Some HFT 500 info.

[mikeygasbag]

I’m putting this post up as it may be of interest to Air Arms HFT 500 owners.

After experiencing a little frustration with not seeming to be able to get at least 40 consistent shots with my HFT 500, I decided it would be best to spend some time shooting a fill over the chrono. Since buying the rifle, I have been filling it to around 175 BAR for competitions.

Right, the sun was shining this afternoon, so what better time to get stuck in. First I adjusted and set the power to 765 fps, using prepared JSB Exact 4.52 pellets, whilst the on-board pressure gauge read 150 BAR. Then I topped the rifle back up to 180 BAR, read from the divers air cylinder gauge. At this point, the on-board pressure gauge was reading almost 200 BAR. So the first thing I’ve learned is that the on-board gauge is somewhat optimistic and reads around +20 BAR.

Then I proceeded with a string from 180 BAR down to where my Combro indicated a drop off of power output. I carried out the test by firing a pellet, taking the chrono reading and writing it down. I then dry fired four shots. Thus, the first shot was recorded, then shot 5, then shot 10, then 15 and then 20 and so on.

756, 759, 759, 760, 758, 763, 763, 768, 766, 763, 768, 769, 772, 769, 769, 771, 762, 765, 762, 763, 766, 761, 758, 752, 746.

Interesting to note that I also checked the on-board gauge around every 20 shots or so, and found that my gauge is a bit like a car petrol gauge, in as much as it doesn’t seem to drop very quickly, then when it eventually reaches over halfway, drops down very quickly. So I won’t be using the on-board gauge to help me with topping up procedures. Just out of interest, the on-board gauge read 175 BAR after 25 shots, then 170 Bar after 40 shots, 150 BAR after 60 shots, 140 BAR after 80 shots, 130 BAR after 100 and 120 BAR after 120 shots. I guess that the pressure inside the air cylinder was more like 100 BAR as the shot to shot power started to drop quite rapidly. If my calculations are correct, I got 120 shots out of using 60 BAR of air. Can that be right? That seems very frugal to me.

Anyway, the whole purpose of this exercise was to get some idea of shot count and also discover where the sweet spot is on my HFT 500. It’s looking like I will be filling to 150 BAR only, which should give me around 50 decent shots to play with, before needing to top up again. Again, just out of interest, the power deviation over 50 shots within this sweet spot was 762 fps lowest and 772 fps highest. So, that’s 10 fps variation over 50 shots. If you take away the one highest and one lowest reading, which I believe is the correct way to calculate fps deviation, then it's only an 8 fps deviation over 48 shots! All this with an un-regulated rifle. Mind you, I have cleaned, smoothed and polished all the internal moving parts.

Mike.

[wezdavo]

Good job, at least now you can be more confident with your setup, the only down side is....... You will now have to take the blame when you miss

To find the sweet spot I leave the rifle connected to the bottle with the fill valve shut off, without bleeding obviously...

I find it much more accurate to read the pressure from the dive bottle, takes no time at all to find where the sweet spot starts and ends..