hatsan bt65 looses velocity in every shot

[navi99]

Hatasn BT65 .25cal with after market shroud.

Hi I'm new to PCP and I was testing my bt65 on a chronograph, i started at 200bar  and the 1st shot was 1008fps
2nd 990
3rd 978
4th 966
5th 955
6th 942
7th 937
8th 926
9th 913
It keeps decreasing as i keep shooting. Is this normal or do i have to to adjust anything.
Can anyone give me ideas.  Thanks

[Ribbonstone]

Lot to learn with PCPs.

There is a “battle” going on between two forces inside the rifle.

Holding the valve closed is air pressure.  Knocking the valve open is the hit of the striker to the valve. If they are out of balance, then you're results are out of balance.

When the hit to the valve is very strong, it will fully open the valve against max pressure.  But the pressure of the next shot is lower (by the amount of air the first shot let go) so that same hit to the valve results in a little lower speed.  The 3rd shot has even lower air pressure…etc.

If the striker’s hit to the valve is a lot lower than the closing force of air pressure, it might not even open the valve at all (called valve-lock).  Normally, it will open the valve part way, so the shot is slower than expected.  Each shot after that works on progressively lower air pressure, so opens the valve a little farther, and the velocity is a little higher.  Problem is, could have many many low powered shots until pressure drops enough to form a blance between the two forces.

Balance forces would have the first shot be a little under max speed, but not by much.  Then it builds to a level area where the speed is pretty constant.  Once the pressure drops enough, the hit to the valve still opens it all the way, but the air pressure isn’t high enough to produce full velocity, so velocity starts to decline.


In your case, you’ve got way more hit to the valve than you have closing force holding the valve shut.  As you are already at max pressure (200BAR) the only way to get it in balance would be to decrease the striker’s hit to the valve.  Easiest way would be to decrease the spring tension powering the striker (it will have a way to adjust that lower).

[Hobbyman2007]

Here's a link on how to adjust the hammer spring tension on a BT65. Skip the first part with the regulator and go to the picture of how to access the adjustment screw. Very simple once you take the action out of the stock. http://www.gatewaytoairguns.org/GTA/index.php?topic=33033.0
Just out of curiosity ,what pellets are you using ? You may want to get some heavier ammo if using anything under 25 grain. The new jsb mk2 king heavy pellets are just about perfect  at 34 grain.

[Mod90]

yup, thats how they come set from the factory, & luckily, its very easily adjusted & theres a lot to be had from them with a little bit of simple tweaking and tuning.

[navi99]

Thaks a lot guys. I use JSB Match Diabolo Exact King 25.39gr .25       I to try the JSB Match Diabolo Exact King MKII Heavy 33.95 Grains and see how they perform

[oldpro]

Great explanation!!!
Lot to learn with PCPs.

There is a “battle” going on between two forces inside the rifle.

Holding the valve closed is air pressure.  Knocking the valve open is the hit of the striker to the valve. If they are out of balance, then you're results are out of balance.

When the hit to the valve is very strong, it will fully open the valve against max pressure.  But the pressure of the next shot is lower (by the amount of air the first shot let go) so that same hit to the valve results in a little lower speed.  The 3rd shot has even lower air pressure…etc.

If the striker’s hit to the valve is a lot lower than the closing force of air pressure, it might not even open the valve at all (called valve-lock).  Normally, it will open the valve part way, so the shot is slower than expected.  Each shot after that works on progressively lower air pressure, so opens the valve a little farther, and the velocity is a little higher.  Problem is, could have many many low powered shots until pressure drops enough to form a blance between the two forces.

Balance forces would have the first shot be a little under max speed, but not by much.  Then it builds to a level area where the speed is pretty constant.  Once the pressure drops enough, the hit to the valve still opens it all the way, but the air pressure isn’t high enough to produce full velocity, so velocity starts to decline.


In your case, you’ve got way more hit to the valve than you have closing force holding the valve shut.  As you are already at max pressure (200BAR) the only way to get it in balance would be to decrease the striker’s hit to the valve.  Easiest way would be to decrease the spring tension powering the striker (it will have a way to adjust that lower).

Great explanation!!!

[Ribbonstone]

Thanks.

Switching to a heavier pellet might give a longer shot count and very likely to earn more energy (would still slower than lite weights, but not slow enough not to end up with more energy), but the shape do the curve would be about the same: starts off at max speed at the very first couple of shots and falls in speed with each shot.

That's pretty much a sure indication of being out of balance; the striker hit being much stronger than the force holding the valve closed.  IF the air pressure wasn't at max, might be able to get one in balance with a higher pressure fill...that's "OUT" with a 200BAR fill....so pretty much have to lower the striker energy.

Are also likely to slightly decrease over all speed.  Might end up with the firsts shots clocking something like 900fps, having it slowly work its way up to where it hangs around 935-940fps, and then decline until it gets to about 900fps again at shot #20 or #25.  No matter, all of them would be within a useable velocity range.


ON this rifle, the only adjustment given is for spring tension, and that one kind of feels like it runs backwards.  The adjuster is at the front end of the spring....so as you turn it COUNTER CLOCKWISE it results in  MORE TENSION to the spring.  Clockwise results in LESS

[rsterne]

The AT44 is CCW for more tension.... is the BT65 the same?....

Bob

[Mod90]

No Bob, unlike the AT44 the BT65 is cw for more, ccw for less. And there is no need to insert anything into the hammer to stop the hammer from turning while making adjustments either.

[Ribbonstone]

OK...bad assumption.....same idea to decrease the striker hit, but the adjusting screw must act on the rear end of the spring rather than the front end.

[navi99]

Hi guys thanks for the help, today i  the adjust hammer spring i went back 1 turn and it looks more stable and the air last longer. the sad thing is that i didnt record the first 3 shots. the 4th shoot was 964fps
5nd 967
6rd 973
7th 968
8th 972
9th 968
10th 962
11th 967
12th 960

tomorrow in going to do more testing, thanks again 

[mista meener]

OK...bad assumption.....same idea to decrease the striker hit, but the adjusting screw must act on the rear end of the spring rather than the front end.

this confuses me. if you turn  CC on a bt 65 does velocity increase or decrease?

[rsterne]

AT-44 CCW to increase.... BT-65 CW to increase....

Bob

[Mod90]

Before you turn that bolt CW, iirc the guns spring adjustment screw only has 11 turns from when the screw is backed out and its back end is flush with its adjustment access hole. Meaning, when the back of it is flush with this space you can only get 11 full CW rotations inward till the screw is unseated from the adjustment threads. If this happens, it may not be able to be backed out by turning CCW & you may have to disassemble the gun to fix it.

when sending that screw inwards, go slow & keep track of your rotations.