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LT gen 2 M4 LT-19B-G2

#1 User is offline   aotsukisho 

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Posted 12 November 2020 - 06:07 PM

Got another commission gun, this time it's a Lancer Tactical 2nd gen M4. LT-19B-G2 to be specific, 10" keymod rail and I believe there were options for low/high FPS - this one is high FPS model, which LT claims 370-395fps (guessing M120 spring).

First impressions of this gun is that it's a plastic fantastic, for better or worse. Plastic receiver, plastic rail system, I think even the buffer tube is plastic to lower production cost. However, this means that the AEG is very light, and the owner has made it a point that he wants to keep it that way.

User has installed a Prometheus purple packing but there are no further performance modifications done.

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Photo of the AEG after receiving it from the owner, with some multicam wrap up front. Covering up keymod is fine by me lol

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Hooked it up to a 2s li-ion to chrono and was met with performance inconsistencies.

Full-auto results showed an FPS range from 323.4 to 370.2, meaning a variance of 46.8fps (14.47%). RPS is around 10, which is on the lower end of stock AEGs, which I've noticed have an RPS of 12-14 out of the box on 2s.

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I fired off a bunch of shots in semi-auto to see if there were timing issues leading to poor full auto performance. I did not bounce off of the min or max FPS values so perhaps this means it is more reliable, but seeing the average FPS only change by 1.6fps (0.47%) it is statistically likely that the gun is performing 'how it should'.

Notice the strange dual-FPS groupings, it seems to fire off shots clustering around both 370fps and 325fps. I would've thought that large FPS variance would be all over the place, but I took some time to plot out the 16 visible data points and the results are kind of surprising

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The variance of the FPS when bundled per 'cluster' is very tight. This data is meaningless and we will need to focus on the actual results (all FPS readings grouped together) to gauge true performance. This is just surprising empirical data for me, perhaps falling into a secundum quid fallacy.

Also, I'm aware that this gun has some games under its belt, but only one out of 16 FPS measurements were in Lancer Tactical's claimed 370-395fps on their product spec page.

Owner has states that he wants this gun built for semiauto play, so we will focus on reducing the cycle time as much as possible.
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#2 User is offline   aotsukisho 

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Posted 13 November 2020 - 05:18 PM

Cracked open the gearbox last night and cleaned it out. Fortunately most of it can be reused, but there are several weak points that can be upgraded for better operation.

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Ready to open up the receivers. Front takedown pin is the screw type, rear takedown pin is just push-through.

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Separated the receivers to see the v2 gearbox, it's pretty neat to see the red/black color scheme inside.

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Looks the same on the other side, very standard M4 setup

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Removing the buffer tube showed a very short receiver post, but the buffer tube internals was standard v2 length (for a long post).

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Opened up the motor grip and they routed the positive side wire incorrectly, it's supposed to come up the same side as the negative and cross over again. Here they just drew it up the front and folded it.

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The 10rps was explained by the motor, which has magnets on the weaker end of the spectrum. Upgrading the motor should be the first thing you do to these LT Gen2s. Motor grip screws could be longer as well, but they'll suffice after being threadlocked.

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Brush dust all over the motor endplate, the motor is likely struggling pretty hard. I don't think it has enough torque to be pulling the spring in the 'high fps' version.

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Rear takedown pin is painted black up top, and is actually knurled to prevent it from loosening itself. I needed a punch to get it out, which is a good thing.

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Gearbox finally free. I'm digging the red/black color scheme even though it's not possible to get them all the exact same hue.

Unfortunately the nylon air nozzle has some casting imperfections so we will swap it out with a POM or aluminum one. Stock nozzle measures 21.24mm long

Another good thing about this gearbox is that the bushings are epoxied in. I'm not sure of the quality but they should hold up to moderate use.

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It's also got a quick change spring guide, which is very welcome. I'm not aware of any standard among QSC design gearboxes regarding the key shape of the spring guide, but my Krytac and ZCI spring guides fit. Retro Arms does not, the wings are slightly too long.

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Cracking open the gearbox reminds me of a CA one. I'm digging the cutout for wires around the motor tower and pinion gear. My only real complaint about this configuration from the factory is that the ARL spring is a bit strong and the trigger spring is a bit weak. Everything functions as it should, although like I said before the compression needs a lot of work.

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Degreasing the gearbox and internal components, drying them out before reassembly.

Those blue SHS pistons are there too since I needed to degrease them before gluing in the steel racks.

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Measured the inner barrel, Lancer Tac claims it's a 6.03mm tightbore so overall it's a 370x6.03mm brass barrel.

I'll check with the owner on what weight bb he wants to run, it might need changing along with the cylinder.

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Stock piston head seals quite well and I will be reusing it so here's an Air-Pad going on for AOE correction and shock absorption

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The gearbox seems to be slightly narrower than Marui spec since the SHS and G&G pistons I had laying around do not freely slide back and forth under their own weight during testing. The LT piston actually has a full metal rack, even though I think it's MIM not solid steel it should still be useable as this will not be a high stress build. I need to replace the horrible LT piston head however, which would not seal under any conditions I threw at it. GMT aluminum piston head, cleaned up the flashing from drilling the vent holes and stuck on a NBR #011 O-ring and it seals up great.

The stock cylinder looks nice but I think it's out of spec as well, the O-ring sits extremely tight inside and binds up the piston's movement slightly. There is no problem with the piston assembly sealing or moving in a ZCI cylinder which is Marui spec.

Looking at the owner's desired build direction, here's a list of potential parts to put in:

- SHS piston head (already installed)
- ZCI QCS spring guide (already on hand)
- M120/SP110 spring
- M4 air nozzle
- 22t neo motor
- 13:1 (13.65) gears
- Steel cutoff lever
- MOSFET controller with a full rewire
- Cylinder, type dependent on target bb weight
- Inner barrel (possibly, depending on target bb weight)
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#3 User is offline   aotsukisho 

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Posted 19 November 2020 - 05:54 PM

Talked to the owner and we are going with a full cylinder build. I will have to replace the cylinder anyway so finding another full cylinder will be cheaper than a ported one.

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Shimmed the hop arm to center it as there was quite a bit of play. Used two 0.1mm gearbox shims on either side

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I was intending to reuse the tappet plate but I found that it doesn't quite fit the Lonex POM air nozzle. Could the Lonex nozzle be out of spec?

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Nope, same story with the Retro Arms air nozzle. This is definitely the tappet plate

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I found a Dream Army nylon tappet plate that also happened to keep the red color scheme that fits the Lonex nozzle perfect. Shame that there was molding issues with the Lancer parts, they look like they'd otherwise do the job fine.

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Inner barrel will be a 455x6.03mm Black Python to save weight, and being undervolumed will have a quieter muzzle report.

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Reused the Prometheus purple hop rubber and nub, very tight fit that required grease to get in properly without binding up. Removed hop unit seal spring, instead I used O-rings to apply pressure against the gearbox in the receiver. This one used quite a bit of O-rings but they are very low cost 70D NBR so it's literal pennies to do so

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Predictably, the inner barrel now pokes out of the flash hider. That extra 55mm is much longer than the birdcage

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Owner had painted over the orange, but this is a standard plastic flash hider. Twisted with an AR wrench and the pin popped itself out

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Plastic and glue residue since I'm guessing it was CA'd in. There was also a translucent material at the base like a crush washer, but it was removed easily as well.

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Cleaned up best I could without using stronger solvents than alcohol

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Teflon tape around the outside of the inner barrel to lock it in, the hop chamber does not fall out when the upper is tipped anymore

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Used a simple GK Tactical 140x35mm suppressor to hide the inner barrel. This one is threaded on both ends for both -14mm CCW and +14mm CW, but the can itself is not reversible (CCW end is threaded such on the can and the base, same with CW). Red loctite on the CCW side, and blue loctite on the CW side so I can remove it to adjust the foam inside, but it won't rattle off during play.

Lower outer barrel has the GKT 120x35mm suppressor, it's actually very nearly the right length but it doesn't look as good in my opinion - I'll keep it for pistol builds instead. I think this rifle could use a shorter outer barrel and longer suppressor like a 180mm ones but the aesthetics are mostly up to the owner, and I'm not sure if the LT outer barrel is proprietary to the handguard like how the G&G ones are.

I will have to order majority of the parts in that list still, I might bump down the gear ratio to 12:1 (12.7) so the gun can be used on 2s lipo and stay competitive on full auto should the need arise. 3s lipo is the way to go however, although may be unbalanced in the crane stock since the MOSFET unit will probably reside in the buffer tube.
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