A&K M249 Hop improvements

[Docv400]

I have a couple of A&K M249s in for some work, one (MKI) primarily to improve the pathetic hop performance, the other (Para) for a rewire, MOSFET, Deans etc.

I've done the longer, firmer nub mod, but felt it could be improved further.

Remembering my experience with one of the STAR versions, where there was a 1.5mm gap between the hop rubber and the nozzle, I looked at these, and one had a slight gap, maybe 0.5mm, the other was better but the inner barrel had approx 0.5mm fore and aft movement due to the flimsy retainer clip.

I machined off approx 1mm from the rear shoulder of the hop unit on the worst one, so it sits further back, the nozzle now just slips inside the rubber.

I also had to open out the feed hole slightly to allow it to feed reliably.

Chronoed it, and it's giving 408fps!

I'm waiting to hear from the owner what it was before...

 

On the second one, I only took off approx 0.7mm.

It's giving 365fps now, before it was at 310~ish.

I also modified the hop with a grubscrew to retain the inner barrel (I really hate those plastic clips ).

I machined a small flat on the barrel for the grubscrew to bear on, keeping it perfectly aligned.

I also had to cut a section out of the brass ring that centralises the barrel in the outer.

It's solid as a rock now

[TwinTurboCH]

Some nice ideas there, I'll be trying them on my M249 as soon as I can get over to my mate's house (with his very nice lathe )

[Docv400]

Ideally, you'll need the use of a Milling Machine as well .

 

The MKI has a flat-nosed hop rubber, without the formed lip of the 'normal' AEG rubber, I'm assuming that's the original fitment?

 

The nozzle also has a much flatter end than the more usual tapered item of most AEGs.

The Para has an aftermarket hop rubber (Red), which I'm sure is the reason for the excessive end-float of the inner barrel.

 

With the barrel fully inserted in the hop unit, the locating slots were right at the front of the retaining clip slot in the unit, whereas the flat 'lugs' on the clip are central, meaning the barrel had to be moved back out slightly for the clip to lock in place.

[morugatu]

Out of curiosity, how did you modify your hop up nub? Also, do you think using a harder rubber (like a promy hard bucking) will be a better choice over the soft rubber I have in it right now?

 

My mod was to toss the nub into a spare parts bin, take an ink tube from a pen, cut it to the length I wanted, and inserted it instead. After two attempts I got it right. My only concern is that the rough plastic and the high velocity the bbs are going at is going to tax the soft SRC rubber too much.

 

I really like your idea for the fitment issue (it is most likely the aftermarket hop up rubber as I have the same problem with the SRC rubber I have installed), I'll try to machine mine as well since I have a relative with tons of equipment in his packed garage.

 

Sort of noobish question, how does one measure how much is needed to take off? I just want to be absolutely sure before taking any sort of power tool to it.

[Docv400]

Out of curiosity, how did you modify your hop up nub? Also, do you think using a harder rubber (like a promy hard bucking) will be a better choice over the soft rubber I have in it right now?

 

My mod was to toss the nub into a spare parts bin, take an ink tube from a pen, cut it to the length I wanted, and inserted it instead. After two attempts I got it right. My only concern is that the rough plastic and the high velocity the bbs are going at is going to tax the soft SRC rubber too much.

Exactly the same as you did .

I usually replace the nub with a section cut from a suitably sized o-ring, but with the way the nub is set vertically in these, I thought a more 'solid' option would be better.

I cut the plastic tube by putting a piece in my Dremel chuck, switched it on, then used a craft knife to cut it to length.

By resting the blade against the face of the chuck, and holding it at a slight angle, you can get a very smooth and slightly rounded end on it.

I don't think you'll have a problem with it damaging the rubber if you do this.

Up to around 350 fps, I'd stick with a softer rubber.

 

Sort of noobish question, how does one measure how much is needed to take off? I just want to be absolutely sure before taking any sort of power tool to it.

I just did it 'by eye'.

Do the inner barrel retaining screw first (if you are going to do that), as that will determine how much you need to remove from the hop unit rear end, you may find that alone is enough to give you a good seal.

 

If you need to machine the unit, take approx 0.5mm off and try it, you only need to place the parts together by hand to check, you don't have to assemble the whole gun each time.

Do it with the gearbox out of the gun, then when you're happy with the nozzle contact with the rubber, assemble it in the gun to check the alignment of the feed hole in the magwell casting.

On the MKI, I needed to open out the front slightly, and just trim a tiny bit off the hop rubber corner (It has the square-ended rubber).

The Para has a 'normal' rubber and didn't need any trimming.

 

I've just done the retainer mod to the MKI as well...

[cjfield1129]

Did you consider trying a longer nozzle for the hopup/nozzle fitment issue?

[morugatu]

Any longer and it doesn't feed correctly. At least in my setup it doesn't.

 

Thanks Doc! That was a great help! I'll get on it and if I'm not too lazy, I'll throw some photos up as well. No promises.

 

EDIT: One question more, slightly off topic, but you mentioned cutting o-rings to replace nubs in normal AEG platforms? A link or a short explanation would be greatly appreciated!

[Docv400]

Did you consider trying a longer nozzle for the hopup/nozzle fitment issue?

Any longer and it doesn't feed correctly...

That's correct.

The front end of the nozzle is flush with the rear of the feed hole, any further forward (i.e. longer) and it would block the passage of the BBs into the hop unit.

[ertgunner]

very nice i was looking for a way to replace the clip ... i put 2 grub screws on the hopup rubber but it tossed the barrel off to a side i would have never thought to put it on the ring

 

thanks i will change my setup to something similar to yours

 

EDIT:

one question why did you cut the stabilizer ring ? (mines is a coil of wire looking almost and it can not be cut)

[Docv400]

...one question, why did you cut the stabilizer ring ?

Because it sits right under the grubscrew, and I wanted the grubscrew to bear directly onto the flat I made on the barrel.

You could just tighten the grubscrew down onto the ring and barrel, ensuring the barrel was aligned correctly (rotationally).

[appslapp]

The barrelock tends to wear out, and make the barrel(and the hopup) to drift forward a few mm........I always screw the barrel tight in to the receiver.

[Docv400]

Do you mean the outer barrel locking mech?

As you're aware, the hop unit is held against the gearbox by a fairly strong spring, which is compressed by at least 6 or 7mm when the outer barrel is fitted, so even if the outer were to move forward a mm or so, the hop unit would still be held securely enough.

 

It might be a problem if the wrong spring was fitted (too weak or too short, or both), or possibly if the alloy sleeve that sits in front of it were missing.

 

The main problem with the latch, as I see it, is that the groove that the roller on the latch arm locates in is too shallow, and totally the wrong shape.

The roller is approx 7mm diameter (by only around 4mm wide), yet the groove is approx 1mm deep, if that, and approx 5mm wide.

I have a nice little radius cutting tool for my lathe, which is around the same diameter as the roller, so I may look into opening out the groove with that at some point, or maybe just Milling a groove in the appropriate position with a ball-nosed cutter.

 

EDIT: One question more, slightly off topic, but you mentioned cutting o-rings to replace nubs in normal AEG platforms? A link or a short explanation would be greatly appreciated!

Sorry morugatu, you edited this just after I'd posted my reply...

The standard hollow nubs are just too 'squishy' (sorry to use such overly technical terms ) to give consistent results I reckon.

I'm sure they're used to make the hop 'idiot-proof', because even if you wind the hop full on with one fitted, it rarely causes a jam.

Replacing it with a section cut from an o-ring, with a cross-section of around 3.5mm gives much more consistent performance I find, you don't have to adjust the hop for ages once it's set.

You do, however, have to be careful not to over adjust the hop, as it will cause a jam.

Never adjust the hop until it's properly 'warmed up' (by firing off at least 40-50 shots in Auto).

If you adjust it 'cold', it'll over-hop, or possibly jam, once it's had a few dozen shots through it.

The only exception is possibly a Sniper rifle, the hop on those never really get 'warmed up' in a game, only maybe when test-firing or zeroing a scope.

 

Use an o-ring with an overall diameter of at least 50mm or so, if you use a small one, the section you cut will have too much of a curve on it, which can affect the hop performance.

You can buy lengths of o-ring 'cord', it's used for making custom-sized rings.

Example...

[Danke]

The barrelock tends to wear out, and make the barrel(and the hopup) to drift forward a few mm........I always screw the barrel tight in to the receiver.

 

I think you mean the quick change outer barrel ejects, that was happening to mine.

 

There are 2 grub screws that sit in the outer barrel, and 2 cosmetic dome head bolts on the receiver opposite them. You remove all four and source longer grub screws, line up by eye and thread the grubscrews in from the outside pinning the receiver and barrel together.

 

Don't go too long though so when you've got them in flip up the feed cover and push the hopup to be sure you still have movement against the spring. Then put the outer dome bolts back on.

[ertgunner]

wow i seems to have over tightened the screw and now it has a dent in my barrel gladly i was going to switch over to a stock one because i wanted it to spread a little more

[Docv400]

Has it actually deformed the inner bore?

Look down the barrel with the end pointing at a strong light, you'll be able to see if there's any deformation of the inner bore.

[ertgunner]

i dont see it but the BBS don't rill smoothly anymore

[slyguyfry]

This thread is full win. The grub screw is genius. When you machined the "key way" flat where the screw touches the barrel, does it allow the barrel to be adjusted front to back or is the screw held by the flats? Also, how do you when you check the engagement of the hop up rubber and nozzle? Do you just feel and check the engagement by looking through the feed hole or do you have another way? Can't wait to through my star m60 hop on my mill I have always been getting inconsistent results due to the clip.

[Docv400]

The flat is large enough to allow some front and rear adjustment if needed.

You could make it as long as neccessary of course.

I assembled the inner barrel as far into the hop unit as it would go, then wound the grubscrew down onto it to make a mark, then Milled the flat where the mark was.

 

You can feel when the nozzle contacts the hop rubber, I just did it with the gearbox and hop unit/barrel in my hands.

The rear end of the hop rubber should just be visible at the forward edge of the feed opening when it's fitted, you can see if the nozzle goes far enough when you slip the hop unit into the gearbox (ensuring of course that the nozzle is fully extended, by removing the spring and winding the gears by hand).

 

Does the STAR M60 have a plastic hop, like their M249?

If it does you may be interested in this...

STAR M249 hop fix...

[Docv400]

Another little mod I did was to the spring retainer on the MK1.

I machined a second groove in front of the original, so it can be set further back, and machined the back of it down so it doesn't protrude beyond the rear face of the gearbox.

On the front slot (low setting), it gives 350 fps, and on the rear one (high) 390.

So now I can trim the spring (or fit another) to get around 310 (for lower site limits) and 350 for the owner's site limit (or whatever he wants it set at).

[Docv400]

I have another A&K M249 here at the moment for a little work.

The hop is fine on this one, the nozzle contacts the rubber OK.

I was interested to find it's had a grubscrew fitted since I had it in last time.

It's fitted it underneath, so the point of the grubscrew (if you use a pointed, as opposed to a flat-nosed grubscrew) locates in the hop rubber locating slot in the inner barrel.

I might do any further ones like this, although it would be better if the screw was in the section forward of the slot (for the original clip).

As it is, the barrel centralising ring is free to work it's way rearwards (as it had slightly in the pic), and also, the grubscrew is pushing the inner barrel downwards (compressing the rubber), which means it won't be perfectly central in the hop unit.

[3vi1-D4n]

Darn it, I was hoping the A&K wouldn't have the same issue as the CA. Essentially the CA249s I have had to modify all had this issue.

 

My solution was similar in principle but different.

 

- Swapped out the CA hop rubber and replaced with guarder clear

 

- superglued the plastic barrel clip in place, so no forward movement of the barrel inside the hop-unit.

 

- I modified the tappet plate to sit a little more forward by filing down the front of the tappet plate. I have it an extra 0.3mm forward movement.

 

- I filed down the front of the feeding block that the hop unit pushes against and the mags engage into to feed BBs into the hop unit. an extra 0.2mm

 

- I bore'd out the area around the hop unit, like what you did to allow BBs to feed.

 

It seems to work well after that.

 

But on burst/sustained fire, do you guys notice the hop up being irratic? First 2 rounds may be on target, but subsequent rounds are either hopping high and low. A G3 with a drum mag seems to be more consistent on sustained fire than the CA249s.

[Docv400]

...I modified the tappet plate to sit a little more forward by filing down the front of the tappet plate. I have it an extra 0.3mm forward movement.

I toyed with this idea myself, but rejected it for two reasons;

1. I didn't need to dismantle the gearboxes for the work I was doing (I'd like to have done, to set the AoE, but the owners were on a tight budget).

2. I've always thought that tappet plates seem to be on the verge of breaking, the way they're designed, especially the versions with the nozzle at the top of the cylinder head.

Add an o-ring nozzle, and a tight hop rubber and I'm surprised they last at all .

I think most tappet springs are way too strong for what they have to achieve as well, but that's a topic in itself...

 

I filed down the front of the feeding block that the hop unit pushes against and the mags engage into to feed BBs into the hop unit. an extra 0.2mm

I forgot to mention that I had to do that on the MKI as well. The hop adjuster wheel was pressing tight up against it, and was very hard to turn.

The Para was OK, as I didn't take so much off the hop unit.

 

But on burst/sustained fire, do you guys notice the hop up being irratic? First 2 rounds may be on target, but subsequent rounds are either hopping high and low.

What's the ROF like on the 249?

My own theory is that as ROF goes up, the hop rubber has less time to 'recover' after each BB contacts it.

It's less of an issue if you use a solid nub in place of the standard hollow type, but it's still there to a degree.

The hop rubber 'warms up' as well of course, which is why you should always fire off a good 30-40 rounds before you adjust it, the first time you use an AEG after it's been inactive for a while.

Especially if you have a solid nub fitted, if you adjust it cold, it'll jam, or severely overhop, the first time you give it a prolonged burst.

It's more critical on lower powered AEGs, as they don't have the 'puff' to force the BB past a tight hop.

[3vi1-D4n]

The reason most tappet plates break is because it smacks against the sector gear axle. Other reasons are of ROF and material of the tappet plate. It never breaks from the front.

 

I examine the tappet plate as soon as an AEG/SEG comes into my hands even if its brand new so near all AEGs passed to me never need a tappet replaced after they have been seen by me.

 

So what I do is I sand down the front, then sand off the area that the tappet plate contacts the sector gear.

 

I also keep the ROF under 20rps.

 

Strong tappet springs are good. The reason its not good for you is if you haven't done the mod as in the 1st paragraph then the tappet will slam into the sector gear shaft and break. In my case it creates a better airseal without breaking.

 

As for O-ring nozzle, while it does make a difference, you can do the same with a little bit of super glue to thicken the insides of a non-o-ring nozzle. A just sufficient layer of super glue does not wear yet still allows the nozzle to freely move will seal up most of the air leak.

 

 

As for the ROF to do with Hop up variance, I dunno if that is the issue. It could be, not disregarding it outright.

 

I have had AEGs where I fire a burst and all the shots have hit quite consistently, say on a G3 or on a G36. The variance on the CA249s seems to be quite horrid. Is it the same with an A&K M249?

 

It could be a barrel issue, mine has guarder clear hop-up on it so I don't think hop is the issue. Plus my hop unit has been modified to have the Hop nub mounted sideways now as a normal AEG and the issue still occurs so its not the hop design.

[Inkblot]

Nice, My A&K '249s hop-up never was quite right. Good work!

[Docv400]

The reason most tappet plates break is because it smacks against the sector gear axle....then sand off the area that the tappet plate contacts the sector gear...Strong tappet springs are good. The reason its not good for you is if you haven't done the mod as in the 1st paragraph then the tappet will slam into the sector gear shaft and break...

Do you just take a little off the face, or shorten it vertically?

Thinking about it now, on quite a few gearboxes I've done, the tappet pushes on the axle, whereas others don't.

I take it you don't get any feeding issues after doing this mod?

 

I've also have had a few tappet plates where the spring hook has snapped off, or the spring itself has failed.

A friend's ICS (they seem to use the strongest springs, or at least they used to) MP5 went through 3 plates fairly quickly, until I put a weaker spring in, it's still going strong 2 years later.

 

I have had AEGs where I fire a burst and all the shots have hit quite consistently, say on a G3 or on a G36. The variance on the CA249s seems to be quite horrid. Is it the same with an A&K M249?

The few I've worked on seem pretty good, grouping-wise, using 0.25g BBs at around 350fps (chronoed with 0.20g).

A friend has an early CA, that's all over the place at around 20m and beyond, his A&K is much tighter.

 

Edit; Meant to post this earlier, the modified spring retainer with the 'high' and 'low' settings...