M16A1 low fps - theory?

[JinxDuh]

21 minutes ago, ButcherBill said:

Is it possible to trim it?

Trimming it runs the risk of ruining any air seal it has and for the sake of saving £6, not really worth it. Once the new parts turn up it’s bound to fix the issue. Seems to just be a nozzle issue now.

[Adolf Hamster]

15 hours ago, JinxDuh said:

To be fair, it's a nasty old brass one so I'll be ordering a nice new shiny one along with the nozzle etc. I also realized when I was talking about nozzles needing different lengths for inner barrels I was actually thinking of the cylinder which I'd read on AK2M4's site. 

 

Tbh, a brass cylinder isn't the worst, i'd take brass over alu any day.

 

The key is internal bore quality, smooth and clean with no scratches, the enigma of e&l cylinders notwithstanding (they look like they should be crap due to machining marks but are anything but)

 

15 hours ago, JinxDuh said:

Luckily I did open it, as my piston was chewed to pieces and didn't look like it would survive much more. Lovely specna quality. Great, until it's not great. 😂

 

Hmm, might be indicative of an issue.

 

Aoe is one option, other is pme although if it's a stock box and motor then probably not the latter.

 

Whilst a lot is said about aoe correction and the whole ethos of stuffing 5mm of sorbo into every gun you touch is kinda bs, it is worth taking some time to look at how the sector engages the piston and that it's a smooth pickup. I like to aim for flat-flat, ie the face of the tooth and the face of the piston meeting with full engagement.

 

A good way can be to "feel" the pickup, assemble the box with the piston, cylinder and geartrain but no spring/tappet plate/col, then turn the box by hand and feel how the initial pickup of the piston is compared to once it's been picked up and being pulled back.

 

Ak2m4 does shims to adjust aoe which are the best method imo, although you want to take care that the piston head can still centre on the piston properly if it needs a bunch of them.

 

13 hours ago, JinxDuh said:

So as I removed the case, the tappet plate had somehow bent itself up, right around where the damage to the piston is. Air seal is all good, replaced the piston and tappet and still all good. FPS is also consistent, just low.

 

Consistent is good.

 

It might be time to consider the spring, if you don't know what strength it is then it might be as simple as it's just wimpy, plenty of pews i've had with fps downgrades have been a case of pull the stock spring, slap in some coiled pasta and call it a day.

 

Ofc springs aren't always going to be what their rating is on the packet, mis-labelling or different box configurations for preload can end up with quite a range, eg having a thrust bearing on both spring guide and piston versus no thrust bearings.

 

That said, what you don't want is to be compensating for leaks by just throwing energy at the problem as it'll just beat up the box with no benefit so the goal is to get yojr energy target with the weakest spring you can.

 

12 hours ago, JinxDuh said:

The longest I have is 25mm, the closest I got to a 20.7mm was 21.03mm

 

Generally the only downside to a longer nozzle is when it gets long enough to cause feeding issues by not clearing the feed tube, might be worth dropping the 21.03 in see what it does.

[JinxDuh]

53 minutes ago, Adolf Hamster said:

 

Tbh, a brass cylinder isn't the worst, i'd take brass over alu any day.

 

The key is internal bore quality, smooth and clean with no scratches, the enigma of e&l cylinders notwithstanding (they look like they should be crap due to machining marks but are anything but)

 

 

Hmm, might be indicative of an issue.

 

Aoe is one option, other is pme although if it's a stock box and motor then probably not the latter.

 

Whilst a lot is said about aoe correction and the whole ethos of stuffing 5mm of sorbo into every gun you touch is kinda bs, it is worth taking some time to look at how the sector engages the piston and that it's a smooth pickup. I like to aim for flat-flat, ie the face of the tooth and the face of the piston meeting with full engagement.

 

A good way can be to "feel" the pickup, assemble the box with the piston, cylinder and geartrain but no spring/tappet plate/col, then turn the box by hand and feel how the initial pickup of the piston is compared to once it's been picked up and being pulled back.

 

Ak2m4 does shims to adjust aoe which are the best method imo, although you want to take care that the piston head can still centre on the piston properly if it needs a bunch of them.

 

 

Consistent is good.

 

It might be time to consider the spring, if you don't know what strength it is then it might be as simple as it's just wimpy, plenty of pews i've had with fps downgrades have been a case of pull the stock spring, slap in some coiled pasta and call it a day.

 

Ofc springs aren't always going to be what their rating is on the packet, mis-labelling or different box configurations for preload can end up with quite a range, eg having a thrust bearing on both spring guide and piston versus no thrust bearings.

 

That said, what you don't want is to be compensating for leaks by just throwing energy at the problem as it'll just beat up the box with no benefit so the goal is to get yojr energy target with the weakest spring you can.

 

 

Generally the only downside to a longer nozzle is when it gets long enough to cause feeding issues by not clearing the feed tube, might be worth dropping the 21.03 in see what it does.

 

This brass cylinder is nasty, like... stored in a shed for years and rusting nasty. It was just a "Hm let's give it a clean and chuck it in to test" case. 

 

Aoe does seem to be a major factor in this as the damage doesn't make sense any other way. The spring inside is an XT 100, so should be giving me the joule output I'd like.

 

I ended up ordering this kit from AK2M4 https://www.ak2m4.co.uk/m4-compression-upgrade-kit with a closed cylinder, a 21mm nozzle and a standard pad to replace the lot. Once it's fitted I'll check the air seal and then AoE. I'm still 90% sure it's just the nozzle but for the price I might as well replace the lot with new parts!

[JinxDuh]

Update:

 

Fitted the kit supplied by AK2M4, sadly the longer nozzle was out of stock so used a short green one. Up to 312 fps on .2g as the spring suggests it should be. Turned out Specna just had some bad air seal I guess 😂