Hatsan .35,137fpe

[Flyonwall]

I got my Hatsan .35 to 137fpe(95 grain Hunters Supply bullet@807fps); probably the high point of the power curve. I used about 18 1/2 loops of .5"O(utside)D(iameter,.062" wire diameter (hard drawn steel) for a new hammer spring . I got 12 10" lenghts by Small Parts on Amazon for about $14.50 delivered (August 25) now Sept.29 they want $43.56 delivered. Basically I replaced most of the pre-load adjustment with more compressed spring.You cut off the very tip of the wire because you don't want any flat/bent part like on the tapered spring. Compress your cut wire using a rod as a guide. It'll go from 4 1/2" free length to about 3", ( it's drawn steel); this will make it less likely to bend  and be ruined  being put in.With no pre-load the back of the spring follower is visible through the set screw hole.Each turn of the pre-load adjuster is about one thread. Mine is in about two turns of spring load. I could maybe go about 1 1/2 more loops of spring but I don't want to stress the valve anymore. If it won't cock decrease the pre-load 1/8 to 1/4turn,repeat if necessary. Loosen it another 1/8 or more so it won"t be hard to cock. This should get you to around 770-780?fps(95 grain bullets). I broke my hammer movement bushing probably from pulling too hard(it should be made stronger). I also broke my cylinder channel connection screw from dry firing;had to do a work around of it till Hatsan USA sent me another.I  plugged all but two valve holes which I enlarged a little( I could have shimmed for just one to line up with the  transfer port).I enlarged the valve poppet hole by about .015 and ground about .015 off the valve stem. I made a  transfer port about .030 larger to about .258. My new transfer port O-rings are partially exposed and the bottom of the port epoxied/super glued to the trigger/hammer housing(no leaks there!)This might be just 25 to 30 fps more than the spring alone, didn't really keep good track. A person might try putting a few loops of another spring( with out the flat end on the extra loops for more compressionable space)  between the stock spring and the spring follower(One of the springs has to have a tapered flat end or the springs will intertwine). Where the springs meet has to stay within the trigger assembly housing.You don't get nearly the efficient energy transfer  as from just one spring.Well I had some labeled pictures that might have helped but don't how to load them,maybe some technical person can show how to replace a hammer spring on the BT65 platform. My shot count now is probably 9 or 10 instead of thirteen. I'm leaving my Bt65SB .25 alone; its powerful enough stock. 

[Flyonwall]

I'm adding to my own post already. I didn't state my bullets diameter ;it is.356 a hollow point from Hunters Supply with a hardness of 10. Their solid points have a regular bullet hardness of about 15 to 18. A Carnivore .35 review at Airgun Depot said not to use .357. I think PyramidAir use to say specify 9MM or.357. I got some of PA's 9MM/357 95Gr Hollow points and 115 Hollowpoints they wouldn't chamber in my Carnivore .35 unless I rezized them;now their 105 Gr round nose did chamber. PA in their bullet description of 9MM/.357' 95 &115 Gr HP's  did list their diameter as .356 but they wouldn't chamber.

[KnifeMaker]

A shorter bullet will be able to chamber easier, even though it is generally too large for your bore. We size our bullets to the last ten thou.  If both bullets are the same dia, you are actually using the barrel to size the shorter bullet that only fits the short inlet of the lede.  Not good! 
 To say that a bullet is .356 or .357 is mostly meaningless. The bullets need to be, say, ..3563 for a actual bore of .356 for optimum performance, in both accuracy and air uses-age.

Test-Test-test! Every barrel is different. And some will want .0003 over. Some will want .0006 over, and some will want exact. or even slightly under. But NOT .001 over!
Simply skipping from .356-.357 is completely missing the whole point of fitting the bullet to the barrel.  It works in powder burners.
It exactly the wrong way to go in Air Guns. It will result in lower shot count, and lead the barrel much quicker, along with lower fps and fpe. All things we don't want.   

Knife

[Flyonwall]

  If anybody wants I could send them two compressed springs of about 18 1/2 loops. I have 8  10" springs to spare. If you find them useful donate a few bucks to GTA if you want or not.

[Bwalton]

Martian you did not do any internal mods?

[Flyonwall]

.  This picture might help.

[Flyonwall]

  I took about .015 off the valve stem and drilled the poppet out( in place) about .015(which probably was not a good idea might affect sealing). The air channel through to the pellet is about.030 bigger now.I made  a new transfer port from brass. You might drill out the stock transfer port by .015 but it would be getting thin in front of the O-ring. There is a YouTube video on taking apart the BT65 platform from Spain? I depress the hammer with something to disengage the drop safety when working with it. Be sure the valve doesn't start unscrewing when removing  the air cylinder after the stock. Enlarging the air channel alone like this should only make 35-40 fps more  in this .35 caliber? Most of the increased fps is from the stronger spring.  The 95 grain bullet topped out at 807 fps,82 H&N Grizzley  at 828, and the 115 grain hollow point will be about 710 fps.

[Bwalton]

Those numbers are low for me, I would like to see the 95gr @ 920-930fps I do not think that that platform can do it without a bottle over the tube that it has.