FAS 6004 Project

[WyoMan]

Here’s the gun:


It became a project when I finally ran it thru the chrony - a disappointing 301 fps with 8.44 gr pellets. I knew it was going to shoot less than advertised... especially at 7000’ elevation… but this was way less than the 400 fps. Got me thinking there might be something wrong...

Started the disassembly process and wasn’t expecting this... the end cap is threaded into the cylinder - nice:


Pulled the piston out and found this:


The factory lube appears to be a mixture of oxidized aluminum with dirt in some kind of media. But I really don’t mind stripping a gun for cleaning, even if it’s new.

Cleaned the piston, felt wick, cylinder and flushed the valve. Then I added a modest shim (.015”) to the piston because the compression seemed a little low.


While I had things apart, I cut a recess (leade) in the barrel breech:

The barrel had one, sort of, but it was pretty sad.

All of the o-rings looked fine and I saw no other problems so I reassembled. It’s now shooting jsb 8.44 gr at 326 fps.


I did three different things without testing in between, so I’m not sure what helped the most. But it’s a promising start...
Up next, sight aid. I want to see how well this gun can shoot. The velocity ES is now under five. The lock with the rotational hammer is fast. The barrel looks to be well machined... Thanks-

Wyo

[WyoMan]

Finished the sight aid...

Started with some flat dovetail stock, cut off a piece 4-3/8” long, made some clearance cuts, and then drilled for #8 close fit.
Shown below, it’s ready for the counter-bore:


Counter-bored at .250”:


Top view of what I'm trying to fit:


Lever was drilled and tapped 8-36:



Not much room for screws on the inside – these extend about one thread beyond flush:


Feels pretty solid with this compact scope:



Hopefully it shoots well (there’s very little info on this gun) -

Wyo

[a1rgunz]

That's a good looking one now! Hope she shoots as good as she looks.

[GP2004]

That looks really nice!

[WyoMan]

Thanks, guys. It's been a while since I shot a pistol like a pistol, LOL. But the gun seems very capable.

Five, 10-shot groups from 10m:


I was using a two-hand hold, arms extended, and hands resting on a bag. Need to practice some more, but no problem because this is a fun pistol to shoot.
It does have less power than a P-17 (mine anyway), but it's way easier to cock (pump).
The trigger is light, crisp and very precise. I'm not a big fan of 2-stage triggers in pistols but this one makes me a believer.

Wyo

[TF89]

Very nice, great groups.

[Benny]

I really like the scope mount that you made for your pistol and it's good to see that you got the velocity up a bit. The JSB 8.4gr pellets are heavier than the match pistol pellets usually used in this type of pistol. Most match pistol pellets are 7-7.5 gr so your pistol probably wasn't as far under spec as you may have thought.  

[clarky]

Wyoman, you have made an error with the piston packing mod but i did exactly the same as you.
I got virtually nothing from a 0.2mm shim so i experimented with a different approach.
The key, is the area directly infront of the transfer port hole needs some clearance, where part of this lost volume is part of the required compression....which is not quite as one might assume...i.e...zero head space must equal max pressure.....not so.
Its apparent to me that your improved breech leur has provided your 20 fps then.

Another gunner on the crosman forum similarly modded his HW75 with such a shim, but also went with a 10 inch barrel increase but only got 380 fps for all the effort.
I similarly modded a HW75 barrel length only but got 420 fps.....suggesting his compression mod may have robbed him of some of his barrel improvement..

So heres what to do....
You can reduce volume but you need to retain a tiny resevoir....to do this put an O ring of the same size as that fitted in your piston groove down inside the bottom of the piston.....this will reduce volume but leave a small resevoir around the transfer port...i got 35 fps alone from the O ring compression mod.
Incidently a similar O ring mod to a P17 over on the Crosman forum gives 30 fps.....its the way to go.

[clarky]

....a further mod which worked for me is replacing the barrel. Its very heavily choked which probably does impart good accuracy but re machining up a stock Crosman 1377 barrel gave me 22 fps ....and no noticeable loss of accuracy.....perhaps 1 to 2mm increase in group size possibly owing to not liking the same pellets but a lot less pellet sensitive than the FAS Barrel due to that tight bore..
Hobby pellets ( tight skirt pellets) providing the best example of this, with them barely firing through the FAS barrel at 280 fps but giving almost 400 fps through the crosman barrel with the O ring mod.....Old English Wasp pellets at 4.6mm do not even fire through a stock FAS barrel incidently.

Its a shame this beautiful FAS didnt have a bit more oomph as stock as its a bit of a pain to have to mess with such a nice pistol but it just dont cut nice clean cards without the work.
I so not think there is anything more to do beyond this as the valve is a sealed unit.

[WyoMan]

Hi Steve - An error you say? ... Well, it wouldn’t be the first time, but thankfully it wasn’t this time.

I did make the egregious deadhead error a few years ago.
Fessed-up to it here:
http://www.network54.com/Forum/79537/thread/1448652986

Some more info about avoiding trapped air:
http://www.network54.com/Forum/79537/thread/1499997122
Perhaps there are several ways to skin this cat? And it may depend upon the amount headspace in each individual gun.
The 17% increase (fpe) I experienced here was pretty consistent with similar efforts in the past - a P17 and HW75.

Was it me on the Crosman forum with the HW75? (sorry about the lost pics). I got 380 fps with the longer, but choked barrel… and at 7000’ elevation.
http://www.network54.com/Forum/275684/thread/1408856593
I subsequently increased that shim to .020”. It was done to restore the lost performance that occurred sometime later. I think there is some initial, fairly rapid break-in where headspace can open up.

The different piston shims…

FAS 6004 (.015”)


HW75 (.020”)

P17 (.025”)


I’ve also got a longer barrel build in the works (for another FAS 6004). Plan to use an 11” un-choked section from an LW blank.
My 6004 oem barrel doesn’t have a choke – and I’m not sure I’d want one. You’re right about the energy loss from the choke. With an LW choke I measured a consistent 7 lb force over 1-1/8”… that’s 0.7 fpe… significant with a gun that only does 3 fpe at best.

Thanks for ideas, etc… you guys from across the pond do good work -

Wyo

[clarky]

You have done some nice work too. Sorry buddy i hadnt realised it was you with that 75 mod.
It was some lovely work and we need more people like us performing these experimental ideas.

My own findings is that if one bases their piston packing on closing in the diameter around the transfer port, instead of lengthening the piston.
An ideal set up of this would be a tiny resevoir of say 1/4 " in diameter around the transfer port but stock lenght to the front of the piston...creating a tiny initial resevoir....maybe in rubber sheet instead of an O ring which would also act as a seal on the bulkhead wall.??
Incidently i did do a clever barrel increase mod for a friend using the old sight extension unit.
I simply drilled it out to take a section of barrel.....got 23 fps from it.
Using the smallest diameter pellets being the most important consideration for the FAS tight barrel, my O ring mod piston gave me 390 fps with H&N Excite or Falcon 4.49 pellets.....

I wish there was more we could do.
Have you tried tackling the excess of angle on the valve seal. Its got to be restricting flow.
What is the highest performance you have achieved?

[clarky]

there appears to be great inconsistancy with HW75 velocity down the years.
Ive had as high as 430 fps from a completely stock pistol from the first year of its introduction with decent weight pellets too. This was also realised by Todd Cooper (424fps) from an early version he had.
Some disappointingly low as 330 fps mid production period until recently . ...
THe very latest specimens all seem to be hitting 400 fps.
Ive never been able to quite figure it out.
I wonder if they slightly shifted the air inlet port hole to aid cocking ...mid production pistols did seem very easy to cock ....then recently put it back due to complaints of low power.....even 1mm of shift would do it but easily gone unoticed by users down the years..
Cocking a new one recently i did encounter a slightly stronger cocking force and the thing giving 402fps with Hobby pellets.
I just could not tell by way of visual check if that inlet hole the reason......
What do you think

[dan_house]

Sweet! like an all metal p17 only better....

gotta ask, where did you get it?

[Hoosier Daddy]

Not being a smart alick... but all that only yielded you another 26 fps?
While it was nice to clean up the inards, Do you think it was value added for group improvement... or was your mounting the scope that made the difference in accuracy?


dan_house Only place I have found here in the states is Airguns of Arizona. http://www.airgunsofarizona.com/fas/
But I would LOVE other options.

[WyoMan]

Getting back to this, I'm a little late..was going to update this when I finished the new FAS 6004 (longer barrel) project. That's going to be awhile. Still don't have the gun.
This 6004 was from Air Gun Depot. They were out-of-stock when I ordered and it took two months. The next 6004 will be from AoA. They were out-of-stock when I ordered and it's only been three weeks so far...

They're the only two places I know of... and they're mostly out-of-stock.

Scott, it wasn't much work - about 4 hours total. Can't say the accuracy improved because I'm blind without sight aid. But the holes in the paper are cleaner. The 26 fps gain is only meaningful when compared to the initial velocity... it's a 17% increase in fpe. Yes, 1.17 times nothing still looks like nothing 

I did modify the shim (following Steve in NC's work) to hopefully get more gain. I increased the shim to .018" (total measured dead space is .022") and ported the shim to evacuate the air on the piston side...

Ready for JB Weld:


Finished:


Results...


I think that's all I can get up here (7000' elev.) with the oem barrel.

Steve, the HW75, like most SSP's, is under powered and can have dramatic changes from small things... e.g. >>
Bad Piston Seal - lack of compression
Too much Headspace - lack of compression
Valve and Porting flow restriction - debris, grease, etc. in the works
Breech Seal damage or large gap - loss of firing charge, blow-by
Poor Leade geometry - too much engraving stiction or blow-by
Bad pellet fit to the bore - blow-by or stiction... etc.

Wyo

[anti-squirrel]

FWIW, I believe this "Gamo" is a very similar gun, and notably less expensive: linky.  Shipping from Aceros ain't too bad.

Watcha bet it has a BSA barrel?  Prolly designed for 5M or 10M comp with a speed of 400fps

[WyoMan]

Thanks, Peter .. another one that I have to put on the list.   

Wyo

[dan_house]

Right after I asked Wyo where he got it, i ran across them on line.....

nice guns, tempted........

[WyoMan]

Hey Dan... what I like best is the ergonomics of the grip and trigger.. it's a perfect fit to my hand. And also the trigger pull - one of the best for a crisp, predictable and precise pull. It's shooting fast enough now for clean paper holes, so I can't say anything bad.

The next one on order will get a longer barrel, which will require a bigger scope   

Wyo

[clarky]

Wyo buddy ...u maybe interested in my final mad on this gun for one of my shooting compardres..
I machined an O ring groove into the front face of the piston ...no shim....just a std O ring groove with slightly radiused tool with no rolling clearance ...as its not a moving seal and so the O ring fits with a shove fit....I used a thick 14mm Id ring ....which sits proud approx 1/16 (2mm) on the front face....i can actually feel it touch down on closure of the pump stroke ....and i believe its sealing on the front face around the transfer port hole....
I cannot compare with you because of your elevation issues but like for like against my last mod with a 1mm shim, im getting 15 fps more....this one running 385 fps with Finale pellets ....im on coastline elevation...a few feet i should think