MK12 DMR project - suggestions appreciated

[suzenonest]

So over the last few months, I've been in the process of upgrading my VFC MK12 SPR into a rifle with more accuracy at longer distances.

The first thing I did was replace the spring with the downgrade spring that came with the gun, correct AoE and change piston head o-ring. After some games, it was shooting good but still lacked some range.

I decided to swap out some stock parts to improve this.

-Prometheus ms135 spring (advertised on ehobby to shoot 380-390)
-SHS tappet plate
-element silent piston head
-5KU double o-ring tapered cylinder head
-modify o-ring air nozzle
-Madbull shark hop up rubber
-Madbull 6.03, 509mm inner barrel

The gun was shooting much better. Never got to really game it tho, since the stock VFC piston stripped. So I swapped it with a madbull polycarb piston.

Now comes my issues...

Even with a fully charged 9.6 NiMh battery, after 5 min of shooting, there's not enough juice left to fully pull back the piston. After some research, I'm concluding it's because this spring is shooting closer to 440-450 fps.

So i'm thinking to either swap back to original spring, which was only shooting approx.360-370fps, or get a stronger battery/motor.

I'm also thinking about going back to the stock piston head/cylinder head + spacer, since I've been reading that the silent piston head/cylinder head is just a gimmick, and I cant have corrected AoE with the silent combo.

I've also read of other mods, like shaving the tappet plate to allow the air seal nozzle to seat farther in the hop up.

So now I'd like to turn to the experts, and hear what they have to say

it would be nice to get more range out of this gun. Otherwise I'd rather play a gun that isnt so long lol

[Stealth]

Originally Posted by suzenonest

Even with a fully charged 9.6 NiMh battery, after 5 min of shooting, there's not enough juice left to fully pull back the piston. After some research, I'm concluding it's because this spring is shooting closer to 440-450 fps.

So i'm thinking to either swap back to original spring, which was only shooting approx.360-370fps, or get a stronger battery/motor.

How's your shimming?

Do you know how much charge your battery actually holds? Ferrous magnet motors are not suited for pulling high-powered springs and are remarkably inefficient at doing so. You're most likely over-drawing from your battery. You basically answered your own question: Consider a stronger battery and/or motor. Also, you may look at using a FET and/or upgrade your wiring to increase your electrical efficiency.

Originally Posted by suzenonest

I'm also thinking about going back to the stock piston head/cylinder head + spacer, since I've been reading that the silent piston head/cylinder head is just a gimmick, and I cant have corrected AoE with the silent combo.

You can get a sorbo for silent piston heads.
http://www.airsoftstore.ca/index.php...products_id=10
Alternatively there are other methods of correcting AoE (plastic washers on your piston head)

Originally Posted by suzenonest

I've also read of other mods, like shaving the tappet plate to allow the air seal nozzle to seat farther in the hop up.

How's your air seal in the first place between nozzle and hop rubber? If your fps is consistent and is hovering around your rated FPS, then this mod is unnecessary.
If you don't own a chrony and aren't able to test all of the changes I'm talking about, then this discussion is moot. Shot-to-shot consistency is the key to accuracy and range. If you can't stabilize the FPS of BBs coming out of the gun with zero hop-up, how do you expect to be able to test consistency of hop-up rubbers? All of my personal AEGs have a maximum variance of +/- 2fps (most of them 1fps), a "good" build should not swing by more than 3fps either way.

Originally Posted by suzenonest

it would be nice to get more range out of this gun. Otherwise I'd rather play a gun that isnt so long lol

There have been a lot of discussions about increasing overall range, most suggestions have been to:
* Establish a consistent perfect seal starting from your piston head o-ring all the way to your hop rubber
* Get a better hop rubber (R-hop, REAPS, PDI W Hold, etc.)
* Use heavier ammo (.3g+ BB)
* Increase FPS (see below)

Originally Posted by suzenonest

it would be nice to get more range out of this gun. Otherwise I'd rather play a gun that isnt so long lol

I hate to beat a dead horse but a long gun (>455mm) != increased range. AK owners have known for ages that 455mm length barrels are the most efficient length barrel out there and watched as Armalite fanboys ran out to buy 509, 590, and 650mm length barrels because they thought 363mm wasn't long enough.
High FPS also does not = increased range either. Time to target is obviously longer and the shots seem to "float" to their target but my 400fps, 363mm R-hopped M4 outranges my much higher-powered 509mm "DMR" noticeably. We can get into a discussion about joules of energy upon target impact when using a 400fps gun vs 450+fps gun at 250 feet and whether the target will "feel" the shots at such extreme ranges, but I think that's outside of the scope of this topic.

[suzenonest]

1600 mah, which I know is really low. But I've always had 2 and never even went through one battery in a game with the other spring. A lipo would make more sense, but then I'd have to invest in different wiring and a new charger.

I'm kinda iffy on that sorbo pad. I've considered it before, but I have a feeling it wouldn't hold up, and would kinda make the whole silent piston head setup useless anyway. Spacing the piston and piston head seems the most practicle

[suzenonest]

So I took the gun apart yesterday and made a few changes:
-swapped back to original spring
-stock vfc piston head
-shaved a little off the front of tappet plate
-modded 5KU tapered cylinder head with orings. Thats right, i got a 3 ring cylinder head :P

crazy compression out of the cylinder now and sounds nice



It shoots just as far now, maybe just lacks the power at longer range. I might leave it this way until I game it once and see how I like it.

[Stealth]

You probably don't want "crazy compression" as it introduces excess drag on the piston o-ring and will actually decrease FPS.

If you're getting compression while moving the piston through the cylinder slowly then you have too much.
Good compression comes from the piston o-ring sealing only when the piston is moving quickly.

[suzenonest]

It moves freely when the air nozzle is not covered

[JLiang]

But when you cover it, does it move in and out freely, slowly?

In other words, when you cover the nozzle, moving the piston in and out at a slow rate, does it give notable resistance, or is it still hard?

Also, did you check your shimming for the gears? That might explain some taxing of the motor and battery, as Stealth said.

[Styrak]

Originally Posted by suzenonest

1600 mah, which I know is really low. But I've always had 2 and never even went through one battery in a game with the other spring.

A mini is not suited to pull much more than a 400fps spring. Doing so will GREATLY lower it's life/shooting time. Even if you had a 1600mAh large, it would be much MUCH better than a 1600mAh mini.

[Dynamo]

that M135 spring should put you up in the 445FPS range. stock gears and motors will strain hard to pull that, and your battery will suffer for it. stock ratio gears can be used up to 400FPS with the use of a good high torque motor. anything above that, and you should look into getting higher ratio gears.
as for airseal, make sure to check the loading nozzle o-ring (most come with them these days). some brands fit kind of loose on the cylinder head nozzle tube, and other really tight. you want the nozzle to have just a bit of resistance to being pulled out. i test mine by cleaning any and all grease from the nozzle and the cylinder head, then tip the assembly down. the nozzle should not slip off the tube on under its own weight.
for the piston head o-ring, you don't want it to drag inside the cylinder. when not compressing air, the piston should move freely with little to no resistance. the o-ring will expand and seal once the cylinder is pressurized, then contract when the piston is being drawn back.
the seal between the hopup & loading nozzle should be good without modifying the tappet plate if you have the correct length nozzle. most of the air leaks around these two parts are not from where they seal. the nozzle will leak around its o-ring, and the hopup will leak around the barrel underneath it. to eliminate leaks around the hopup, use wax coated floss. wrap it around the hopup tightly starting just behind the cut out on the barrel. you don't even need to tie it, once it's installed back into your hop chamber, it'll be held in place.

[suzenonest]

Some good advice here.

The piston does move freely when pulling back, but with the cylinder head is plugged with my finger, it stops moving forward, therefore good compression. This is with the air nozzle on and off.

I will be getting new gears soon, torque up helical. My shimming is as good as it gets with the self shimming gears. The bevel gear needed the most shimming. I just added some shims to the sector and spur gear because the spring lost some tension. I put the shell back together and the gears spun freely with no drag or play.

I have a hard time getting the barrel and hop up sleeve into the hop up chamber without it getting all bunched up. It's in there now, but could be sitting better. I lube when I put it In, lol. Can the inside of the hop up be opened up with a round dremel bit? Or is that a no no?

[Rabbit]

I wouldn't dremel anything in the hop up chamber - customizing stuff with a dremel has too much margin for error and will more than likely cause an airleak and you'll just end up throwing the part out.

For the hop up sleeve i use SPIT. simple enough. It dries and leaves no greasy residue - anyone who lubes their hop up insides with some form of grease is a boner. It ruins the friction the nub/sleeve combo when contacting the bb - however some will argue but i've read plenty of posts that say its a no no on here.

I also remember txting you saying that spring is too stiff and if ANYTHING - to stretch it out a bit, hence the appearance of your battering dying - it just wasn't able to pull it.

[suzenonest]

I only lube the outside of the inner barrel and the outside of the sleeve. But nowhere near the actual point of bb contact

Also how would stretching the spring help? If anything wouldn't that just make it harder to pull?

[Rabbit]

Well, you just pull it from both ends.

Think about when you were in school and you'd dismantle a pen and get to the spring - you would pull that shit from both ends and the coil would strech out and there wouldn't be a lot of tension making that spring/bounce weaker.

If you compare most aftermarket springs to stock springs - you'll notice they are smaller in length and a tighter coil.

Mind you - this probably wont make much of a difference and you're probably just better off buying an s110 / s120 to get you around 400fps.

[suzenonest]

Haha I'm under the impression it would be the exact opposite. The longer the spring (more stretched) would require more work to compress to the same size, and put out more power

[ThunderCactus]

I've read nothing here so forgive me if I'm repeating you guys

Better motor + mosfet = ridiculously good battery efficiency
torque or double torque gears would be best for 400-440fps range on a 9.6v battery
consider upgrading to a 7.2v lipo with the standard gear ratio if you don't want a new gearset. You get significantly more amperage out of a lipo vs 1800mah NiMH, therefore more torque.
ehobby is really bad for rating their spring 100fps under their actual performance, please go by the ASC spring chart or reviews!
highly recommend Falcon or firefly hop rubber
highly recommend ARS piston head/cylinder head, they get the best performance I've seen of any heads
Use .3g BBs up to 440fps, .32 if you can get those, .28s if you're cheap
And the 509mm barrel works, but 420 is about where your accuracy peaks, anything past that is just a waste. 6.03 is the perfect bore IMO though