21 lbs.).... With the sudden increase of force at the end of the bolt travel previously, you had a tendency to not complete cocking the gun, which could lead to double loading pellets....

So, how did it work?.... did it make the gun any more efficient?.... Yes, it did, and there is no longer any audible hammer bounce.... When I first assembled and tested the gun, I got 17 shots with the 44.8 gr. JSBs at just under 94 FPE.... However, with use and wear, the gun has developed an audible hammer bounce, and in addition a leak between the barrel and receiver at the transfer port, and was down to 13-14 shots per fill.... It still leaks, but you can no longer hear any hammer bounce (a very obvious difference in the sound), and the shot count is up to 15 shots before any velocity loss.... It is only down 10 fps at 17 shots, by which time it is down to 2000 psi (the setpoint is 2200).... Basically, by changing from a short, heavy spring (which gave me the previous best efficiency) to a lighter, longer spring with preload and a positive stop that holds it 1/8" clear of the hammer when uncocked, I gained at least a shot or two, putting me back over 2 complete magazines, even with the present leak.... I can say with confidence that using a preloaded spring that is not sitting against the hammer can reduce or eliminate hammer bounce.... We now have one more arrow in our quiver for tuning PCPs....

Bob

[stalwart]

Nicely done. Are you going to try it on your other beasts?

[stalwart]

I asked about rebuild kits for the MF-19s, for the guys who plan to do over 2 million cycles... here's the reply:

"HI Eric,
 The price for MF19 repair kit is $75.00 each.

Thanks,
Kristen".

  For a $45 part? Maybe Kristen hand delivers it? Is she really cute? Might be worth it...

[rsterne]

As time allows, and the gun can utilize it.... It will be a balance between room for the spring and hammer travel, and the necessary hammer strike.... Some guns may not have the room for a longer lighter spring with enough force.... I may seriously think about making future designs to allow for it, I really like the feel of the way it cocks.... much smoother, and feels a lot easier, even though it's only about 10% less peak force.... Any guns I have where I can hear a B-r-a-a-p will get the first attention....

Bob

[QVTom]

I asked about rebuild kits for the MF-19s, for the guys who plan to do over 2 million cycles... here's the reply:

"HI Eric,
 The price for MF19 repair kit is $75.00 each.

Thanks,
Kristen".

  For a $45 part? Maybe Kristen hand delivers it? Is she really cute? Might be worth it...

Eric, you might give some consideration to making your own ram for your application.  First, the operating pressure for your application is considerably less than industrial gas spring applications.  Second, you could uses the largest diameter casing with  a smaller piston diameter allowing a higher fill pressure and a more constant force.  The N-Forcer uses a lip seal but I bet you could get away with an o-ring.

If you haven't seen this thread........ http://www.gatewaytoairguns.org/GTA/index.php?topic=48062.msg452463#msg452463  it will give you an idea of what's involved.

Tom

[stalwart]

Bob, I strongly suspect the next gun to burp at you will get it's guts ripped out, in short order. Please LMK when it happens, and the result? Sure is a clean setup.

Tom, excellent thread! I always thought there would be a check valve in there, too. Clever, elegant system.

An in-house ram is a really good idea. The case is already there, in a thru-tube set up... and, it could be made to be externally adjustable at just about any point on the gun, with a little tubing.

I've been looking for the smallest Foster nipple made, for a P-17 MSP project, and got this. It could be stuck anywhere on a 60.

Is it Wednesday yet?

[stalwart]

Guys, Tom just lit a fire under me.

Think about it... the ram would use the same o-rings, at the ends, as the valve does. No special seals... I could toss in a few rebuilds worth at shipping. The additional weight would be a significant fraction of the N-Forcer's. Any changes needed in the forces, while proto-ing, can be made right here...

Although $45 is cheap compared to my time, a production item could be turned out here at a competitive price, given enough quantity. There would be little, if any, cost penalty to the customer.

I gave some thought to the possibility that someone would hook a tank to the wrong Foster, and blow the thing up. I decided that rather than make the GRS 4.5K capable (everyone pays), if I made the rear cap take twice the pressure the front can, the front would shear its retaining screw... and the pressure would vent through the trigger and bolt slots (only the perp pays... a little).

I think I'll pass on my usual geezer nap today.... homework to do.

[stalwart]

A good pressure release could be made from a Foster coupler, with a thumbscrew and vent hole, no?

[QVTom]

What about a vented set screw?  Most fosters have a poppet with a stem that may be able to be pressed in far enough to release some air.


The N-Forcer's valve look like this.  The valve has been moved to piston from the end of the casing.  Parts are still factory N-Forcer.

[stalwart]

The Ninja fosters I stock have a valve stem that goes almost flush to the top of the nipple. A coupler with a standard plug installed that is tapped for said set screw (cheapest by far) or thumbscrew... Done.

NEXT!

 

Any gut feelings about what kind of pressure would give us the best performance, so I can (try to) figure out piston sizes? This is ALL new to me.

[stalwart]

Just a teaser for the QB7x guys here... I love mine, too, and as soon as I can get this design working, I'll build a GRS setup, in an HPA tube for them.

I have a good feeling about this... we shall see.