AAP carbine – failure to feed at 15c but cycles fine?

[The_Lord_Poncho]

So I have recently built up an AAP carbine – based around the TTI PCC kit, with a 210mm inner barrel. In terms of working parts, the only upgrade has been to add in a 150% nozzle return spring.

In testing, everything worked absolutely fine – reliable feeding, good accuracy/range, and shooting at about 1.2 j (its semi locked). However, in its first game at the weekend, it initially chronoed at 0.7J for a couple of shots, and thereafter entirely failed to feed on any of my mags. Weirdly the bolt still happily cycled back and forth with every shot, it just wouldn’t feed. It was circa 14/15 oc, and the mags were filled with pure propane from a plumbers bottle. Back home at the end of the day – everything now feeding happily again at circa 20 oc.

I know that gas guns don’t do well in the cold, but at 15 oc surely it isn’t that cold? And why would the bolt still be happily cycling but the gun not feeding – this doesn’t make a lot of sense to me?

Any thoughts welcome + any potential solutions!

 

[Cr0-Magnon]

15c is certainly getting on the cooler side for GBB's, especially if you're out all-day using it. Initially it sounds like it's just that bit too cold for the gas to bring the bolt back far enough to load a BB. The big drop in power also demonstrates it's suffering negative affects of the ambient temperature.

 

[The_Lord_Poncho]

Thanks ! I'll order some MAP gas, and give that a go then. Would swapping to a lighter bolt help matters (e.g. the 5KU lightweight bolt)?

Do we think that the 150% nozzle return spring is helping or hindering?

 

[Skara]

15° isn't warm but it's still well within the operating temperature range of an AAP.

Both of mine work flawlessly down to near 0° temps on WE mags and Green Gas.

The "upgraded" nozzle return spring doesn't do anything for feeding except pulling the nozzle away from the hop chamber faster/harder.

Did the gun lock back on an empty mag on the field? Which mags are you using?

AA mags aren't that great, they behave well with TM guns but on the AAP they tend to lose some power (compared to WE mags).

 

[The_Lord_Poncho]

I was using a mixture of mags- the original Action Army one, three army armaments, two We standard sized, two we extended. They all behaved the same, albeit the extended ones did feed a few times first before failing....

I'm wondering if somehow the TTI receiver is two tight, causing the metal to shrink slightly when it gets cold, and rub on the bolt- just enough to stop it fully retracting to pick up a BB? Albeit I didn't notice a loss of the full range of movement of the bolt, and it wouldn't even fire the first BB after manually cocking it...

 

[Sewdhull]

Propane should be giving you 7 bar, around 90 psi at 15C, MAPP might give you another 20 psi as its only 50% propene. Each shot will reduce that percentage until there's no propene left, just propane and the other 'p' gas. Pressure will reduce as the mag is used.

Propane should be enough at 7 bar...

 

[The_Lord_Poncho]

Hmm - that's interesting. So what you are saying is, the MAPP gas isn't likely to be a workable solution, at least not for the second half of the mag? I would have presumed that the gas would stay mixed and the different components of the gas vent equally through the mag's load.

I think I need to look at more of the mechanical side of things then, and work out why, (only) when cold, it's not scooping a new BB from the mag.

 

[Sewdhull]

Mixed gasses with differing vapour pressures evaporate unequally. 

Some camping gasses mix butane and propane so that the butane is squirted out with the propane, but if it used in cold conditions the butane is left behind and the propane evaporates using all the propane before the butane is exhausted.

MAPP is a hotter gas than propane and not formulated for pressure reasons. The pressure you will see in your Mags is the propene pressure which is not massively more than propane.

Whilst the outside temp might be 15C you will see alot of cooling even after the first shot if you have gas supply, over supply, issues. 

I have an AAP and it cycles just fine at 15C on green gas.

 

[The_Lord_Poncho]

Thank you for the explanation, this makes sense!  I think i am going to strip it down, sand and polish the inside of the receiver where the bolt tracks, check the barrel and hop is seated correctly, and re-tighten the upper (TTI carbine) receiver to the lower, in case the mags aren't sitting quite right for some reason. I might also try withy a range of different BB's in case the .32g Vorsk i was using aren't playing ball when the metal work gets a little colder than room temperature.

Trouble is, i need to wait until a coldish day outside to try to replicate the game conditions again!

 

[Sewdhull]

Put the mag in the fridge in a sealed bag.

Or part fill it and empty the gas out. That will chill it.

 

[The_Lord_Poncho]

I managed to do a little more testing yesterday. Conclusions:

 - WE mags producing around 80-FPS higher than Army Armament mags and the original Action Army mag that came with the AAP.

  - The Vorsk .32 bb's don't seem to feed as well as alternatives (ASG blaster .25)

 - Mags generally aren't feeding when filled to capacity. 75% filled mags are feeding better.

any thoughts?

 

[Piman]

What bucking (brand and grade) and nozzle are you using? Are they the orignals or aftermarket?

The AA mags have a poor reputation with many commenting on their poor performance. I also found them them bad with my AAP-01 and so I now only use WE or TM.

Also don't underestimate the impact to performance and reliabilty due to low quality BBs.

James

 

[The_Lord_Poncho]

Thanks Piman. To be fair, I hadn't even considered the bucking - I'm not sure why not (its my first thought for feeding issues in an AEG). The bucking I'm using is by Unicorn Airsoft - it came packaged with the unicorn inner barrel. Looking  it up online, seems like the bucking is a 60 degree. The nozzle is the original stock AAP one. Whilst the Unicorn Bucking seems reasonably well regarded, perhaps  ought to swap it for a Maple Leaf and see if that makes a difference to the feeding....