I need an air guru

[Brookesy]

I'm a little perplexed.

I have a hatsan at44-10 which for some reason has lost power. ( can't identify why )
In my investigation I shaped out my full size transfer port for one about half the size and saw a near 300fps increase in speed.

Far as I know that shouldn't be the case. Anyone care to educate me a little?

[Jim-in-UK]

The narrower port is allowing more efficient expansion of the air into the barrel, liberating more of the air's internal 'U' energy which, in springers, I believe can account for up to 40% of muzzle energy.

[ER00z]

I'm no guru...

I'd give your o-rings a look. One just in the barrel leade may be worn or missing, think there are two on the valve... might need a bit of silicone grease or replacing. Depending on how many shots through the gun, the hammer spring may have settled or fatigued. Those older Hatsan designs (AT44, BT65 and etc.) were massively over ported... Reducing transfer port diameter might just be reducing the pressure drop between valve and pellet... Essentially "hitting" the pellet a little harder from the start, although with less volume (Simpler explanation to what Jim-in-UK posted, more or less).

[JPSAXNC]

As mentioned, the bigger the transfer port the lower the pressure, and the slower the air speed through the transfer port.

[Brookesy]

Wouldn't the lower air pressure from the larger port by irrelevant because of the speed of the air delivery? (more air behind the pellet quicker)