Breech O-ring leak on new PP700; with statement of intent

[subscriber]

Posting here on the Darkside because the Chinese airgun gate does not seem to have PP700 threads, so no owners will see it there...

I got a .177 PP700 from Krale yesterday.  Was impressed at how quickly I could hand pump it, from completely un-pressurized, to the top of the green zone on its manometer; 200 BAR.  Left it overnight to check for leaks.  The needle did not move.

After shooting two "wet" felt pellets and two dry ones I declared the very shiny bore ceremonially clean enough for lead pellets. 

I started shooting H&N 7.3 grain wadcutters because they seem to be very efficient out of many other platforms, probably due to low friction because their head size is .176".   These shot around 670 FPS for 10 shots.  Then I decide to try RWS 9.3 grain Supermags as a heavier version of the same thing.  These shot at 615 FPS. 

After about 20 shots total, I went back to the 7.3 grain H&N wadcutters to see how close the velocity would be.  Interestingly, the velocity had risen 15 FPS to 685.  So, 7.6 and 7.8 FPE for the 7.3 and 9.3 grain wadcutters, respectively.  I don't know if the velocity increase seen with the lighter pellets was due to the poppet valve breaking in, or evidence of less than perfect regulator action.  Not enough shots to allow the mechanism to settle - so time will tell.   

The pistol arrived with oil on the O-ring face of the swinging breech block.  I decided to leave it there.  My first shot was over the chrony.  I noticed what looked like oil mist spraying from the O-ring seal between the barrel breech and block sealing faces.  After the first 10 shots I placed some tissue paper over the gap and fired a shot.  As you can see, the leaking air cut a rough slot in the paper; indicating what I think is a significant leak.

I was expecting the pistol to be loud, and it is.  However, I am convinced that half the apparent report to the person firing the pistol is coming from the breech O-ring area.  Improving that should reduce noise and improve performance and efficiency.

When you push the closed breech block forward at the top edge, the apparent gap against the barrel face closes, with perhaps 10 lb of force applied.  I tried shooting it over the chrony while pushing the block forwards.  Once I figured out how to avoid having the hammer whack my thumb, a clean hammer strike actually produced a 100 FPS loss in velocity.  Either tweaking the breech block opened the gap at the bottom of the O-ring, or perhaps the kinking load somehow partially bound the valve pin.

The gauge pressure dropped from 200 Bar to 120 BAR after about 25 shots total.  Not sure what to expect here yet, but am pretty sure that efficiency should improve, if the gross breech O-ring leak can be eliminated.

So, the question is how to reduce the gap and increase the load on the O-ring?  Is there a shim washer one can replace with a thinner one to close the gap?  Is there a spring washer pushing the parts together; or how is the gap controlled beyond parts tolerances?  Will the O-ring seat accept a taller O-ring?

This experience tells me that screwing in the TP throttle screw will increase the leak; and as such is contraindicated.  As noise reduction is one of my goals with for this pistol, throttling the TP will reduce pellet velocity, but in my case, probably increase noise exposure.  Better to reduce what seems like a very energetic hammer strike. 

I figured that setting the PP700 up as an indoor 10 meter plinker, shooting at perhaps 5 FPE should reduce the report noticeably.  Certainly, my IZZY MP-46M SSP pistol shoots these same pellets at 4 FPE, and produces only a muted pop, from a barrel that is 0.5" longer.  As the PP700 is shooting 7.5 to perhaps 8 FPE, it does not seem that it would be as quiet as the 46M when detuned to 4 FPE.  Not until the breech leak is reduced by 98%.  The 46M does not have any kind of air stripper.  Just an exposed muzzle.

So; I either want a quiet indoor 10 meter plinker, or an outdoor "ratupuncture" with 10 to 12 FPE.  In the latter case, I would investigate ways to improve the shroud.  Either way, tuning strategies would be appreciated.

General comments:
Pellets load almost too easily into the breech.  As if the land diameter is on the large end of the spectrum.  Admittedly, this is with pellets having head diameters on the small end of the spectrum. .177" head diameter pellets should be "perfect".

Trigger is very heavy compared to the IZZY SSP.  More like an out of the box Weihrauch springer. 
I like the fact that there are sights; even if there is no elevation adjustment.
Overall impression; a very nice airgun for the money.  Will need a little bit of intervention to make it closer to perfect.


https://www.krale.shop/en/artemis-pp700s-a/

[subscriber]

Perhaps some of you folks with a PP700 could drape tissue paper over the closed breech to see if it fluffs up, next time you shoot your pistols?  Mine did not so much fluff up, as it turned part of the paper into confetti.

One can feel the air blast from the O-ring with your hand held at a distance.  I was not going to place it directly over the gap between breech block and barrel...

[JohnnyPDX]

Mine did fluff but no paper tearing. I will test again later today.

[nervoustrigger]

I dealt with the issue on mine by substituting the metric O-ring for the nearest fit imperial size.  The increase in cross-section from 0.059” to 0.070” worked perfectly and has held up for thousands of shots.

[subscriber]

Thanks Jason

Do you know the nominal O-ring diameter?  (specified directly or not, in my opinion both the ID and OD matter)

What material and hardness would you suggest?

[elguapo99]

When I received my PP700S/A one of the two o-rings under the breech block was totally destroyed. I removed the valve and cleaned up all of the surfaces (they were full of brown colored preservative oil). I replaced the two o-rings with Buna-n and it has been shooting perfectly ever since. It also helps to add a set screw to hold the barrel sealing block from rotating. Here's the exploded view/parts diagram.

[subscriber]

Thanks Robert.  That exploded parts diagram is sure to come in useful.

Makes me wonder if my leak might be at the lower O-ring.  Although the air blast seem radial and even with the upper green one.

[Kerndtc]

I've had leaking breech seal issues with almost every one of these I've had.

I generally pull out the old barrels and put in a better one and replace the seal with a large oring or a breech seal for a HW97/77 and machine the barrel to fit the seal.

You can play with valve tension to lessen a gap (if any) between the sliding gate and the breech face.

I like the slightly larger oring fix first. I buy most of my orings from theoringstore, very very useful site.

[subscriber]

Thanks Cameron

[JohnnyPDX]

Gentlemen,

What does the red-outlined screw on the bottom port side of the breech block do?

[nervoustrigger]

I didn’t write down the size…going from memory, if you have an assortment in the range of -013 -014 -015, go with the one whose ID is ever so slightly larger than that of the metric one it is replacing.  The goal being that the O-ring’s diameter gets compressed very slightly as you stuff it into the groove.  (If instead you use an undersized O-ring, the act of stretching it makes it more apt to jump out of the groove when the block is swung open.)
 
If you want to measure the OD of the OEM one, I expect I can tell you the size you’d want.  The exploded diagram doesn’t appear to call it out. 
 
The optimal durometer is going to be determined by the gap.  If the OEM metric O-ring almost works, a soft replacement (50 durometer) will probably be best.  If the metric one has massive blow-by arising from a large gap, you’ll probably want to use a 90 durometer.
 
BTW on occasion I’ve abraded down an O-ring’s ID or OD or thickness slightly to deal with problematic sealing issues.  That was not necessary for my particular PP700’s breech seal but it’s an option you may want to consider if you encounter a slight mismatch.

[nervoustrigger]

Johnny, that grub screw looks like it is placed to act as a stop for the rotation.  That way you can just swing it closed and not have to worry about it over rotating. 

[JohnnyPDX]

Johnny, that grub screw looks like it is placed to act as a stop for the rotation.  That way you can just swing it closed and not have to worry about it over rotating.

Ok cool. I will carefully give it a turning.

Mine over rotates slightly CCW into the close breech position

[JohnnyPDX]

It doesn't seem to affect rotation stop. It definitely goes in and out. I'll leaver her be for now.

[JohnnyPDX]

Forgot my manners.... Thank You.

[Ribbonstone]

Not comfrotable....but if you take the grips off and fire it that way,you'll pretty quickly sort out upper/lower leaking breech....without the grips/grip plates covering it up (and redirecting the air "up"to the breech), if the bottom o-ring was leaking you'll detect it.

Not sure  about the little set screw shown above....the old version doesn't have one and I just assumed it had to do with rotation.

The only ones I've checked did show at least a tiny leak. Nine included, but I'd need to put my face there to notice it (remembering at least one recent picture that showed a shooter using that kind of "on the face" hold.)

Sorry...don't have the sizes...had acouple of random o-rings from one gun or another over the years that worked in the breech seal.  Replaced once....I'll need to replace it again eventually.

Did find out that at least on mine,if I over tighten the  lower cap,it the barrel seal leaks worse....evedently, can "cock eye" the breech.

[JohnnyPDX]

I don't want to start yet another thread and we are all fans of this gun so I'm barging in. If you want to kick me out that's fine I understand. Being very new to this gun and messing around quite a bit I made a couple observations.

MOUNTING OPTICS:
1) On my PP700 SA, 177 II noticed that when using scope mounts, the forward one mounted onto the shroud, will slightly deflect the shroud when tightened up during re-assembly and installing barrel. I know this because I had the barrel adapter for the LDC removed and the shroud removed so I could re-crown the barrel. That meant I had to loosen the forward scope mount screws and I did so to remove the shroud. Upon re-assembly I thought it wise, logical, to slide on the shroud then see if the scope mount moved the shroud up or down when tightened, it moved the shroud up relative to its natural relaxed position resting on the air tube and drawn into the receiver and the position it needs to be in for the LDC adapter to align properly. In my case I will not attach optics to BOTH the shroud and receiver.

BARREL TO SHROUD RELATIONSHIP:
2) The shroud and barrel, when the barrel is screwed snugly into the receiver do not align properly. With the barrel snug into receiver the LDC adapter, in order to align with the barrel AND shroud requires that the barrel be slightly deflected upwards. The barrel points down relative to the opening of the shroud and the shroud rests on the air tube. No where for the shroud to go so the barrel gives way. To over come this I loosened the barrel approximately an 1/8 the turn CCW. Now the barrel is somewhat loose as in NOT snug to the receiver. When the LDC adapter is screwed into the barrel everything is aligned and the barrel is effectively snug to the receiver because it is being pulled out by the LDC adapter.

[nervoustrigger]

Hi Johnny, are you sure about the grub screw?  Mine doesn’t have that feature but I think you can back it out so it stands proud and it will act as a stop to prevent the block from over-rotating CCW.  For mine, I have some 0.020” thick tape that turned out to stop it perfectly vertical.  I applied it in the same location where the grub screw is located on yours.
 

[JohnnyPDX]

Hi Johnny, are you sure about the grub screw?  Mine doesn’t have that feature but I think you can back it out so it stands proud and it will act as a stop to prevent the block from over-rotating CCW.  For mine, I have some 0.020” thick tape that turned out to stop it perfectly vertical.  I applied it in the same location where the grub screw is located on yours.

Not 100% sure. I backed it out a couple turns to see if it would actually allow for over-rotation and it did not.

Maybe I'm being clueless about it. Mine is well-positioned and only overhangs on port side by .015".

It does SOMETHING.

[nervoustrigger]

Regarding the scope rail, in my case I didn’t detect any misalignment issues however the 2pc construction made me leery about possible sensitivity to POI shift if I bumped it into something.  I decided to install a 1-piece mount bridging the two pieces of the rail to lock everything together.  I have not had any POI shift issues after a lot of handling so that seems to have been effective.