Hammer Spring/Regulator Set Point GSX200

[kena]

Gents,
Just had the need to strip and rebuild my Impact GSX200 as it had sprung a leak. The rifle is .20 calibre and has a 350mm barrel. Although I bought it secondhand as far as I am aware the original owner hadn't tampered with it.
Prior to the leak it was running at 11.3ftlbs with 13.5gr pellets. Before rebuilding the regulator I put it in the tester and to my surprise the set point was just above 70bar/1000psi. Another strange one was the hammer spring set up which had two springs, what looks like a shortened standard Rapid spring and a smaller spring inside the other. The spring guide was also stepped to accommodate both spring sizes. The rifle had been reasonably accurate with the odd flyer but always had quite a wide shot spread in the region of 25 - 30fps over a full magazine
I am led to believe the regulator set point for a .20 calibre should be in the region of 100bar/1450psi to achieve the best results. With the spring tension at maximum I have tried changing regulator set points with the following results, 80bar/10.0ftlb, 90bar/9.5ftlb, 100bar/9.0ftlb. Your thoughts would be greatly appreciated

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[Fergie777]

Could it be the springs binding when compressed, have you tried a single spring ?

[Squinty]

Dual springs not really necessary at sub-12 unless it's not breathing too well.

velocity/energy decreasing with increase of regulator pressure...
I'd say valve porting. Front port possibly too small and the pressure makes it harder to open..hence dual spring set up?

30fps variation; reg creep, could be hovering at the closing point or if rising after resting.
Possible that the pressure is too low (70 bar) and there wasn't enough preload on the belvilles
 
Or lazy/slow recharge  if you get say 2 shots and a drop, it's not reacting until the pressure is really low.
Could be that the core nose end float is too tight.

[angrybear]

I think that idiot on fleabay sells dual spring sets, so I'd say it's very much been tampered with,
IIRC my .20 reg is about 90bar.
That spring looks pretty short, & pretty thick, so I'm wondering what exhaust spring it has, & what valve come to that 
It's a balance of everything to get them running nicely, so it probably all needs to come out to check what's there & in what condition, could it have a bent valve stem ?

[kena]

Barrel port looks standard as does the valve and valve spring. Changed to a single Theoben spring and guide. The new spring was cut down to about 50mm and ,although slightly higher fps than where I want to be , seems to perform better than the dual spring set up. The anti tamper was still in place with no sign that it had ever been removed so I can't be sure if it's previously had a spring change. Did a lane 5 bar step on the rifle from 50 bar up to 100 bar. It seems to plateau at 90 bar, although I only had the gauge on my air bottle to go by, then starts to fall off just over the 90. There is a definite fall off between 90 and the set point which was 100 bar. I reset the regulator to 90 bar, but it still had a 25fps shot spread and above where I wanted to be. Thinking the hammer might have been slamming the valve I trimmed another coil off the hammer spring and ran a couple of strings over the chrono.

String 1 H&N Baracuda .20 13.58gr

Shot count: 14

Low: 616 FPS
Hi: 620 FPS
Avg: 618 FPS
Spread: 4 FPS
STD Dev: 1.7 FPS

22-Sep-2026 12:59:06,616,FPS,11.4,FT Lbs,
22-Sep-2026 12:59:14,616,FPS,11.4,FT Lbs,
22-Sep-2026 12:59:21,616,FPS,11.4,FT Lbs,
22-Sep-2026 12:59:29,620,FPS,11.6,FT Lbs,
22-Sep-2026 12:59:36,620,FPS,11.6,FT Lbs,
22-Sep-2026 12:59:45,618,FPS,11.5,FT Lbs,
22-Sep-2026 12:59:56,618,FPS,11.5,FT Lbs,
22-Sep-2026 13:00:04,618,FPS,11.5,FT Lbs,
22-Sep-2026 13:00:11,616,FPS,11.4,FT Lbs,
22-Sep-2026 13:00:17,620,FPS,11.6,FT Lbs,
22-Sep-2026 13:00:24,620,FPS,11.6,FT Lbs,
22-Sep-2026 13:00:29,620,FPS,11.6,FT Lbs,
22-Sep-2026 13:00:35,620,FPS,11.6,FT Lbs,
22-Sep-2026 13:00:55,620,FPS,11.6,FT Lbs,

String 2 .20 H&N Baracuda .20 13.58gr

Shot count: 13

Low: 611 FPS
Hi: 620 FPS
Avg: 614 FPS
Spread: 9 FPS
STD Dev: 2.6 FPS

22-Sep-2026 13:09:21,613,FPS,11.3,FT Lbs,
22-Sep-2026 13:09:31,613,FPS,11.3,FT Lbs,
22-Sep-2026 13:09:41,611,FPS,11.3,FT Lbs,
22-Sep-2026 13:09:51,613,FPS,11.3,FT Lbs,
22-Sep-2026 13:10:02,613,FPS,11.3,FT Lbs,
22-Sep-2026 13:10:10,616,FPS,11.4,FT Lbs,
22-Sep-2026 13:10:18,613,FPS,11.3,FT Lbs,
22-Sep-2026 13:10:25,620,FPS,11.6,FT Lbs,
22-Sep-2026 13:10:31,613,FPS,11.3,FT Lbs,
22-Sep-2026 13:10:37,611,FPS,11.3,FT Lbs,
22-Sep-2026 13:10:44,613,FPS,11.3,FT Lbs,
22-Sep-2026 13:10:49,618,FPS,11.5,FT Lbs,
22-Sep-2026 13:10:54,611,FPS,11.3,FT Lbs,

I think I'm pretty close with this now. Taking it down the range in the morning to put about 10 magazines through as it may need a small tweak once everything has settled down. Thanks to all who contributed gents, much appreciated

[angrybear]

Pretty sure the std hammer spring length is 67mm, so a bit baffled as to why you'd cut it down to 50mm.