My first P35 was a P35-25 caliber and I was recently gifted a P35X in 25 caliber that was not shooting right. I have not fixed it, actually it is worse at least for now, but in the process I've moved parts to my P35-25 and thus had the opportunity to compare parts. SPA makes changes in their guns during manufacture so there may be some differences between your gun or one you would buy today and the guns I compared but I think the changes will not invalidate most if not all the observations I made.
I think people buy P35Xs either for the prettier stock or more power or perhaps both. The stock is stained beech, I believe, and is nicer than plastic. It doesn't fit me so I have not shot using it but I think the stock is nice. I was mainly interested in getting more fpe out of my P35-25. Stock I have touched 50 fpe but I cannot keep the gun at this power level consistently. The regulator has to be absolutely as high as the hammer spring can open even though I drilled out the passages through the block and barrel port to 85% of barrel diameter and ovalized the barrel port (stock is 75%). I have several times got it set to about 50 fpe, put the gun away and pulled it back out and velocity had dropped to 600 fps or something. So I adjust the reg, get it back up and then it happens again. I consider the stock P35 to be reliably capable of maybe 40 fpe. I have an aftermarket plenum on mine about half the size of the P35X and it still was only giving me about 42 fpe at least somewhat reliably.
The barrel on a P35 is 450mm long, the Bullsharks is 490 and the P35X is 531. With only the longer barrel helping velocity my P35 went from 865 or so on 25.4 grain pellets up as high as 924 fps. I have it tuned to about 905 fps average right now and it is very accurate. I shot a 198-14X on the 30 yard challenge this morning with it using a 14X scope. ES was 10 with 2.8 std deviation over 24 shots. I am liking this tune even though it is "only" 45-46 fpe. I will try 34 grain at some point and preliminary testing indicates it will add about 2 fpe. I want to also test both for penetration too and the final choice will be accuracy and penetration.
The hammer spring of the P35X is also heavier than at least my P35's hammer spring. They are both 8.5mm OD. My P35 had a 55 mm long spring and the P35X spring is 48 mm long. But the P35X uses 1.3mm wire whereas the P35 uses 1.2mm wire. When I put the long barrel on my P35 I got a low velocity because the regulator I thought was safe for the stock hammer spring was up a little too high. So instead of turning it down I put in the P35X hammer spring. Velocity was up over 900 fps. I turned the hammer spring down over 2 complete revolutions and was still well over 900 fps. So I should not have to turn the regulator down ever again.
The valve on my P35X is 7mm in diameter and my stock one is 8mm. The P35X is brass and the P35 is plastic. I have enough hammer spring to turn my regulator up but I am a bit worried about the stock plastic valve. Today's target convinces me it is accurate where it is so it will probably stay there. But with a smaller valve the P35X will flow better. I cannot say how much because the valve does not work now and the passage from the valve to the barrel in my P35X is 75% of a 22 caliber bore (.190 inches) instead of 75% of a 25 caliber bore like it should have been made. 85% works fine in my P35-25. I tried to pull the valve to drill it out like I did on my P35 but apparently damaged the valve in the process (it leaks) and didn't get it out. I may try sticking it in my oven. I am pretty sure the valve seat is stainless and the block is aluminum so the block will expand significantly more than the valve seat possibly permitting me to remove it. So maybe someday I will really fix my P35X. But for now I have to guess how much the smaller valve would contribute.
The P35X has a 60 cc plenum, I think the stock one is about 12cc and the Extreme Airguns one I am using is about 30cc. Having twice the plenum would no doubt add some velocity to the P35X. I tried using the P35X airtube with the P35 valve but velocity was low, something was not working right. It screwed together fine and didn't leak.
One thing that isn't exactly a positive about the P35X is the air capacity. I believe SPA claims a higher air capacity for the P35X but I think that is misleading. If I measure the distance from the spring loaded port cover to the start of the plenum (end of the airtube) for my P35 I get 19 inches and for my P35X 17.25 inches. I think SPA includes the plenum in their size and that is a bit misleading. The airtube has higher pressure air than the plenum and thus more control over shot count. Shoot 24 shots this morning at my current tune I went from 250 bar to 180 bar. If I assume linear bar/shot I will get about 34 shots on reg at this tune. I think a P35X would get less. My P35-25 falls off reg very gradually and will be me another 10-12 shots less than 20 fps lower than the on reg shots. I'll have to get the P35X working to know what it does as it comes off reg.
Another more minor thing I did not like about my P35X is the lack of a sear adjustment screw. The threaded hole was there, however, so I just added a 3mmx.5mm screw and lock nut so I can adjust sear engagement like my P35s. My bullshark also lacked that screw.
So in summary, I think the changes in the P35X that give it ~15 fpe more power are: 81mm longer barrel, about 5 times the plenum, stiffer hammer spring to permit higher regulator pressure, and 1mm smaller valve. The longer barrel goes into my P35 action just like it's shorter barrel does and the hammer spring drops in just fine. So those changes can be retrofitted if you can find the parts. I ordered a 450 mm barrel from Krale so it might be possible to get these parts from them or possibly Airgun Archery Fun who I noticed sells P35Xs now.
I think people buy P35Xs either for the prettier stock or more power or perhaps both. The stock is stained beech, I believe, and is nicer than plastic. It doesn't fit me so I have not shot using it but I think the stock is nice. I was mainly interested in getting more fpe out of my P35-25. Stock I have touched 50 fpe but I cannot keep the gun at this power level consistently. The regulator has to be absolutely as high as the hammer spring can open even though I drilled out the passages through the block and barrel port to 85% of barrel diameter and ovalized the barrel port (stock is 75%). I have several times got it set to about 50 fpe, put the gun away and pulled it back out and velocity had dropped to 600 fps or something. So I adjust the reg, get it back up and then it happens again. I consider the stock P35 to be reliably capable of maybe 40 fpe. I have an aftermarket plenum on mine about half the size of the P35X and it still was only giving me about 42 fpe at least somewhat reliably.
The barrel on a P35 is 450mm long, the Bullsharks is 490 and the P35X is 531. With only the longer barrel helping velocity my P35 went from 865 or so on 25.4 grain pellets up as high as 924 fps. I have it tuned to about 905 fps average right now and it is very accurate. I shot a 198-14X on the 30 yard challenge this morning with it using a 14X scope. ES was 10 with 2.8 std deviation over 24 shots. I am liking this tune even though it is "only" 45-46 fpe. I will try 34 grain at some point and preliminary testing indicates it will add about 2 fpe. I want to also test both for penetration too and the final choice will be accuracy and penetration.
The hammer spring of the P35X is also heavier than at least my P35's hammer spring. They are both 8.5mm OD. My P35 had a 55 mm long spring and the P35X spring is 48 mm long. But the P35X uses 1.3mm wire whereas the P35 uses 1.2mm wire. When I put the long barrel on my P35 I got a low velocity because the regulator I thought was safe for the stock hammer spring was up a little too high. So instead of turning it down I put in the P35X hammer spring. Velocity was up over 900 fps. I turned the hammer spring down over 2 complete revolutions and was still well over 900 fps. So I should not have to turn the regulator down ever again.
The valve on my P35X is 7mm in diameter and my stock one is 8mm. The P35X is brass and the P35 is plastic. I have enough hammer spring to turn my regulator up but I am a bit worried about the stock plastic valve. Today's target convinces me it is accurate where it is so it will probably stay there. But with a smaller valve the P35X will flow better. I cannot say how much because the valve does not work now and the passage from the valve to the barrel in my P35X is 75% of a 22 caliber bore (.190 inches) instead of 75% of a 25 caliber bore like it should have been made. 85% works fine in my P35-25. I tried to pull the valve to drill it out like I did on my P35 but apparently damaged the valve in the process (it leaks) and didn't get it out. I may try sticking it in my oven. I am pretty sure the valve seat is stainless and the block is aluminum so the block will expand significantly more than the valve seat possibly permitting me to remove it. So maybe someday I will really fix my P35X. But for now I have to guess how much the smaller valve would contribute.
The P35X has a 60 cc plenum, I think the stock one is about 12cc and the Extreme Airguns one I am using is about 30cc. Having twice the plenum would no doubt add some velocity to the P35X. I tried using the P35X airtube with the P35 valve but velocity was low, something was not working right. It screwed together fine and didn't leak.
One thing that isn't exactly a positive about the P35X is the air capacity. I believe SPA claims a higher air capacity for the P35X but I think that is misleading. If I measure the distance from the spring loaded port cover to the start of the plenum (end of the airtube) for my P35 I get 19 inches and for my P35X 17.25 inches. I think SPA includes the plenum in their size and that is a bit misleading. The airtube has higher pressure air than the plenum and thus more control over shot count. Shoot 24 shots this morning at my current tune I went from 250 bar to 180 bar. If I assume linear bar/shot I will get about 34 shots on reg at this tune. I think a P35X would get less. My P35-25 falls off reg very gradually and will be me another 10-12 shots less than 20 fps lower than the on reg shots. I'll have to get the P35X working to know what it does as it comes off reg.
Another more minor thing I did not like about my P35X is the lack of a sear adjustment screw. The threaded hole was there, however, so I just added a 3mmx.5mm screw and lock nut so I can adjust sear engagement like my P35s. My bullshark also lacked that screw.
So in summary, I think the changes in the P35X that give it ~15 fpe more power are: 81mm longer barrel, about 5 times the plenum, stiffer hammer spring to permit higher regulator pressure, and 1mm smaller valve. The longer barrel goes into my P35 action just like it's shorter barrel does and the hammer spring drops in just fine. So those changes can be retrofitted if you can find the parts. I ordered a 450 mm barrel from Krale so it might be possible to get these parts from them or possibly Airgun Archery Fun who I noticed sells P35Xs now.
