Working on the transfer port --> help !!

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james.dean

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Apr 2, 2015
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Hi guys..

I've just ordered a .25 bullpup knowing that I'll have to work a lil bit on it as the stock velocity is only 240 meters per second and I'd like it to be 280 meters per sec and I know there is nothing to do on the Hammer and Hammer spring to achieve this kind of tuning.
So... I'll have to work on the transfer port a'd maybe the barrel port to allow more air to pass.

My question is... What's the best way to drill your transfer port? Is there anything I got to know before I start the tuning please???
 
Assuming you are using a drill press....
Use higher RPMs
Set the max depth of the drilling so you don't dimple the opposite side of the bore. (so you don't drill too deep)
Use cutting oil. Use a new drill bit.
​De-Burr the edges, slightly, and by hand, not with a power tool.

​Make sure the hole you drill isn't so large it affects how the O-Ring seats and seals

And for my questions...
What is the current hole size? If it is 0.187 inches or larger, consider leaving it alone.
​The increase in size needs to include:
Valve outlet
Transfer port (tube)
and
Barrel

​A restriction anywhere largely negates the advantage of increasing other areas.
 
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Thanks for yours answers !!
So for your questions:
I've ordered a Kral puncher Breaker with a CZ barrel and it'll be Huma regged. It has a 280 cc tube and a max fill pressure of 200 - 210 bars. (I guess I'll be able to go up to 250 once regged no??)

So I still don't know anything about transfer port size etc as I don't own the pup yet ^^
But... I asked the persons who sell it and they told me that from a stock 50 joules with 25.4 grains JSB, they could only tune it up to 55 joules (but, when I told them I'll work on transfer ports to gain power, they told me no to as I'll Risk to affect negatively the rifle, which drives me to think they have not even TRIED to open up a bit these TP. Plus, I know someone who pushed it to 110 joules with deeper mods, so I guess 65 joules is just minor mods.)

Another thing is that, sadly, I don't own a drillpress (I've perfectly drilled through my s510 cylinder to reg it though, 'cause I'm the kind of guy who pays a LOT of attention when it comes to modify something he loves ^^) so... I'll still be able to drill..

What I'm really not sure about though, is the way I'm gonna clean the burs off the inside of the bore... I definitely don't want to scratch it but I'll need to use hard metal tools.. I've seen Robert Lane video about it but it didn't really convinced me..

Finally, there is the "shape" of your holes. I've read that, depending on it, the airflow will be affected.
Your drilled hole could look like a cylinder with sharp edges, or like a 3 steps stairs (drill through 3/3 with the small drillbit, through 2/3 with a bigger one and through 1/3 with an even bigger one) etc etc..
These are Just examples but I've seen much more shapes so.... Can you lighten me on this point please ?? =)
 
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You can de-burr the holes via very fine sandpaper rolled up, or a very fine diamond needle file. Be sure to ruthlessly clean up all of the metal bits and grit out of the air rifle before using. Grit and metal bits in an air rifle is very bad news. Your main goal when widening the airway is "port matching" (google the topic, which mostly speaks of car engines). Any change in size (larger or smaller) causes less than ideal flow. 
 
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Yes, with port matching the holes need to be the same diameter, and perfectly aligned. No sharp burrs, and polished if possible. Also, make sure the bolt is not significantly obstructing the transfer port. This may require some gentle shaping of the bolt "AT" the transfer port. As I am not familiar with the actual construction of the Kral, I cannot give any more specific suggestions.
 
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