New S400 classic strange chrono results

[BJS]

The rifle is just a month old, I purchased it from my local RFD who chrono’d it while I was there and it averaged out at 11.2ftlb - happy days!

After a few days I did a full shot string from 190bar, 90 shots (jsb exact) before it started to fall, averaged out at 11.3 with a spread of only 18fps - even more happy days!

I then found that the barrel seemed to love QYS streamline 9.56 and I was getting amazing groups. I then chrono’d the gun using the QYS and I was just over 12. Thinking my chrono was broke I used someone else’s chrono with the same result.

Today I went back to my RFD and we did a 5 shot test on their Skan with the gun filled to 160bar, the results were:

AA 8.4
1. 11.20
2. 11.25
3. 11.25
4. 11.17
5. 11.25

QYS Streamline 9.56
1. 12.33
2. 12.19
3. 12.19
4. 12.33
5. 12.19

Obviously I am getting the RFD to lower the power slightly, but I would be really grateful if someone could explain why this is happening. We tried different guns in the shop and got the similar results, we also tried heavier pellets than the QYS and they were well within the legal limit.
Thanks in advance,
Barry

[tonyc]

I'm not surprised. I've tested about 90 different pellets/batches in my S400 Classic and found the power peaks with pellets in the 9.5-10.5gr range. The highest were JSB Heavy 4.51 (10.34gr) but H&N Hunter Extreme (9.57gr) were close. Generally the looser and softer pellets are faster - 4.51 faster than 4.52, JSBs faster than H&Ns at the same weight - but I've not tested many 4.50 as the accuracy and consistency is not good in my guns. I'm guessing the QYS are 4.50 and low in antimony so fairly soft.

One thing I suggest you check is the actual weight of the QYS. They may be lighter than specified.

[BJS]

Thanks Tony, that theory seems to ring true with my testing, yes, the qys streamline are 4.5, I really want to stick with them so I’ll get the power turned down a touch...

[tonyc]

I'd check the weights first. Until you mentioned it, I'd have thought 11.2ft lb with AADF or JSB 8.44gr is ideal for a new gun, and it will drop off a little. So I wouldn't adjust it without first confirming those QYS numbers are correct.

[BJS]

I’ve checked the weights, they ranged from 9.4-9.6 on my scales, I have separated some out and re-tested putting the weighed figures into the chrono and it’s still 12 to 12.3, I’m stumped! I called Air Arms and asked to speak to a tech, basically his answer was don’t use them (very technical answer that....)

I’m just going to turn the gun down to around 11.5 with the QYS Streamlines, I’ll be in the 10s if I decide to use a different pellet but that’s fine, I’m not chasing the full 12 I just want to stay the right side of legal....

[tonyc]

A sensible move, I think.
My S400 Classic is putting out 10.8ft lb with AA DF and due for a service. I'd be setting it at 11.3 but I think I might try some QYS first!

[BJS]

Try the streamline 9.56, I think you may see 12s from 11.3!