Best mounting method for aftermarket barrel for PP700?

[darkcharisma]

you can buy a mini lathe for less than $300 from ebay. it would pay for itself after a couple of parts.

mounting where the pressure gauge is perfect. you can make a plenum bigger to fill the gap between the gauge and trigger guard

[freddich]

Yeah that was the idea! Unfortunately less than 300 tends to turn into ~$650 when not in the US. In any case, I'll see if someone I know has access to one for now!

[freddich]

well I'm now the proud owner of a 7 by 14 inch mini lathe for these kinds of projects... More to come!

[freddich]

I'm currently in the process of making the extended plenum, and I have to say the lathe is working better than I expected. This is 7075-T6 aluminum, so pretty hard stuff as far as aluminum goes and it works really well. (I haven't even tightened, lubed or adjusted anything from factory yet, which will no doubt improve it further.)

I haven't bored out the inside of the plenum yet as I'm not sure how thick the walls should be. It will only ever hold 150 bar, but it needs to not explode instantly if it's exposed to 230, if the reg fails.

Obviously it needs a cap, on which I think I'll put the pressure gauge.

[darkcharisma]

what is the OD of that plenum? and what is the thread size of that plenum?

you can google “pipe burst calculator” and use engeneer’s edge website to calculate the thickness of your plenums. use 4 safety factor.

[freddich]

The outside diameter is 28mm (1.1 inches). and after some calculations, the wall thickness will be 2.5mm (0.1 inches), which translates to a safety factor of 4 with a working pressure of 2500psi. In other words, still plenty of room if it's ever exposed to 230bar.

The thread is 1/8 bsp, with a 5mm hole drilled in it.

[darkcharisma]

seems like the 1/8bsp threads will have 8.7mm-5mm through hole will give ya 1.75+mm wall. plenty good for 4500psi working pressure.

My pp700 25 cal set up has less than 10cc plenum and doing 835fps with 34grains. you may not need alot of plenum thus you can probably machine a thread for a burst disk on the bottom for another layer of safety. gauge mounts to the right side of breech. if you so desire.

[freddich]

This plenum, plus the already existing plenum volume will give me slightly more than 20cc in total. Should be more than enough, especially given the smaller caliber.

Can't wait to try it!

[freddich]

Well it looks like I'll have to wait for some time for the tailstock drill chuck to arrive, so I can start making the hole in the plenum. In the meantime, I'm making the cap for it which is, so far, turning out well.

After this mod, is there any other performance mod even left to do? I'm considering making a new valve from scratch if that will help performance. The goal would be making one that takes a lot less pressure to open at a given pressure, so I can use a lighter hammer spring (and thereby also get a lighter trigger pull). Since this is somewhat beyond my skillset, has anyone made such a valve I could get inspiration from?


please don't bring up how much off center the lathe tool is, I'm waiting to get a quick change tool post and will adjust the height properly then.

[darkcharisma]

before my quick change tool post arrived. i shim to center. may even save you $100 there by shimming up.

as far as the valves, the smaller the valve poppet OD, the easier it is to crack open. you may have to experiment a little bit depending on your caliber. i have made one valve with larger ID from 416 SS. works as intended

my 25 cal gets a 7mm valve seat and 8mm poppet. you can try 6mm seat and 7mm poppet.

[freddich]

Plenum is done. To be honest, it didn't really make any notable difference to performance at all, so that's a bit disappointing I guess. The caliber might be too small, or the barrel too short to use all that volume.

In either case, I'm sat here thinking what project I'd like to tackle next.. Building an airgun mostly from scratch sounds pretty fun...

[Rob M]

Plenum is done. To be honest, it didn't really make any notable difference to performance at all, so that's a bit disappointing I guess. The caliber might be too small, or the barrel too short to use all that volume.

In either case, I'm sat here thinking what project I'd like to tackle next.. Building an airgun mostly from scratch sounds pretty fun...

the reg pressure might be the limiting factor..

[Rallyshark]

Plenum is done. To be honest, it didn't really make any notable difference to performance at all, so that's a bit disappointing I guess. The caliber might be too small, or the barrel too short to use all that volume.

In either case, I'm sat here thinking what project I'd like to tackle next.. Building an airgun mostly from scratch sounds pretty fun...

the reg pressure might be the limiting factor..

It could be reg pressure, hammer spring's ability to crack the valve as well.  The other thing that jumps out to me is the location of the plenum.  I'm assuming the pass through to the valve was drilled out where that plenum is mounted?  Otherwise, there's just a tiny hole there for the air to get through, which would negate all that extra plenum. 

[darkcharisma]

I think a bigger valve and stronger spring will help increase the FPE output. Mines 45Fpe with 34NSA

[freddich]

the reg pressure might be the limiting factor..

I would think so too, I need to make a heavier hammer if I want to run higher pressures though, the hammer springs (I have dual springs) are about as tight as they can be. In fact they're already a bit tighter than I think they should be, and it results in a heavier trigger than I'd prefer.

I'd like to make some sort of balanced valve, but fitting one in this form factor would be VERY hard.

It could be reg pressure, hammer spring's ability to crack the valve as well.  The other thing that jumps out to me is the location of the plenum.  I'm assuming the pass through to the valve was drilled out where that plenum is mounted?  Otherwise, there's just a tiny hole there for the air to get through, which would negate all that extra plenum. 

Yes, that tiny hole is drilled out to 6mm. I also made a 6mm hole through original reg body so there's a straight line up from the plenum to the valve.

[MJP]

Nice looking stuff, very interesting little gun, but I need to add one important fact.
All of the parts in your system need to be designed to withstand the full fill pressure with adequate safety margin for that pressure.

One part with lower pressure design is very dangerous if a reg fails!
Either use fill pressure in you design or a burst disk that is rated properly.

Marko

[freddich]

Yes of course, I think I even mentioned that earlier in the thread. The only parts that are changed in a relevant way is the screw that attaches the reservoir to the gun and the plenum, both of which will take way more than the fill pressure.

[freddich]

Now that the carbon barrel shroud is completed, I can proudly say this project is finished! And I must say it turned out really well!

Now on to the next project: I've decided to buy an Edgun Leshiy (gen 1). Originally the price was a bit steep for me, but I just realized Edgun makes really cheap spare parts for their guns, so I can get a complete action plus the valve and hinge assembly for a total of about $650 instead of the whole gun for $1350. The things that need to be done at that point for a complete "franken-leshiy" would be:

1: machine one of the barrel blanks I have at home to fit.
2: Buy a Huma reg/plenum combo for another $90
3: Make an air tank and stock for it, I really want to make a Leshiy 2 like stock, for more air capacity.

And boom, Edgun Leshiy for about $800, plus a fun project for me to do!

Will be making a separate thread about this once I get the parts!

[sb327]

I’m in the process of building a Leshiyesque contraption. I had started the build a month ago and kept looking at the Leshiy2 for design features. Looked at it so much I just bought one.

My build will be single shot though.

Anxious to see your build!

Dave