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Gifted a Classic Army: need advice on upgrade replacements

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Good evening, thank you for taking a minute to read and or help. My 13yo son was gifted an old classic army M15A4 rifle. We immediately tore it apart and identified a few things that should be addressed before use in the field. Now with a week of troubleshooting, learning, reading, testing, and searching online parts we're moving ahead with what purchasing and forums are a great place for advice...


Identified issues:

1. We're missing the spring assy. and the C-clamp from the hop up unit.

2. It's shooting about 30 feet and a very low FPS (will not puncture cardboard box at 30').

3. Bucking is pretty old, not deformed that I can tell.

4. Battery has been replaced

5. Gear on motor inside pistol grip is worn/chipped at center of splines ///I\\\ but no issues with skipping

6. thorough cleaning and lubrication



New hop up

New bucking

New tight bore inner barrel (.603) possibly

anything else?


We need some insight on what works best with this weapon and what has worked best for you. We have some ideas and have already searched Evite, AirsoftGI, amazon as well as googling several others for pricing and reviews. We think asking a larger forum might also help making the best possible choice on the subject. Please tell us what you think if you have a minute to help. We both appreciate your time and thank you for supporting the next generation of airsoft players.


Richard & Ayden


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Guest alberty

Great, it's good to know you're learning the internals.


1. Replacement c-clamp and springs are fairly easy to get for M4 style guns. You can either get those individual parts, or source them from another hop-up unit. For the spring itself, you could almost substitute any random spring.


2. For the low FPS, that could be many possibilities since it's a whole system that works together. For example, some random possibilities:


- stripped piston

- worn/broken bucking (or packing)

- inner barrel not seated correctly in hop-up unit

- hop-up is adjusted too high

- air nozzle is damaged

- etc!


You'll have to do more investigation of each part and that is involved in generating that power to send the BB flying. Please note, these ones I listed are not necessarily your problem, I cannot tell without actually seeing your parts in person.


3. Is it sort of dried out and stiff?


4. Should be OK.


5. That's the pinion gear. If it's bad, it could be replaced individually, or you could considering upgrading to a more powerful torque motor.






- You don't need a new hop-up unit if you can source the missing parts alone. Sometimes switching to a new hop-up unit could cause new problems, be aware.

- New bucking if your current one doesn't work or seems to be not working well

Edited by alberty

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So hold off on the TBB. I can do that.


The FPS still eludes me though...

- Not sure how to tell if the piston is stripped but it is firing somewhat.

- Bucking is just old. Not brittle or deformed but old.

- Inner barrel should be installed correctly aside for the clip that holds the hop-up and barrel together as a unit. (my best guess to be the main issue at hand)

- Hop-up setting... we shot a few hundred rounds and adjusted as we went with very little difference from high to low.

- Air nozzle damage isn't noticeable that I can see unless I missed a crack. Not chipped, dented or deformed currently.


I may check to see if anyone at the next event has a clip I can purchase or I'll just have to hold off. The pinion gear (thank you) is good for now but is likely to be replaced. If I'm replacing the gear or motor or anything major I'm likely to replace much of the insides to give it a little extra boost. Why not...


Thanks again gentlemen. Please let me know if there are good options to purchase. Any advice is better than nothing.

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Could it also be the way the inner barrel is seated in the hopup unit causing compression issues? I suppose it wouldn't be that bad, but it's one thing to look at. Try some compression tests and see how that goes. But, just in case, I would get all the new parts you need, and then look at FPS because it could be one of those that's giving you problems. Like Guges said also, check out the cylinder in case someone(or even you) put it in backwards. I've taken my gearbox apart probably 8 times, and just recently I forgot to attach the tappet spring, so I wouldn't be surprised if someone reassembled some things wrong.

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