Hatsan BullBoss .25 Modifications and Slug Testing

[Jzizzle]

I have Bullboss in .25 wood that I want to both increase power and shot count for plinking and hunting. I want to eventually shoot slugs with this platform.  Pellet wise, only the JSB 25.39gr did well for me out past 50 yards. 

Bone stock with hammer adjustments my baseline string is here.
01. 899
02. ER
03. 895
04. 890
05. 894
06. 891
07. 885
08. 888
09. 887
10. 891
11. 886
12. 888
13. 884
14. 882
15. 880
16. 880
17. 873
18. 874

[Jzizzle]

Thanks to Rsterne, a well know member here on GTA, I have reviewed his modifications for a while and finally performed very similar mods. 

After much testing I am right where I want to be in terms of fps and am surprised how consistent this has become.  This is unregulated!

New shot string is here.

JSB 25.39

01. 928
02. 931
03. 930
04. 933
05. 932
06. 932
07. 938
08. 940
09. 936
10. 918
11. 935
12. 935
13. 933
14. 931
15. 932
16. 930
17. 931
18. 930

[Jzizzle]

Testing results from yesterday with various slugs at 22 yards.
28gr and 30gr that appear to be replicas or similar to fx hybrid.
28gr with deep hollow base
30gr dish base

initially dialed strings over the chrony.  I was trying to achieve 950+ fps but that wasnt happening.  910 was max but shot count was only 12 with 3% spread. To get 2 mags ave velocity is 875fps with 3% spread.  At these speeds the only one grouping well is the 28gr hollow base slug.  Whats interesting is I will get a few hole in hole and then a flier.  I think this may be due to how the slug is loaded in magazine and what the probe does while loading into breech.  They are a tad too long for the magazine.

I will order these hollow base slugs in 25gr to have a better fit in magazine and to sling at higher fps.

to be continued...

[Jzizzle]

Some more testing today.  Concluded that the magazine fitment is not the issue.

Today is 17mph sustained winds with over 25mph gusts.  Accuracy testing at 22 yards off a bipod and cheap walmart table.

5 shot groups

30gr Dish Slugs (.250)  Avg 829 fps  1.02"
28gr Hybrid Slugs (.249) Avg 863 fps  .85"
30 gr Hybrid Slugs (.249) Avg 834 fps 1.09"
28 gr Deep Hollow Base (.250)  Avg 873  1.57"


All not acceptable at 22 yards!   Oh and 25.34 JSB  955 fps .47"

I just got in Varmint knockers 32.8gr Flat Base Slugs.
(.250) Avg 855 fps  .39"
 This is good news and will test further out on less windy day.  Best slug out of the Bullboss so far.

Also on the way is the Hades pellets, AVS slugs, NSA slugs and lighter weight Deep hollow base slugs.

More to come...

[GoneShootn]

Thanks to Rsterne, a well know member here on GTA, I have reviewed his modifications for a while and finally performed very similar mods. 

After much testing I am right where I want to be in terms of fps and am surprised how consistent this has become.  This is unregulated!

New shot string is here.

JSB 25.39

01. 928
02. 931
03. 930
04. 933
05. 932
06. 932
07. 938
08. 940
09. 936
10. 918
11. 935
12. 935
13. 933
14. 931
15. 932
16. 930
17. 931
18. 930

Great string! In some guns, this may be a bit hot for good accuracy with diabolo pellets, or so I have read. Care to outline your specific mods and what problems you may have encountered along the way?

[Xraycer]

Thanks to Rsterne,  I have reviewed his modifications for a while and finally performed very similar mods. 

Care to share, in details, what mod you did?

[Jzizzle]

All ports and valve seat were enlarged to .180"   
5 of the valve ports being plugged and epoxied.
Barrel shrouded with carbon fiber tube and tensioned by Ronin LDC.

No issues encountered with the modifications.

With my particular gun, I have 7 turns of usable preload.  What I found is once I go to 6 or 7 turns out shot count suffers with 12-14 shots within 3%.  Sweet spot for JSB 25gr is 4 turns out and now I'm running 5.5 turns out with the slugs.

[Jzizzle]

Also to note the trigger was super heavy out of the  box.  Hatsan triggers have good adjustability though and its now much lighter with a crisp 2nd stage.

[Jzizzle]

Has anyone swapped hammer springs for one that is stiffer?  Id like to get another 75+ fps with these varmint knockers 32.8 gr slugs.  Possibly a lighter valve spring would give some more power too.

I'm a bit disappointed velocities are not higher with the slugs.  Goes to show you how much bearing surface impacts the speed.  JSB pellet can go over 1000 fps if you wanted it to.

[Xraycer]

Donny aka Rally Shark should be chiming in on this. He's done a lot of mods to his Bullboss.
I'm paying close attention to these threads because I'm looking to make it more efficient. I'm green in regards to airgun modding, but I am mechanically inclined.

[GoneShootn]

All ports and valve seat were enlarged to .180"   
5 of the valve ports being plugged and epoxied.
Barrel shrouded with carbon fiber tube and tensioned by Ronin LDC.

No issues encountered with the modifications.

With my particular gun, I have 7 turns of usable preload.  What I found is once I go to 6 or 7 turns out shot count suffers with 12-14 shots within 3%.  Sweet spot for JSB 25gr is 4 turns out and now I'm running 5.5 turns out with the slugs.

By "turns out" do you mean that you begin from all the way tight and then back off "X" turns?

[Jzizzle]

Yes that is correct

[Rallyshark]

John, you're still leaving a lot of fps on the table with your BB, and the speed is definitely there if ya want to get it   It just needs some more elbow grease and a couple of minor parts.  A stiffer hammer spring will help, but your bigger issue is porting at the moment.  Below is a list that will get your power up by a good bit, and just ignore me if I mention something you have already done    :
- Open up the cylinder carrier hole to .180-.185(brass piece that the valve/cylinder screw into, and transfer port sits)
- Open up the transfer port to .180-.185( hard to do, but worth it). 
- Open up the barrel port to .180-.185
- Open up remaining hole in the valve to .200
- *Most important on the top end of power, replace the factory derlin valve seat with one made of PEEK, and
    open that up to .210.  That factory seat is just too squishy to open up much more than it is without failing. You'll
    have to make the PEEK valve seat yourself.  Another benefit of the PEEK is that it is easier to crack the valve open
    since it is harder.
- Install a stiffer hammer spring(I found one at Ace). 
- Open up the holes on the valve spring retainer on the back of the valve as much as possible, without making it too
   weak.  I'm assuming yours has the newer style that screws in with multiple little holes around it? 

Or to put all of that into simpler terms...  Make sure all the porting past the valve to the barrel is .180-.185, install a PEEK valve seat at .210, stiffer HS.  If you do all of that stuff, that Bullboss will really wake up!  I think 75-100 fps improvement is entirely possible.  I hope that helps

[Jzizzle]

I pretty much have done all the above with the following exceptions:

-The 1 remaining valve port is .180,  so I can bring this up to .20

-Im still using factory valve seat opened to .180, so this can be improved.

-Valve spring retainer is the newer style with multiple holes in it.  So I can slightly enlarge all holes?  Would this negatively impact efficiency?

- Hammer spring is stock, will source a heavier spring.

[Rallyshark]

I pretty much have done all the above with the following exceptions:

-The 1 remaining valve port is .180,  so I can bring this up to .20

-Im still using factory valve seat opened to .180, so this can be improved.

-Valve spring retainer is the newer style with multiple holes in it.  So I can slightly enlarge all holes?  Would this negatively impact efficiency?

- Hammer spring is stock, will source a heavier spring.

Okay, so you did the cylinder carrier, transfer, and barrel then?  If so, then all you need to do is tackle the valve seat and spring retainer.  I don't recommend opening up that factory valve seat much more, because it will fail if you do. You can take it to .190 safely, but I wouldn't recommend any more than that.  PEEK can be opened up quite a bit more safely though, and it will also just work better all around.  None of this stuff will hurt the efficiency at all.  The larger valve seat opening may actually help a little, since the larger opening will result in the valve closing a little faster.  The PEEK valve seat is one of the best mods you can do to almost any Hatsan!  Let me know if you need any tips on that.   

[Rallyshark]

Donny aka Rally Shark should be chiming in on this. He's done a lot of mods to his Bullboss.
I'm paying close attention to these threads because I'm looking to make it more efficient. I'm green in regards to airgun modding, but I am mechanically inclined.

Your biggest determining factor will be what power level do you want it to be efficient at?  I sold my Bullboss to VaporTrail, but he tells me he's putting it to good use   I think he said he's shooting 28 grain slugs around 960 fps with it currently(.22). 

[VaporTrail]

Yes, this RallyBoss is mine. There aren't many like it, and it is my own.  (FMJ reference)

Slight correction: 28gr at 974 and they're literal hole in hole at 30yds. This Boss can hit with authority. 

Zizzle, you should be getting a little more zing from your Boss. I bought bear air's .25 version, and that was slinging 28.5gr Benji domes at 925-917FPS with the HS maxed out. No mods done to it at all; stock ports and valves.

My stock Boss (.22) before I goofed it was slinging 25gr JSBs at 947. After the goof it went down to 927. 28gr EJs were traveling at 840.

I agree with Donny...there's more power available. Especially with your mods.

[Jzizzle]

I pretty much have done all the above with the following exceptions:

-The 1 remaining valve port is .180,  so I can bring this up to .20

-Im still using factory valve seat opened to .180, so this can be improved.

-Valve spring retainer is the newer style with multiple holes in it.  So I can slightly enlarge all holes?  Would this negatively impact efficiency?

- Hammer spring is stock, will source a heavier spring.

Okay, so you did the cylinder carrier, transfer, and barrel then?  If so, then all you need to do is tackle the valve seat and spring retainer.  I don't recommend opening up that factory valve seat much more, because it will fail if you do. You can take it to .190 safely, but I wouldn't recommend any more than that.  PEEK can be opened up quite a bit more safely though, and it will also just work better all around.  None of this stuff will hurt the efficiency at all.  The larger valve seat opening may actually help a little, since the larger opening will result in the valve closing a little faster.  The PEEK valve seat is one of the best mods you can do to almost any Hatsan!  Let me know if you need any tips on that.

Yes the cylinder carrier, transfer port and barrel have been ported already.


I have reached out to Hatsan for a Nova star hammer spring.

What size should I enlarge the holes in the newer version valve spring retainer cap?  Currently I have left this alone.

My valve hole is currently .18"  I will enlarge this to .2"

If I attempt to enlarge the delrin valve seat from .18 to .19, is this even worth my time?
Anybody sell the Peek valve seats?

[Jzizzle]

I went back in my notes and I actually used a 3/16 drill bit.
I apologize for previous inaccuracies.

Here is what I have written down as far as completed modifications.

-5 valve holes tapped and plugged with 4-40 set screws that I cut down to length.

- Valve top port drilled to .1875".  Then epoxied the space over the set screws.

- Cylinder carrier port drilled to .1875"

-Transfer port drilled to .1875"

- Barrel port drilled to .1875"

- Valve seat drilled to .1875"

Next I will enlarge the Valve spring retainer holes a bit.  There isn't much material available without thinning the walls too much.  Perhaps only enlarge 3 of the 7 holes.

Do you feel it's worthy to further enlarge the Valve port from .1875" to .203"?
Perhaps do the same to the delrin valve seat or is that too much material removed?

[Rallyshark]

I went back in my notes and I actually used a 3/16 drill bit.
I apologize for previous inaccuracies.

Here is what I have written down as far as completed modifications.

-5 valve holes tapped and plugged with 4-40 set screws that I cut down to length.

- Valve top port drilled to .1875".  Then epoxied the space over the set screws.

- Cylinder carrier port drilled to .1875"

-Transfer port drilled to .1875"

- Barrel port drilled to .1875"

- Valve seat drilled to .1875"

Next I will enlarge the Valve spring retainer holes a bit.  There isn't much material available without thinning the walls too much.  Perhaps only enlarge 3 of the 7 holes.

Do you feel it's worthy to further enlarge the Valve port from .1875" to .203"?
Perhaps do the same to the delrin valve seat or is that too much material removed?

Looks like all your porting is spot on!  The biggest reason I suggested opening the remaining hole in the valve is to give you more room for alignment, since it won't really negatively affect anything.  It will also help, if you get a valve seat that can flow more air.  The alignment on that valve to the cylinder carrier/transfer area is KEY.  If it is off even a little, your top end will suffer.  You can loosen that stock screw on the bottom of the trigger tube and shift that carrier fore or aft if needed to adjust the alignment a little.  The slightest difference in where that carrier sits can make or break that valve hole alignment.  The same goes for the barrel port/transfer alignment.  Oh, did you open up the hole in the breech above the transfer?  That could be choking it down too, if you didn't.  I forgot to mention that one...

I don't know that opening up the factory valve seat anymore is going to make much difference at this point.  Another issue with that factory seat is that is has so much of the valve pin in contact with the surface, combined with it being a little soft, makes it require a lot of hammer force to knock it open.  You can order some 3/8" PEEK rod from Amazon for not much money.  If you have a lathe, they are easy to turn down a little and make, if not they are a bear.  I made the first few I did without a lathe.  I chucked that PEEK rod into a drill and used sand paper to get the OD the same as the factory seat.  Drilling the hole dead center is another matter of ingenuity,lol.  Then I used a coping saw blade to make the o-ring groove while spinning it in the drill.  It is a good job for an octopus,lol.  I'm shooting you a PM on that seat