AA S400 velocity fluctuation

[merpo]

Hey guys,

So I recently bought myself Air Arms S400 (that's unregulated, as it came from the shop) for FT/HFT and I did some measurement on the overall velocity of the rifle from full fill (190 bar) down to something around 110 bar.
I used FX Chrono, shot JSB Exact 4.51 mm; 8,44 gr.

The total shot count was around 110 or similar.
Some shots were missed by the chrono for unknown reason so we see 94 down below.

Is this normal for unregulated PCPs?
I find it too much floating, like one shot is 789, the next one drops down to 754 then we go 763 etc ...
Reason I'm asking is, that just recently I watched a review of HW 110 and the guy there had this sort of issues with his HW and so he was sending it for warranty repair a couple of times etc.. in the end he had to fix it himself. That one is regulated though ..

Anyway, from the accuracy point of view, it's very good, it's just the numbers that got me worried but maybe it's normal for a gun without a regulator ...

Thanks!

---

Here are the results:

Shot count: 94
Low: 735
Hi: 791
Avg: 768
Spread: 56
STD Dev: 13.2

---

789
763
784
770
784
784
754
779
759
768
754
784
784
786
759
786
784
784
784
786
756
786
768
759
786
765
756
759
759
772
772
772
765
770
775
752
791
772
789
777
763
768
754
789
759
772
756
763
749
768
770
791
775
756
756
768
786
765
772
772
765
756
761
779
772
768
789
768
770
763
786
772
761
789
747
784
763
754
759
777
754
756
752
749
761
759
763
779
752
747
745
749
754
735

 

[John "spysir" Blount]

I have the old bolt action, better trigger, 16mm shroud AA410 (long cylinder). It was modded for a strangle screw on the transfer port as it was shooting 23+ fpe stock, otherwise no tinkering.
32 shots just under 20fpe within 20fps ES which seemed to be in line what most everyone else was getting.
Air tubes are still available and not too costly, I do know people who have dropped regs in them, not sure of the shot count though.

John

 

[Emu]

That is is the way it shots.

 

[merpo]

if that's the case I can live with that. Accuracy is top notch, I just wasn't sure about the figures.

 

[Sc0-]

Clean the hammer guide and hammer bushings... Any grime or lubricant can cause further fluctuations.

 

[nervoustrig]

The first thing I notice when scanning over the string is that several of the first shots are equal to the peak velocity of 780-790fps. That means there is some number of useful shots available above 190bar. That isn’t a problem per se but when diagnosing a problem, it’s better to be looking at a full string showing both the leading and trailing sections of a bell curve.

Moving on to your core question about the fluctuations, I wish there were a simple answer for you but there are several possibilities that may need to be explored before you find the culprit (or culprits plural). But firstly it’s not a bad idea to consider the possibility the fluctuations aren’t real. Do you have confidence in the chronograph? And equal confidence about the way in which it is being used, e.g. lighting and a consistent angle? Does a 750fps land lower than a 780fps at 30 or 40 yards? By the expected amount predicted by Chairgun or Strelok?

If the velocity fluctuations are indeed real, then…

1. Hammer friction is probably the most obvious possibility. Does the hammer move freely about its full travel? Are both the hammer and its runway clean and smooth? Not necessarily polished but free of burrs and glaring machining marks?
2. How is the hammer lubricated? Oil or grease can cause fluctuations due to their viscosity, or in some cases contribute to an air cushion (or vacuum) effect as the hammer flies forward. Try a dry lube burnished onto the parts (graphite or molybdenum disulfide or tungsten disulfide).
3. Another potential source of hammer friction is sometimes the trigger group. I don’t remember the S400 trigger group clearly enough to speculate on whether it can be adjusted into a marginal state but if it has an overtravel adjuster, it wouldn’t be a bad idea to back it off to rule out the possibility.
4. A breech seal or bolt O-ring that is marginally sealing. That could be because the seal is nicked or worn or cracked. Such swings can occur long before it becomes obvious enough to detect as a puff of air in your face. Try replacing it for good measure. While replacing it, confirm there is a slight interference fit when closing the bolt. If something is out of tolerance, the O-ring may not be getting the little smidge of compression it needs to seal reliably. FWIW I ran into this problem on an S410 a while back and had to sleeve the bolt to bring up its OD so it would work properly.

I hope this helps you home in on it. Good luck and keep us posted on how it goes.

 

[merpo]

Wow, thank you for spending time on such a complex answer to my question. Really appreciate it! I’ll definitely keep you posted.