13xx accuracy issues

[Bill in SC]

Hello folks. My new to me 13xx is giving me some accuracy issues. When I first got it the breech was kinda loose, resulting in scope movement. I tightened it all down and it still won't group too good. I was just wondering from you folks who have dealt with these airguns, what are some of the things to look for that could create accuracy problems. It has a 24 inch barrel with LDC, flat top piston and flat top valve, and extended probe. Everything is tight and working smoothly but it won't group. Seems to hold air fine. Perplexing. Was thinking of maybe installing a 10 1/2 inch or 14 1/2 inch barrel. A friend shot it a day or two ago and he said he could see the pellet in flight and one shot went out a ways and took a hard right. Wondering if the LDC has anything to do with it. Thansk in advance for any tips. BTW: I did look at a youtube video of a 13xx dis-assembly and it will be real easy to work on. Just need to know WHAT to work on. One more thing. I "think" I read somewhere that there is supposed to be some sort of buffer between the barrel and the breech/air tube. Seems like I read that the barrel flexes a tad during discharge and the buffer prevents barrel movement. My gun is open in that area.

Bill in SC

[Hunter2011]

First thing I do when I experience accuracy issues is to remove the LDC. Sometimes the pellet can clip it. If not that, I mount the gun in a vise-grip. Then you know it can't be you. I then shoot a group with it. This way will quickly tell you if something is wrong. If it groups, then unfortunately the problem can only be the scope or yourself.
   

[K.O.]

One of the first things I would check is for clipping at the LDC.

when I first got my PC77 it was having accuracy issues.

Mine had concentric machining marks causing roughness at the lead in,

it also had some small burrs at the edge of the port,

Both probably caused skirt deformation.

After cleanup  with stock peep sight I was getting 3/4"  groups at 20 yards,

then the roll pin started migrating and it went back to shotgunning,

replaced with a good roll pin from Davio here and its back to good form.

Also make sure crown is in good shape.

[K.O.]

one other thought I also had was that if the valve volume is still stock the long long barrel actually slows the pellet down.

[Bill in SC]

one other thought I also had was that if the valve volume is still stock the long long barrel actually slows the pellet down.

I was definitely thinkin' that the longer barrel would create more friction and slow the pellet down. I had also thought about the LDC clipping the pellet. It is held on by two tiny allen set screws and I have lost my tiny allen wrench. I'll get one tomorrow and start looking in that area. I know it's not me as there is NO recoil, and with my follow through, the cross hairs never leave the target. Just to see if it was me, I broke out my Diana 34 and promptly shot a one ragged hole five shot group at 10 meters with open sights. The scope is also questionable as it is a VERY CHEAP NC Star. Anyhow, thanks for the tips and I will get on with eliminating all of the above mentioned, one by one. It did shoot pretty tight after I first snugged the breech down, but has now gone haywire. We'll get 'er right though. Thanks again for ALL the replies!

Bill in SC

[K.O.]

There is a very good  3 part thread in the tech section called building a pumper power plant,  part of it  boils down to optimum valve volume being about 1/3 barrel volume (I think, I would have to check).

[K.O.]

If I remember right the formula for enclosed volume of a cylinder

pi * r² * h

r is radius   (.0885 for .177 &  .11 for .22)
h is length of barrel.

[WyoMan]

Hi K.O.,
formula is fine but I need to clarify the efficiency charts for valve volume vs. barrel volume
The charts compare the input energy (total pumping effort) vs output energy (muzzle energy) for the different barrel lengths and valve volumes....but they don't really tell everything thats important:
problem #1, the max efficiency doesn't mean max muzzle energy...a longer barrel will almost always yield greater ME
problem #2, the max efficiency doesn't mean fewer pump strokes to get the same ME...it measures total pump effort which is # of pumps times the pump difficulty...most folks don't care if the pump stroke is a little easier or not they care about how many times you have to pump!

Concerns about pellet friction and barrel length...here's an example:
using 4 oz of dynamic friction in a 24" .177 barrel and stock valve volume of 0.1 ci ...we need 25 psia at the end of the barrel to offset the 4 oz friction force...that means we need 300 psi in the valve or about 3 pump strokes and we'll still be ok 

Here's some of the source info:
http://www.network54.com/Forum/275684/thread/1123124381/Steve%2C+I+just+logged+on%2C+have+been+working+on++%28see+pretty+plot%21%29

If I go much deeper I'll get confused unless I'm having a really good day  so don't chop that barrel Bill

[Trophyhunter49]

Hi K.O.,
formula is fine but I need to clarify the efficiency charts for valve volume vs. barrel volume
The charts compare the input energy (total pumping effort) vs output energy (muzzle energy) for the different barrel lengths and valve volumes....but they don't really tell everything thats important:
problem #1, the max efficiency doesn't mean max muzzle energy...a longer barrel will almost always yield greater ME
problem #2, the max efficiency doesn't mean fewer pump strokes to get the same ME...it measures total pump effort which is # of pumps times the pump difficulty...most folks don't care if the pump stroke is a little easier or not they care about how many times you have to pump!

Concerns about pellet friction and barrel length...here's an example:
using 4 oz of dynamic friction in a 24" .177 barrel and stock valve volume of 0.1 ci ...we need 25 psia at the end of the barrel to offset the 4 oz friction force...that means we need 300 psi in the valve or about 3 pump strokes and we'll still be ok 

Here's some of the source info:
http://www.network54.com/Forum/275684/thread/1123124381/Steve%2C+I+just+logged+on%2C+have+been+working+on++%28see+pretty+plot%21%29

If I go much deeper I'll get confused unless I'm having a really good day  so don't chop that barrel Bill

???All I know all this went WAY over my head LOL

[K.O.]

Ah I see and would like to hear your thoughts and another thread but basically I mistook it for a general way to balance valve volume  to barrel length.

I would better describe what I mean but developed a migraine late this afternoon and am thinking less clearly than usual...

My oldest daughter is a Jr. on her way to a B.A. in Math and she just passed me last year.

I developed some concentration/memory probs about 5-6 years ago so I may not follow as well  and my last class was in the early 90's but can usually follow...

But honestly it does not even need the math knowing from experience usually does the trick...

[Bill in SC]

I won't chop this barrel, but still may buy a shorter one. The 24 is skinny, long, and awkward.

BB

[DBOdude]

one other thought I also had was that if the valve volume is still stock the long long barrel actually slows the pellet down.

going by this (and other similar results/threads i've read online) it seems even a stock 1322 will keep gaining FPS/FPE over the stock 10 inch barrel even with a 24 inch barrel .

http://www.network54.com/Forum/275684/thread/1318040664/1377+to+1322+upgrade,+my+results.++Need+stronger+hammer+spring-

[K.O.]

Boyle's Law   (a little simplified but for our purpose sufficient, rather than ideal gas eq)

For a fixed mass of gas at constant temperature, the volume is inversely proportional to the pressure.

so I did the simple math which proved my gut wrong,

I felt the .1 C.I. volume  would peter out at about 18"-20" of barrel.

I believe the Figures of  1000-1200 psi are close for 10-12 pumps

so @ 24" there would be 170-200 psi left     (@ 100% efficiency)

So give away an overestimated 100 psi to inefficiencies  and yep the pellet still gets a bit of a push with a 24" barrel.

[WyoMan]

Very good K.O. 
Boyle's Law gets you close enough...expansion is adiabatic so you need the specific heat adustment

[K.O.]

the simple math ( I still have a bit of a headache so may have it wrong so should show how I got there)

Pi * r²* length

boils down to

.02459 * length    (for .177)

add  .1 to that (for valve volume) which gives volume at given inches

the original volume of  .1 cubic inches is convenient because it makes it simple to compare

the volume @ 24" is .690 it is  6.9 times original volume


Boyle's Law

means I simply do  1200/6.9 and 1000/6.9

Hope I have it right