You can make it only with drillpress and tap (to seal the hole in bottom with set screw), but it has some disadvantages. Above all, you will be not able entlarge the pasage too far, because you using the solder as wall of the hole.
Best is resolder the tubes, drill the bore, put here new, larger tube and solder that again. There you can improve the performance really terrifically.
For comparsion, from this, you can presume how big can be the hole in drilled marker: http://www.pgp.mysteria.cz/ladeni_PG...ni_vrtanim.jpg
And from this, how big is the hole on Palmer guns: http://www.pgp.mysteria.cz/ladeni_P6...teci_otvor.gif
The bigger is better here, the only limitation is the valve body width and the half of the diameter of paintball (to seal the chamber in PGP)... what is around 8mm.
There are some old pics of my:
Stock PGP tubes after polishing: http://www.pgp.mysteria.cz/pajeni_PG..._rozpajeni.jpg
After resoldering: http://www.pgp.mysteria.cz/pajeni_PGP/02-rozpajeno.jpg
Halves with new tube from discharged .22 LR shell: http://www.pgp.mysteria.cz/pajeni_PGP/03-rozpajeno.jpg
Drilled and inserted, ready to resoldering: http://www.pgp.mysteria.cz/pajeni_PG...o_k_pajeni.jpg
After soldering: http://www.pgp.mysteria.cz/pajeni_PGP/05-spajeno.jpg
Final polishing: http://www.pgp.mysteria.cz/pajeni_PG..._lesteno_2.jpg
And this is difference on chrono:
(the same gun, the same internals only with stock tube with 4mm ID and resoldered with 5.2mm ID) http://www.pgp.mysteria.cz/ladeni_PG...otvor_22LR.gif
And here is the Airflow with different tubes ID:
(used tubes: stock, drilled, .22LR, 6.35mm Browning, 7.65mm Brownng, 9mm Parabelum) http://www.pgp.mysteria.cz/ladeni_PG...bicky-graf.gif
Today, I use at least 7.65mm Browning cartridges to get better airflow, but preferably .45 ACP cartridges deformed to oval, where is internal dimension ~7x14mm and the results are even far better (30+ shoots above 250 fps)... but it needs likewise changes on valve, naturaly.
Regrettably, I have problems with my computer, so I can not post here the new pics.