Reduce transfer port on Diana airrifle

[garmic]

I am looking forward to getting my first Diana air rifle, a Diana 280 as a matter of fact. So in my excitement I have been reading every tuning guide I can find, unfortunately I sometimes forget to book mark them. One train of thought I have been following is that transfer ports for Diana springers are over sized in diameter resulting in a loss of power by not letting enough pressure to build before the pellet begins to move also that they are in efficient because they are at an angle and that makes them longer than necessary which ads to the volume of the compression chamber also reducing power.
Some tunes have involved plugging and re-drilling the transfer port off set to line up straight with the barrel and others have kept the original configuration but insert and epoxy weld a liner of brass tubing to reduce the diameter from 4mm to 3mm.
Has any one here heard of this before and would you care to share your thoughts

Garth

 

[leonardj]

I haven't experimented with any of the Diana air rifles, but I have done a fair bit of experimenting with trying to hit the 10 FPE level with an FWB 300S match rifle.
These guns are designed to shoot at around 6 to 6.5 FPE, so to get to 10 FPE was a bit of a challenge. Thus far, I have managed a couple guns at just shy of 9.5 FPE, and very capable of taking down field targets at 55 yards.

A few of the hypothetical theorist airgun gurus told me that the transfer port was too large to net any further gain, and despite the fact that these gurus had absolutely no hard data to back up their theory, I decided to give it a shot at altering the transfer port diameter.

The FWB 300S is a very easy to work on gun when it comes to altering the transfer port diameter - you simply pop out the breech seal, and press the top hat shaped restrictor into the transfer port, then re-install the breech seal. I tried several iterations of reduced transfer port, increasing the diameter in 0.0025" increments. After several tests, the transfer port restrictor walls were just too thin to make it practical to continue. The result of the several iterations of reduced transfer port diameter was a degradation of performance from the manufactured diameter. Imagine that, the FWB engineers had it right, and the hypothetical theorists were wrong.

The transfer port diameter on the FWB 300S is within a few thousandths of an inch of the HW97, so I don't believe that a larger transfer port is the way to go.
My experiments continue, but I have effectively ruled out the transfer port diameter as the impeding factor.

Aside from the above, the only other experiences that I can quote are the bad ones, where people have followed bad advice from internet experts, and modified their transfer ports, resulting in an inoperable gun, in need of expensive repairs.

 

[garmic]

Thanks for answering, the posts or articles I was reading said that the optimum size for transfer ports was around .125" the Diana rifle I was concerned about is almost .160", I believe HWs are smaller than .160 and from what I picked up no gains were made in performance or harshness when tried on a HW35. There are some other brands mentioned with large transfer ports, I read "Another airgun blog" where he reduced the size on a Haenel303-super how ever I wasn't able to determine how the successful the author felt it was.
http://www.network54.com/Forum/7953...+Transfer+Port+Size--Some+Preliminary+Results
http://anotherairgunblog.########.ca/2011/04/haenel-303-super-tear-down-part-3.html
I have a spare compression chamber for my QB57 and I know it has a .160 transfer port maybe I'll give it a go just to see.

 

[KEN.]

Hello.........Do yourself a favour and buy a Diana 34.....lots of performance no need to play and experiment to end up with something that's junked afterwards....KEN.

 

[garmic]

Hi KEN, my 280 is on the mail truck it should have got here on Friday but..... The reason I was looking into the transfer port business was to improve efficiency and allow me to reduce the spring force needed to stay at the current power level and in turn reduce the recoil overall making the rifle more accurate. The scope of work is over my head unfortunately as I am finding while I experiment with my QB57, the length of the QB57s transfer port is about 1/8" so what I have tried is a restrictor plate behind the breech seal. The factory transfer port was .170" so for my first attempt I made the hole.125" and it reduced the velocity by 10fps, the shot cycle feels a little smoother for sure and if your familiar with the QB57 thats a good thing since the tube is right beside your head. I've made a new restrictor plate at .150" but I haven't tried it yet. Nothing is permanent and its a bit of fun.
About the 280 its a shorter version of the 34, it has a shorter air chamber, the rifle is almost 3" shorter overall and lighter.