Marauder question?

So I went to the gun show and bought a Marrauder that has been made in to a bullpup. I love it. Light easy to carry ans I have only had on sesion at the bench but it seems to love Crossmam pellets.
The question, The guage seems to be way off. when I biught it the neddle read n the green zone. I fired it a few times and the needle actec the way I would expect. So before I go to the bench I fill it to 3000 psi witth my hatsan compressor, when the guage on the compressor said 3000 so I quit pumping. However the guage on the pup is off the scale,way over in the white. The compressor and my other PCP/s alway agree so I don't think the gauge is off and the gun shoots very well for about 25 shots then I refill it.
I do think thetank has been extented for more capicity. I don't mind the guage so bad but It makes my bottle useless if I can only refill with a compressor.
Thanks for any advice
 
Thank you for the replies. I will try to post picture
I have another Question. My new gun is affected badly buy fill pressure. The shot groups are very narrow but can get really tall. Been shooting 8 shot groups becases of mag capitcy. each group of 8 has a widely ranging point of impact.
I see that the pidstol is easy to adjust. will this help get more consistant grroups?