Disco double with BNM breech - sudden jump in FPS on some shots - why ?

[Geoff]

I shot this string with my disco double with bnm breech and shroud.  I was pretty happy with it, but thought why not try a regulator to see if it could be better.

http://i1039.photobucket.com/albums/a474/AndrewyChase/14-665-54hd2ndregtest_zps215c81d3.jpg~original

I have tried 2 regulators, the seller was most kind to send a second one for me to try in my gun and I could ship 1 back.  Great customer service right there.


I will have a shot string that goes ..
769.70   760.80   
768.40   797.30   
805.80      763.90
792.60      763.20
1800   psi   
765.80      757.10
765.10      800.90
764.50      760.60
766.10      759.20
760.40      800.60
1650   psi   


cut and paste the numbers, please read them top to bottom.  the 3rd row is another start with full psi fill and different pellets.  when it jumped twice in 8 shots I stopped.

I fill to 2kpsi .. my reg is set (according to my disco gauge) to 1100 PSI.
My plenum is 3 1/2 - 4" long which should be good as I am looking for only 22-23 FPE out of her.

I do get a small poof of air in my face when I shoot.  I replaced the oring in the TP block (I guess it would be called on the BNM breech) with one sent to me for this purpose.

Could it be my valve is having issues ?
Could it be my hammer is hanging up, though would that cause an increase in FPS ?
Could it be 2 regs in a row with an issue ?  (it came highly recommended to me by someone on GTA)
Is it possible the breathe hole is too small and too close to the other air tube ?  There is very little space between them and that is where my breathe hole is.  It almost lines up with the hole in the regulator.

I do have a boss valve that came in the gun but ES with it was too wide, so installed the OEM one the seller sent.  Since she is now regulated, maybe I should try that valve ?

The barrel is clean, crown looks very nice, I shot it with and without the shroud just to see.  I also shot with the mags and single shot tray with the same issue with spikes of ~50 fps for 1 or two shots in a 10 shot string.  A string of 40 might have 6 of these spikes in FPS, though always has 3 or 4.

I made certain the hole in the bottom of the barrel is right over the TP when I lock it down. 

I do NOT get any leaks on the sides of the breech, though do get some in my face and around the trigger when I fire.

Thought and suggestions at what I might look at ?  I have been playing with this for two weeks and have traded PM's and GREATLY appreciate their input.  I don't want to drive them crazy by asking 137 questions, so thought I would toss it out here.

Thank you for any ideas you can send my way and yes, selling it and buying an mrod has crossed my mind.  LOL

I have tried

H&N 14.66  5.53 and 5.54 head sizes
JSB 15.89
JSB 18.13
CP domes

all do the same thing with a wildly high shot or two in a string of 10.

Thanks all,

[Motorhead]

Regulation issues highly doubtful ... but easy enough to check by timing shots differently and see if regulator is creeping at set point.
* Specifics of this test am not going to go into here however.

HAMMER issues are nearly where issue lies ... Lubrication, dirt, catchy spring or a burr etc ... alters the speed of the hammer strike.
It takes VERY LITTLE changes in hammer speed / momentum to effect the shot speed.

Also when regulating because you ask for equal power at lower pressure the ballance of port sizes, spring rates of BOTH poppet and hammer need to be better matched.

For instance if you have KEPT the factory poppet spring in place the RATIO of required strike at a lower pressure within valve to energy it take to open it.  In Regulated guns generally you want a MUCH weaker poppet spring in place of factories heavy one.

On & on it's go's finding the balance to archive smooth and consistent shot strings.  keep after it, you'll figure it out   

[rsterne]

The breather hole isn't at all critical, all it does is provide an atmospheric reference in the middle of the piston where the springs are (between the two O-rings on the piston).... It is really only necessary if there is a slight leak in one of the O-rings.... ANY room for air to travel through the hole is all you need....

I agree that having the odd high velocity shot is hard to explain by having the hammer hang up.... you would more expect the odd low velocity shot.... Try shooting by only squeezing the trigger slowly to the release point, and then try yanking the trigger, and see if there is any correlation to the velocity.... If so, and if you have installed the overtravel screw mod in the trigger, you may have it set too close....

Bob

[doninva]

Just a thought as I'm a novice. I had the same thing happen two days ago. Was tuning a 1720T and the sun got over my chorny and I got a couple shots in the 1100 to 1300 range and then in the 600 range. I had to go to town. When I got back and the shade was over my shooting area, everything was back to normal. You might be fighting a chrony issue.

[Geoff]

..

I don't know how the link I posted was not the one I mean to post .. here is the pre reg shot string .. apologies for my mistake

http://i1039.photobucket.com/albums/a474/AndrewyChase/Disco07-03-14HampN5-5314-66g_zps7dd32d75.jpg

thank you for the suggestions I will look into them all.