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.

[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.

[ChrisC]

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.

Worth a go Geoff and an easy check..... it might actually be doing something at the block end although i very much doubt that scenario.

[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

[mrgeoff]

Yes, this is for bench rest, but 2 clicks down on the scope seemed to be more than 3mm, but I was running 20x zoom? ( Hawks tac 30, 6-24 scope)
I will try the barrel band off next time I get to the club in 2 weeks.

The reg was set to the instructions above, I even emailed the reg company just to check this was optimum setup for shot count, seemed too good to be true it all dialled in out of the box with no adjustment.

I was wondering if I should restrict the transfer port screw a bit, if I would get any more consistency with slightly lower power? Say 10fbs rather than 11

Although what does the last comment "too much hammer Spring" mean? It needs an explanation. Why does this make the poi shift up with less power? Less mass in the rifle making it jump?

[Blackbaronfish]

When the hammer strikes it comes against the force of the air in the cylinder and lets out a slug of air. As the cylinder pressure reduces the hammer will let a bigger slug of air through.This results in a higher point of impact. This change shouldn't happen in a regulated rifle as the regulator should keep the pressure even!.
What regulator pressure is your gauge showing?
The way forward is to do a full chrono string with weighed pellets. Fill to 210 bar and record the shots.


BBF

[mrgeoff]

Took the barrel band off, sprayed 5 pellets across the target. I did wonder if this was good advice, with the long and seemingly very twangy barrel.

Put the barrel band back on.

When I last tested, I went from approx 160bar down. Today I went from approx 200.

After about 10 shots go get zero again, I started a BR card spot on zero, then the POI dropped gradually over the first 25 pellets, and then after another 15, the POI came back up to zero....
Next was 35 pellets POI on zero, then they started to drop again and 10 pellets later it was off the reg and dropping 2 inches on the last pellet.

So from a full fill, maybe 90 shots which isnt bad, but a very unusual the POI dropped then came back up! If it was non-regulated, it would be the other way round!

Everyone tells me to drop the regulator pressure down from the 125bar that Altaros recommend? Why do Altaros reccomend, and set the reg pressure at 125bar, if its not right?

I guess I have to try it anyway, no one will let me sleep unless I do.