THE Holy Grail of building your DMR AEG

I often get asked about, and read threads that are dedicated to, the art of making a near stock AEG into a DMR and then the OP sticks a list of random parts like uber long barrels and silent piston heads and newfangled hop rubbers, and then expect it to be bolt action in its performance and groupings, when all they have in fact done is what every other noddy has done to their AEG, with the exception that the OP is now calling it a DMR.

The fact of the matter is that if you want bolt action type performance you need to sink one hell of a lot of cash and hours into it or just end up with something that is average at best and the equivilent in performance to every other M4 people are using.

In real life a DMR is, to a large extent, an accurised and precision manufactured version of an existing rifle platform in one form another made by an experienced armourer. This also needs to be true in airsoft if you want something that is going to perform and compete against bolt action rifles.

If you want to build a true airsoft DMR you have to look at it thus - you want to incorporate what we know makes a bolt action accurate and then combine it with what makes an AEG fast and efficient, and then throw in the same kind of man hours that your average bolt action owner lavishes on his weapon to get the best groupings he can from it.

So, what makes a good airsoft DMR? 3 things..... 1/ a consistent power output, we're talking a 2 or 3 fps deviation as a worst case senario. 2/ a stable and consistent hop plus a well made and straight barrel 3/ high quality heavy ammunition. Obviously I'll be concentrating on the first 2 as you should already be using the latter!

What follows is a 'how to' to give you the kind of idea of the effort required to get the average DMR to be even close to a bolt action in accuracy. It is no way definative for every type of AEG out there, but most if not all can benefit from the same methods described below, as most are based on the same method of operation.

I'm going to assume everyone reading this is competent at gearbox work, or if not is willing to pay someone who is to do this for you, as I am in no way responsible for you f*****g it up if you are not!

The following project is my KART/G&G Mk14 Mod0 hybrid rebuild and is limited to 350fps because of where I live.

Primary Objectives

What we are aiming for in an airsoft DMR is a combination of a smooth, strong (as most people will want to go above 350 fps) and solidly supported gearbox, that is connected to a solidly supported hop chamber that then fires a BB down a solidly supported barrel. You'll notice there a lot of solidly supported going on there. For anyone who has built a bolt action rifle one of the first things that we are told to do and is proven to be effective is fit barrel spacers to take the vibration out of it, but you'd be surprised at how many AEGers just ram a new TBB down their outer and not give it a second thought, resulting in only percentage the accuracy they could be getting!

The Gearbox

As far as possible you should try to buy quality matched parts, and don't be tempted into buying 'silent' piston heads as they are that in name only, stick to flat faced either plastic or metal ones as this enables foam for shock/noise suppression to be easliy fixed to it.

Where possible you should use a 1 piece cylinder head as this guarantees no air leaks at the joint between the head and nozzle tube.

The cylinder should be a full one, there is no need for bore up kits or any of that BS as they are only needed if you plan to use a 600+mm barrel and if you need one of those your just making up for other things that are lacking in your life, and I shall give an over view of why in the barrel section.

Volume balancing, you should when ever possible balance the volume of the cylinder to the volume of the barrel you intend to use, as this maximises the efficiency of the pressure system. A good starting figure is 1.7 times cylinder volume to barrel volume, but this will vary depending on the weight of ammo you use, the heavier it is the more air you'll need, the lighter the less you'll need, You can do the maths and do the drilling of holes yourself if you want , but it is probably easier to buy a ready ported cylinder from the vast array the after market parts manufacturers make.

You should also polish the crap out of the cylinder to the point that you can see your face in it, like so (you'll notice I do all my modding while wearing a storm trooper helmet )....



When you have done this, and cleaned it thoroughly, you need to fit the cylinder head into it, but before you push it fully home a smear of silicon sealant around it will seal it against any air leaks in our quest for a 100% seal, and should look like this when you finish.



To guarantee a smooth feed of the BB into the hop it doesn't hurt to help the tappet plate back a millimetre or so and to achieve this I glued some brass tube over the tappet nub on the sector gear, like so



While we are on about the tappet plate I also ground about half a millimetre of the front face of the plate to enable the nozzle to press forward harder into the hop rubber, helping the seal at the hop rubber lip. You do need to be carefull not to remove the lip that the nozzle sits in/on, as this is bad, and if it looks likely that it will occur the alternative is to take some material from the gearbox, or a combination of both.

Generally, on guns like the M4 and G36 that use a spring that pulls the plate, this is not a problem, but on guns like the G&G M14 where the spring pushes the plate, it will need spacing forward a little, but not so far that the spring cant compress the other way preventing the plate coming back its maximum travel, that you have just extended by doing the mod above causing the plate to snap!(see small white spacers in the above pic)

The following picture shows roughly how much I ground off the front of the plate, it also looks as if its leaning back and that because it is. Its how the G&G M14 tappet plates are made, so when I ground the plate all I had to do was grind the lower half so it became upright.



As can be seen in the picture, the nozzle has been glued into place, this has been done to aid the BB being loaded into the same place every time in the hop and it also holds it in a constant place on the inner nozzle helping stop random air leaks, to aid this further you should always use a nozzle that has got sealing 'O' rings inside.

The method of gluing the nozzle in place is as follows. I firstly cleaned both parts thoroughly to remove all grease deposits. I then smeared some thick silicon grease around the inner nozzle tube on the cylinder head and then fit the nozzle sliding it back and fore to spread the grease evenly, I then applied super glue to the plate groove, then as fast as possible fit the plate into the nozzles groove, then place everything into where it would normally be in the gearbox case in a retracted state so that the inner nozzle holds the outer at the correct angle.

To also aid in the positioning of the BB in the same place each time I also counter sunk the end of the nozzle very slightly, this also act like a barrel crown, helping to evenly distribute the force of air around the BB in the hop. Like so-



Now anyone who is modifying an AEG should be doing this but I'll cover it anyway, the correction of the Angle of Engagement. The ideal scenario is for the first tooth of the sector gear to meet with the first tooth of the piston at the 12 o'clock position on the sector gear, which in turn means the removal of the second and part of the third teeth on the piston, like this...



To get the piston this far back I glue foam from a mouse mat to the front of the piston, and also to the cylinder head, in this case as on my SR25 I plan to fit it to only the piston, if it needs it, ( some will be all fancy and use sorbothane and such but I'm more bush mechanic than elitist) as this helps absorb some of the shock from the pistons forward travel and makes the gun a tad quieter as well.

Care needs to be taken when using this as you need to take into account the compression value of the foam when the spring is fitted. In the following photo the 2 black lines show the amount of compression in the foam on the front of the piston, GET THIS WRONG WHEN YOU'RE DOING THE AoE AND THE TEETH WILL STRIP!!



And before anyone asks, yes I have ground out the vent holes on the front!

The 'O' ring, there are endless threads that tell you what size to use so I'll not bore everyone with it, I'll show you how to stretch it to get an even tighter fit in the cylinder. First stretch your 'O' ring over a old cylinder, like this....



Then using a low intensity flame, like that of a cigarette lighter, gently heat the 'O' ring by constantly moving the cylinder round in the flame until the cylinder becomes warm to touch. Then leave it to cool. When its cold clean it thoroughly then fit it to the piston and sparingly grease it as you normally would.

Other odds and sods that are drive train related

Spring Guides and Bearings, when ever possible you should at the very minimum use a spring guide with a bearing on it, as this helps the spring to collaps without twisting it in its linear line, this means that the spring can produce its power in a smooth straight line, which means it will pruduce a more consistant power everytime it is compressed.

You should also think about fitting a bearing to the piston head aswell, this will add fps for the same spring rate, as the spring will be held under more static tension provided your original piston head wasn't fitted with a spacer of some sort before, and also aid/decrease further the springs rotational stresses. It will also add weight to the piston, which in a DMR is a good thing, and I'll explain why next....

For all those of you who are thinkin 'Oh, I'll Swiss cheese my piston as it will be lighter and spring forward easier', stop it! Leave such mods to the go faster full auto queens, for a DMR it is a good thing to have the extra weight. You will need the extra weight in the piston to help to shift the heavy weight BBs you should be using, as nothing gets things moving like a heavy object with some inertia behind it.

Gears, I'm limited to 350fps so I'm lucky in that I can get away with using a standard ratio gear set, but for those wanting to use a little more power I recommend a set of torque up gears, but not to high a ratio as this will diminish your trigger response time, and for an electric DMR that's a bad thing.

You should also make sure the shimming is absalutely spot on, to tight and you will lose power, to loose and the box will be noisey.

Take special care when setting the primary gear as if you set it to far to left side you could find the motor will bind and there won't be enough adjustment in the motor to relieve it. Oppositly, to far to the right and you may find you cant wind the motor in far enough to mesh properly.

Motors, basically any high torque motor will be what you are after. The reason being a high torque motor isn't very fast but it gets to its top speed faster, so its got the balls to pull the naffing great spring you've fitted and it will do it one hell of a lot quicker than a speed motor, save them for the full auto queens aswell .

Semi auto, yes you should!! Especially if you want to be considered a proper DM! It largely depends on what gearbox you have as to what you need to do to achieve this, varying from selector plate cutting to arm removal, but you need to do it!

MOSFETs, the best by far is an AB MOSFET, either bought or home made.

If you fit an AB MOSFET to your gearbox this then allows you to remove the Anti Reversal Latch and spring out of your gearbox, as the braking part of it will electronically brake the gearbox, using the motor, at the same position every time you pull the trigger.

The reason for the latch removal.... it will mean your gearbox will not lock up again.... ever! Combine over enthusiastic trigger use, a battery that is lacking in power and an AR latch and it could result in a lock up, at best this means releasing the spring, at worst a gearbox strip, with the AR latch removed the worst you get is a double feed!

The ultimate gearbox is by far the Pre-cocking type, the only reliable rendition of this is the TM PSG-1. It can be done to other gearboxes', and I have done it to this very gearbox I'm rebuilding here, but it is complicated and can be unreliable in the extreme.

The Hop Unit and Barrel

The hop and barrel unit is singularly the most important part of the gun the get a BB to fly straight and far, period. So it makes sense to spend a little time tuning it, past just cramming an new rubber, nub and F'ing long barrel in and away to go.

As I'm restricted to 350 fps I use stock TM rubbers as they are good for the price, and common, and as what I do to them before I fit them means they wear quite quickly that's a good thing.
First I take the rubber and turn it inside out. I then sand the internal nub completely off, using a dremel, like so...



The reason I do this is I'm trying to remove all possibility of a variable in the system, and a thick clump of rubber will deform when it gets a BB shoved underneath it and I feel might leave a compression groove or deform as the BB passes under it, so if it ain't there it can't be a factor to instability.

I then turn it back the right way and cut about 5mm off the barrel end, this is done because in the next procedure the rubber can stretch. I then fit it to my barrel and whip it using dental floss.

This method is 100% better than any Teflon tape method that you've heard about, as it literally crushes the rubber on to the barrel thus giving a 100% seal. For those that have never seen it done here's a pictorial of me doing mine....

Take the tape and form a loop.....



Holding the loop VERY tightly, wind the tape around the barrel 5 or 6 times, making sure you pull it tight as you wind....



Cut the tape off the spool and thread the cut end through the loop, pull the loose end of the loop so that it drags the wound end underneath the windings...



Cut of the tails....



Thinly coat in silicon grease and insert into the hop chamber using a gentle twisting motion.

The hop nub, normally that pokey bit of soft rubber tube you throw away and replace with a 'H' nub, which can yeald very good results.

We'll I've been experimenting with a couple of different solid plastic designs, solid plastic is used to, again, try and remove any kind of variable.

I have made 2 different profiles and have had more success with what I like to call the 'U' profile as opposed to the concave profile.

First I'll show you what I mean then explain why I think one is better than the other....

The 'c' or concave profile similar to Shredders SCS nub only mine is fixed directly to the arm then the profile filed into it.....



The 'U' profile...



The 'C' profile works on the entire top of BB contact principle and when right it works really well, it's just an arse to set up correctly! If you have any kind of deformation/mis moulding on your hop arm, your hop arm isn't dead central, or its sat squew whif for any reason, it will send the BB off around an invisible corner, you then have to try and reseat it, with all the hassle that comes with stripping the gun down, and repeat until you get it right. Get it right first time your either lucky or gifted!

The 'U' profile, I designed this because my luck ran out along with my patience and being gifted wasn't helping any, so I thought 'reduce the amount of contact to a single point and halve the hassle fine tuning it! I was right; using shims either side of the hop arm with a small amount of filing to get the profile in the correct position in the window this is tuneable to shoot straight as anyone would care to desire.

Other benefits.... it requires less hop to gain the same lift and its in less contact with friction surfaces so is slightly more efficient.

The down side.... as there is only a small point of contact on the rubber it tends to wear quite quickly, and I'm inclined to feel there is a small chance for the BB to sit 1 side or the other in initial insertion into the hop, so....

I'm currently working on and testing this....



A hybrid of all 3 types, I'll let you know how I get with groupings or even if it just shoots straight on in due course

Shimming the hop arm is very important. It enables you to fine tune your hop and it also holds the arm as stable as it can be. I make my shims from thin brass sheet, but you can use thin plastic card or even tin foil.

Here's my M14 hop unit....



And how I made my SR25 hop shims....



And fitted....



The barrel, most noobs and a suprising amount of not so noobs are under the tighter and longer is king type spell, well sad to say its total bull shit! It has long been accepted in the spring powered bolt action world that anything over 509mm is pointless and actually causes the rifle to be less accurate, and that the VSR performs best with a barrel between 430mm and 480mm. It is also considered widely that an AEGs can be accurate with a barrel as short as 363mm.

While we are considering barrels its also worth thinking about the bore size, most people believe the tighter it is the more accurate it is, if this is so why are most VSR users moving to 6.05mm bore barrels as they are more accurate than a 6.01? And why are some major Japanese companies starting to advertise 6.08mm barrels as their accuracy barrels?

To this end, I used a 440mm 6.04 Systema, I find that with this barrel the length gives the BB a flatter flight trajectory over a shorter one, which helps with the long shots, and is still of a good accuracy at range aswell .

The most important thing to do is polish it, even if its new, extremely well. Here's a guide to how I do mine.....

http://www.ukairsoftzone.co.uk/forums/topic/5319-barrel-polishing-guide/

Now we have the barrel and hop assembly ready to install we need to make it a snug fit, NOT interference fit, into the outer barrel, usually a single layer of Teflon tape or electrical tape is enough, and its best to 'Barbour's poll' it to provide even support along its length, then smear it with silicon grease so it doesn't get stuck, like so....



The final, and most important, part is you must do every thing you can do to support and try to remove vibration from the barrel and hop unit.

If you refer back to my pictures of the SR25 hop unit you will notice a small piece of plastic card glued to the front of the feed tube this, when the unit is installed, forces the unit back against the gearbox and transfers any shock directly into the body rather than adsorbing it itself and then onto the barrel etc, and if you go back to the M14 hop unit the front is bound in Teflon tape, this is nothing to do with sealing it, it does the same thing as the piece of plastic on the SR25.

You should also be looking for ways to support the outer barrel in the stock/chassis/RIS/RAS as the more you can do to dampen the vibration the better.

With the EBR stock it has the advantage of clamping and bolting everything tightly together, with my inner barrel only ending a couple of centimetres past the last solid fixing point. It is also a VERY heavy stock, currently nearly 17lbs/ just over a stone, in weight which keeps any vibrations dulled to a minimum.

The Stock

Everyone knows to fill the stock with something spongy to adsorb the bangs and rattles, but did you ever consider balancing it?

RS shooters have known for years that you should have 75% of the rifles weight balanced evenly in the trigger hand, why should we be any different?

At first it might seam stupid to want to add weight to something that is already quite heavy, but if you get the balance of the rifle right you won't notice the extra weight.

To do this I use silicon sealant and some large nails, but be careful you don't over do it as there's a art to getting the balance right. The secret is to place weight in it, reassemble, test it for even distribution of weight then repeat until your comfortable with it.

Batteries

In the airsoft DM world trigger response is king. You will need a high voltage pack no lower than a 9.6volt ni-cad or similar or a high dicharge rate 7.4 volt li-po or better yet an 11.1 volt li-po which provide blistering trigger response in a very small package.

Sound proofing

Anything you can do to decrease to muzzle rapport and mchanical noise from your weapon is good. This will give you the chance to remain undetected for longer, enabling you to make a second, third and possibly even a forth shot, if you space your shots far enough apart, from the same position.

Foam on the piston face, dyno mat on the gearbox case and foam filled suppressors, if you are allowed to use them, all aid in this, by deadening or altering the sound made from the rifle.

In the following link there is an extreme example of a DMR I built a year or so ago to fulfil this requirement.....

Oops, there was an error! | Airsoft Sniper Forum

So, there you go, a taste of what it takes to make a proper airsoft DMR, not just a sooped up run of the mil AEG

Just to finish off, this is mine after the rebuild....



(I will edit this from time to time to add extra info when I come across/feel the need to add it)

EDIT 1- Volume balancing and Batteries added

EDIT 2- Spring guides and piston weights added

EDIT 3- Barrel Polishing added

EDIT 4- updated hop nubs (another to be added as soon as testing is complete)

EDIT 5- Sound Proofing link added and general tidy up.

Edit by Plaz: Fixed pictures.