Variable power hw99

[Damned 69]

One of my 99s (22) is causing imense frustration, checking power with Ftts 11.3, with RS 10.3 but with hobby 11.9!. My other 99 also 22 gives relatively constant power at 11.3-11.6 with same pellets , similarly my Hw30 remains constant with same pellets 7.7-7.9 so any ideas as to why one 99 can give such very variable power? Have stripped a few times and nothing appears amiss - good seals, bumslide lubed, spring, guides top hat all look good. Both 99s have had tins of pellets through them and I have cleaned bore in case such was cause.

[madcarlos]

One of my 99s (22) is causing imense frustration, checking power with Ftts 11.3, with RS 10.3 but with hobby 11.9!. My other 99 also 22 gives relatively constant power at 11.3-11.6 with same pellets , similarly my Hw30 remains constant with same pellets 7.7-7.9 so any ideas as to why one 99 can give such very variable power? Have stripped a few times and nothing appears amiss - good seals, bumslide lubed, spring, guides top hat all look good. Both 99s have had tins of pellets through them and I have cleaned bore in case such was cause.

Some guns may need a good few pellets through before settling down, I would shoot 10 or 20 pellets through it then try it through a chrony, I have 3 x mk1 80s , 2 of them after the first pellet or 2 settles down with hardly any variance, The other one tends te shoot late 11s for the first 8/10 pellets before dropping around 11.2 Some will take 1 or 2 shots some may take a good few more , Regular shooting normally wears them in quicker,

[maximus]

Probably just due to a particular pellets fit in the barrel. Being light you might expect the RS to give good muzzle energy like the hobby but there is obviously a difference which I think must be how well the pellets are sealing in the bore.

[averageplinker]

I reckon it is either the spring and/or barrel. If you can be bothered try swapping barrels over and see what happens. If that makes no difference try swapping the springs over.
Rich.

[Damned 69]

Have swapped springs and also tried a titan 10. Never thought about barrel but assume different batches and possible minute differences? - older one has registration number stamped on lhs whilst new one has it stamped on bottom.

[harvey_s]

Kind of unclear what you mean really...
It's common for guns to give different power outputs with different pellets.
As long as the same make/model of pellets gives the same power it's not a problem.

[Damned 69]

My query was that with one 99 the power spread between 3pellet types was relatively close, whilst the spread with my other 99 using same 3 pellets was what I would consider rather large at 1.6ft/lbs.

[averageplinker]

I guess another possibility could be a different transfer port size. Have either been adjusted?
Rich.

[Damned 69]

Both have Tbt top hat / guides /slip washer, original HW weights removed, spring ends polished. Seals are good. Older spring approx 5mm shorter than newer one - assumed such 'settled' . Purchased second one to use as self challenge with just open sights. So I guess it's just one of the curious quirks of air rifles.

[BTDT]

One of my 99s (22) is causing imense frustration, checking power with Ftts 11.3, with RS 10.3 but with hobby 11.9!.

One thing that's guaranteed to give those results is autoignition.

The highest start pressure pellet, the Hobby, gains most from an early start thanks to a spike in cylinder pressure/temperature from autoignitrion, the second highest start pressure pellet (around 60% that of the Hobby), the FTT, gains less, and the low start pressure pellet (the RS, around 20% that of the Hobby) is too far up the barrel to gain the full effect from peak cylinder pressure.

[harvey_s]

My query was that with one 99 the power spread between 3pellet types was relatively close, whilst the spread with my other 99 using same 3 pellets was what I would consider rather large at 1.6ft/lbs.

Ah, understand now...if it helps my own .22 certainly shows that kind of difference between say RWS Superdomes & JSB Exact.

[Damned 69]

BDTB - thank you for very informative reply. I guess that I may have not been as carefull on my reassembly of my misbehaving gun and that maybe some lube has managed to get in front of piston causing a degree of dieselling/detonating to a degree (never been sure of the real difference as both are degree of combustion) and unlike my old meteor which would give obvious tell tale puffs till settled down I didn't detect such. Shooting at range tomorrow and will check power on return in hope that surplus lube 'burned' off - if not then strip down. Once again thank you.

[Damned 69]

Appologies for the 3 degrees

[TonyL]

I reckon it is either the spring and/or barrel. If you can be bothered try swapping barrels over and see what happens. If that makes no difference try swapping the springs over.
Rich.

One thing that's guaranteed to give those results is autoignition.

The highest start pressure pellet, the Hobby, gains most from an early start thanks to a spike in cylinder pressure/temperature from autoignitrion, the second highest start pressure pellet (around 60% that of the Hobby), the FTT, gains less, and the low start pressure pellet (the RS, around 20% that of the Hobby) is too far up the barrel to gain the full effect from peak cylinder pressure.

Personally I wouldn't be getting too hung up about it. Many rifles can show fairly wide variations in muzzle energy with different pellet types, although I accept that your other, seemingly identical, rifle behaves differently.

If it was going to be something that you wanted to pursue, I think the answer lies somewhere in BTDT's or averageplinker's theses, the auto-ignition phenomenon very possibly being the likely cause. Or, maybe, just that most minute difference in barrels or spring force.

[BTDT]

BDTB - thank you for very informative reply. I guess that I may have not been as carefull on my reassembly of my misbehaving gun and that maybe some lube has managed to get in front of piston causing a degree of dieselling/detonating to a degree (never been sure of the real difference as both are degree of combustion) and unlike my old meteor which would give obvious tell tale puffs till settled down I didn't detect such. Shooting at range tomorrow and will check power on return in hope that surplus lube 'burned' off - if not then strip down. Once again thank you.

Just to clarify, I'm not saying the issue is necessarily connected with autoignition, just that the symptoms can be caused by it, and it's a strong possibility.