Audrius Regulator@100 AT44 QE long. 14.3 CHPP

What was your starting pressure?
If you start at 2900 and the reg is set to 1450 and you shot the attached string you are getting terrible efficiency.

If you use 2900 and 1450 you should be able to get 50 shots at 860 with ease using the 14.3,

Since you are getting 880 max at your reg setting, you should be able to shoot at 860 pretty efficiently.

First thing I would do is back WAY off on the preload. On the Hatsan this means turning the hammer spring adjuster CLOCKWISE. Keep turning it in until the velocity is 860 while on the regulator (pressure in tube still above 100bar) If you cannot get it to start out as low as 860 you will need to cut your hammer spring or remove any spacers (been a while since I had my hatsan so don't remember if it had any spacers).
Once you get a start of 860 then fill the gun to 200 bar and run a string.
The long tube is 230cc so take away about 30 for the reg and you have 200cc left. 50 shots at 860 (with 14.3's) from 2900 to 1450 is only .96fpe/cuin which should be easily doable.

You will likely need to bump up the reg to get the power you are wanting. Doesn't look like you will reach the 50 shot count though.
Here is a post from a very good PCP guy with his AT44. He regged it at 137bar. Getting 50 shots right at 800fps.

http://www.network54.com/Forum/79537/thread/1319493917/Results+of+regged+Hatsan+AT44+in+.22cal+and+.25cal+target+pics+added

If you have some hands on skills you might want to do the set screw mod where you block off all of the valve ports except for the one directly under the transfer port. You then enlarge that one.
I did it on my .177 and my .25 increasing efficiency a good bit.

There are some pic in this post.
http://www.network54.com/Forum/79537/thread/1393633939/%26quot%3Bcheap%26quot%3B+Hatsan+PCP+(part+5)