Over sizeing transfer port

[Paul]

Hi Guys

I have fitted a larger transfer port to my 2240 (very pleased works well)
Now I'm wondering should I drill out the breech and the air cylinder a bit to allow a less restricted flow?
some what closer in size to the transfer port??

Your thoughts on this please, or if you have done it, drill sizes would be nice.

[WyoMan]

Short answer...yes, the valve, transfer and barrel ports should be the same - if you have the optimum port size for your build.
You would not want a restriction in that case.

[UKairgunner]

As im on the way to a larger transfer port( diy/bought) i wondered what is the diameter of the standard hole into the valve underneath

intuitively i would guess that its best to have every thing the same size  or getting bigger down line from valve to barrel

[CraigH]

I don't have a valve handy, my internet is down and I'm not home.        Crosman TP's are about .140 so I would think the valve is similar.  What is optimal?  The more knowledgeable will chime in.  I recall posts stating for .22 caliber should be about .156 throughout for average builds.

[Rualert]

     Here's some great information posted by Bob (Rsterne) over on GTA, and other places during one of his high power builds, o I kept notes and use this as a reference.
Per Bob: "Barrel port sizes:

The barrel port can be drilled out to 75% of the caliber, and with proper deburring shouldn't cause any loading problems, and will have enough flow for pretty much any pellet shooter.... The transfer port and valve port should be the same size, to provide unrestricted flow.... That works out to a #20 drill for a .22 cal (0.161" - you can use 5/32"), 3/16" (0.188") for a .25 cal, and 7/32" (0.219") for a .30 cal or larger, which is about the biggest hole you can use in a 5/16" OD transfer port.... A .177 cal should be left the stock size (0.134") or you may create loading problems.... I hope that makes you confident enough to try using a larger transfer port, which is pretty much necessary for .25 cal and larger builds, or if you have an aftermarket valve with an exhaust port larger than 3/16".... "

Hope this helps someone out,

Casey

P.S. I used these dimensions for my latest still on-going build. Currently on hold due to needing funds for another acquisition.

[UKairgunner]

Many thanks guys from a lazy sunny public holiday in the UK

So im going for a 4mm ( 0.161"..ishh!) standardized internal diameter transfer port and barrel opening,ill use a good top seal also whilst its in bits

im hoping that along with a hollow probe and warm co2 cartridges it should do the trick with my squirrel guests

Cheers

[quickster47 †]

Here is a handy chart for optimum transfer port size using math.

Carl

Edited for posting a better image.

[UKairgunner]

always flunked maths at school

what is MV in your data and what is it calibrated in please

sorry if its a dumb questionb

[CraigH]

MV is the muzzle velocity in feet per second.  But barrel length has to be considered also.     

Thanks for the chart, Carl.     

[Crosshairs]

I need to try this with one of my 1377s its stock but want to get a bit more juice out of it.
          Mike

[UKairgunner]

Now as I love the short 7 inch barrel and because i have to be discreet removing squirrels on our campsite a longer barrel is out of the question.I understand that using a longer barrel is the best way to use all that energy contained in the co2  but would using a heavier pellet also help" up" the energy i get from my pistol

just an idea, i'm up to speed with the mass  and its speed being what's important and im thinking that the heavier pellet will spend longer in the barrel and gain more energy from the co2

any thoughts

i shoot at 10/15M always head shots and use 14 grain at the moment but can get heavier pellets which will have a slower fps but might grab more energy from the co2,windage/curvature of the shot isnt a problem at such close range

[quickster47 †]

But barrel length has to be considered also.     

Not really in regards to port size as length is taken care of in the Maximum Velocity number.

Carl

[CraigH]

Carl,

Fascinating - have to think on that.  The chart shows a .22 caliber CO2 air gun needs around 0.162 TP to achieve 600fps MV.  Does the statement mean with a specific barrel length, or any barrel length, or ??  I must be missing something.   

Ukairgunner,

Likely more fpe at those distances.

Regarding use of heavy pellets; that means a slower acceleration of the pellet.  With firearms, heavier bullets generally means a slightly reduced powder charge to keep pressure in check (very different levels of pressure of course).  I am wondering out loud what this means with a heavier pellet and the CO2 pressure curve, especially with a heavy pellet.  If the valve operates at the same speed, then the pellet is at a different location when the valve returns to closed (regardless if it is in the barrel or not).  Seems like the volume of CO2 expended is the same, but the pressure curve would be different.  I don't think, in the end, it changes the 2240/7" barrel gas useage, but this pistol configuration may be a bit more efficient with a heavy pellet.     

Lots of    today.     

[UKairgunner]

yep CraigH thats along the lines of what i was thinking,ok its just for my particular circumstances of head shots/.22/10m range/7" barrell....and other peoples needs are different of course.  Im trying to mod because of my  needs ...resisting the urge to ......well you know!

ill get back when its sorted .im trying the common 4.1 mm gas passage first(valve/port/barrel),then its a stronger hammer spring,then its 20 grain pellets
back to work tomorrow and  hundreds of campers and dozens of squirrels

[Paul]

Looks like my post kicked up some thoughts

I drilled my Barrel transfer port out to 5/32"

As I already had a larger transfer port I just removed it and checked which drill fitted it, the closest drill I had was 5/32'' being very slightly larger than the transfer port size. I checked the barrel port it was a lot smaller so drilled it out, then I checked the Air cylinder outlet port it was very slightly tighter than the 5/32'' drill so I ran it through.

Now that means my barrel and air cylinder are just a tiny bit bigger than the transfer port, the prob already had a whole bigger than the rest so no restriction there.

Now with an 18'' barrel, 15.9 grain pellets. I get 580 FPS = a muzzle velocity of 11.38 or 11.86 depending on which calculator you use online.

I also noticed by adjusting the hammer spring tension just 2 or 3mm makes a very big difference to the FPS.

In conclusion I gained about 4/5 MPS from making the ports all about the same larger size I also changed from 14.5 grain pellets to 15.9 so I'm sorry to say none of it is conclusive but I'm happy with the results from a 2240, but i'm still looking for 600+FPS.