Regulated S400 - odd POI results for 25m benchrest

[mrgeoff]

So, I fitted an Altaros regulator to my S400 (very old, pre-anti tamper)

I put it across the chrony, from 160 bar down. As expected, the fps was fairly consistant, but dropped ever so slightly from the start, down to the regulator set point, and then dropped faster below the reg set point. All good, just on fps variations. From 160 bar to 125 bar, it dropped about 10fps.

Unfortunately, this has not translated into great 25m benchrest results.... So I zero in at 160 bar (my bottle is a bit empty) great, first 5 shots perfect. Then the POI starts to climb, and climb, so after 20 shots I re-set the zero down 2 clicks. Its spot on again, until it comes off the reg, and drops as expected. So I re-fill and the POI is low again!

So I dont understand, why does the POI climb, as the fps slightly decreases on the regulator?


Thanks!

[lensman57]

So, I fitted an Altaros regulator to my S400 (very old, pre-anti tamper)

I put it across the chrony, from 160 bar down. As expected, the fps was fairly consistant, but dropped ever so slightly from the start, down to the regulator set point, and then dropped faster below the reg set point. All good, just on fps variations. From 160 bar to 125 bar, it dropped about 10fps.

Unfortunately, this has not translated into great 25m benchrest results.... So I zero in at 160 bar (my bottle is a bit empty) great, first 5 shots perfect. Then the POI starts to climb, and climb, so after 20 shots I re-set the zero down 2 clicks. Its spot on again, until it comes off the reg, and drops as expected. So I re-fill and the POI is low again!

So I dont understand, why does the POI climb, as the fps slightly decreases on the regulator?


Thanks!

As the FPS drops the trajectory becomes more curvy and hence the rise in the POI. With a faster pellet trajectory is more flat. What is interesting is the amount of shift in POI which seems to be significant. Which version of the Altaros do you have if I may ask? These are are also set to 120 bar from the factory which is slightly too high for our pedestrian guns.

A.G

[mrgeoff]

Its the latest reg, just fitted. Yes, the pressure is set to 120 bar, but the firing pot adjusted to give 11ftlbs with the transfer screw fully open. I was advised this was best for efficiency/shot count.

I understand that the pellet will have more curve when the velocity decreases, but what I dont understand is why the POI should be higher, with a lower velocity. this makes no sense to me.

If the curve is flatter with more power, why is the POI lower with more power???? Its almost like the scope has its close zero at 25m, and its 2nd zero beyond that, but I cant get my head around how to test or adjust for that.

[Vowski]

Might be worth checking the barrel clamp is set correctly?

I had the same problem with my standard S400 classic which had a POI change after refilling

Good luck!

Kev

[ChrisC]

Hi Geoff, do those regs sit internally in the tube or are they added at the end by the block?. I'm wondering whether this change might be inducing POI change in relation to cylinder pressure, possibly moving the barrel a gnats?

It can happen with Steyrs where the barrel goes through the 'A' frame.....quite a few FT/HFT shooters have opened the 'A' frame hole slightly as some were getting POI changes related to cylinder pressure and/or temp changes.

[Gecko]

With a 120bar reg im suprissed you not getting a huge power spike as you first come of the reg, on S400 classics ive found peak pressure to be around 85bar.

[mark112]

With a 120bar reg im suprissed you not getting a huge power spike as you first come of the reg, on S400 classics ive found peak pressure to be around 85bar.

Hi,

Bit of a late response but here goes. My S410k in .22 had peak pressure at 90 bar and a humpback whale of a power curve when I first purchased it. After a change of firing valve and springs peak pressure is now at 120 bar and curve is much, much flatter. Mine, I suspect, had the valve extrusion issue that cloverleaf talks about on this very forum.

My four Air Arms rifles at the time had peak (i.e. very tip of the power curve arc) pressures of:-

S410k .22 120 bar
S410k .177 135 bar
S410C .177 125 bar
S400C .177 115 bar (foolishly now sold ... damn the wife)

So I can see why Altaros set regs to 120 bar.

Mark

[RobF]

Hi Geoff, do those regs sit internally in the tube or are they added at the end by the block?. I'm wondering whether this change might be inducing POI change in relation to cylinder pressure, possibly moving the barrel a gnats?

It can happen with Steyrs where the barrel goes through the 'A' frame.....quite a few FT/HFT shooters have opened the 'A' frame hole slightly as some were getting POI changes related to cylinder pressure and/or temp changes.

I'd investigate this.

[mrgeoff]

Thanks. The reg sits inside the end of the cylinder, next to the firing pot.
So I should remove the o ring from my barrel clamp and give it a go?


Cheers.

[lensman57]

Its the latest reg, just fitted. Yes, the pressure is set to 120 bar, but the firing pot adjusted to give 11ftlbs with the transfer screw fully open. I was advised this was best for efficiency/shot count.

I understand that the pellet will have more curve when the velocity decreases, but what I dont understand is why the POI should be higher, with a lower velocity. this makes no sense to me.

If the curve is flatter with more power, why is the POI lower with more power???? Its almost like the scope has its close zero at 25m, and its 2nd zero beyond that, but I cant get my head around how to test or adjust for that.

The transfer screw should be fully open ( I am surprised that AA employs this crude method of adjusting the power ) with the pot set @ 7.9mm proud from the firing block base. The power will then have to be adjusted with the hammer spring. But investigate the suggestions given first to see if it helps with the problem.

A.G

[mark112]

Hi,

I agree with a previous poster that the POI change is probably due to mechanical movement caused with the increase in fill pressure shifting the barrel somewhat. Try it with the barrel band taken off. POI will be probably be a lot lower but you should see if the POI changes over a fill without it. I take it when you say 2 clicks we are only talking about 1/8" (3mm) difference assuming 1/4 MOA scope clicks.

Mark

[Blackbaronfish]

Too much hammer spring

BBF