Updated Holy Grail DMR guide for 2015...

Holy Grail DMR guide by vindicareassassin on the airsoft sniper forum from 2010, updated for 2015....

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 - 2 or 3 fps deviation as a worst case scenario.
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!

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

The biggest change since the original guide was written in 2010 is the rise of HPA. Polarstar and now Wolverine, Kuba, Valken, etc. are proving very effective ways at getting great performance with all the benefits of gas guns - instant trigger response, quiet muzzle report, great tuneability and control.

There’s not much to write in a guide other than these are great systems that are a heavy investment. They have their own tuning requirements that are covered in many guides online so I won’t add anything else on them.

Back on the AEG front this guide was originally written for, version 2 gearboxes are prone to cracking around the cylinder head area on high power builds, so make sure to take a small round file and radius the corners of the cylinder window or get a box with this pre-done.

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 easily 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. Most cylinder heads today are one-piece.

Volume balancing, you should whenever 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 rough figure is at least 1.7 times cylinder volume to barrel volume, but this will vary depending on the weight of ammo and barrel bore diameter you use - the heavier and wider the more air you'll need. For 0.40g on 6.03mm barrel I find a ratio of 2.0+ works well. Overvoluming is preferable to undervoluming.

Most DMRs with a barrel over 400mm will require a full cylinder, not ported. Bore up kits are really only needed if you plan to use a 600+mm barrel or a widebore over 350mm.

You can do the maths and do the drilling of holes yourself if you want to use a fairly short barrel, but it is probably easier to buy a ready ported cylinder.

You should also polish the crap out of the cylinder to the point that you can see your face in it.

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 RTV silicone around it will seal it against any air leaks in our quest for a 100% seal.

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. You can use a sector gear chip, or glued some tube over the tappet nub on the sector gear.

While we are on about the tappet plate, you can shave 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 careful not to remove the lip that the nozzle sits in/on, and if it looks likely it will happen 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 may need spacing forward a little - but not so far that the spring can’t compress!

The air nozzle should be replaced with one with an internal o-ring to prevent air leaking past, which causes more inconsistencies.

The original guide calls for the air nozzle to be glued to the tappet plate - I don’t think this is a good idea as the nozzle should be free to seat where it wants to so it can cycle smoothly and consistently. It should still end up in the same place each time.

To also aid in the positioning of the BB in the same place each time you can countersink the end of the nozzle very slightly. This will also act like a barrel crown, helping to evenly distribute the force of air around the BB in the hop.

Anyone who is modifying an AEG should be doing this - correction of Angle of Engagement. The first tooth of the sector gear should meet with the first (pickup) 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. There are many guides online just for this. This aids long-term reliability.

You can do this with solid spacers between the piston and piston head, to space the piston far enough back to lnie up the pickup tooth. Solid spacers will be the most consistent method, but on V2 gearboxes that are prone to cracking (or when you want more sound damping) you may want to use sorbo or silicone:

Glue foam or sorbo (sorbothane pad) to the front of the piston or the cylinder head. Sorbo will absorb some of the shock from the pistons forward travel, helping to stop the gearbox cracking (especially on V2 boxes), and makes the piston’s impact a tad quieter as well. It may introduce small inconsistencies as the sorbo squashes, so

Or silicone on the cylinder head, although I've personally not tried this.

Care needs to be taken when adjusting angle of engagement with sorbo or silicone as you need to take into account the compression of it when the spring is fitted.

The piston seal o-ring - there are endless threads that tell you what size to use if it needs replacing. It’s a good idea to stretch it a little to get an even tighter fit in the cylinder…

First stretch your o-ring over a cylinder, 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 it’s cold clean it thoroughly then fit it to the piston and sparingly grease it with 100% silicone grease as you normally would.

Finally, polish every moving surface in the gearbox for maximum smoothness - the guide rails for the piston and the tappet plate, and any areas of the shell that contact moving parts. Polish any parts that are rough too.

Other odds and sods that are drive train related

Spring Guides - whenever possible you should at the very minimum use a spring guide with a bearing on it, as this helps the spring to twist freely as it compresses. This means the spring can produce its power smoothly, increasing consistency.

You should also think about fitting a bearing to the piston head as well. This will add fps as it’ll pre-compress the spring a bit more, and also let the spring rotate more freely. Plus it will 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 ;)

Piston, along these lines an aluminium piston (and piston head) is heavier than a plastic one. Whatever you get, ensure it’s got a full steel rack and adequate support for the rack.

Bushes/ings or bearings - Bearings should be avoided in high stress builds at they're more prone to failure. Good quality steel bushes are where it's at.

Gears - depending on your fps you can go for 16:1 or maybe even 13:1 for fast trigger response. Don’t go more than 18:1 (standard ratio) as this will diminish your trigger response time too much, and for an electric DMR that’s a bad thing.

You should also make sure the shimming is absolutely spot on - too tight and you will lose power, too loose and the box will be noisy and lose consistency.

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 it’s 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 as well ;)

It should have 22tpa (turns per armature) or more. More = more torque, slower speed. You can get off the shelf ones or for maximum torque build your own frankentorque beast. Google this if you're interested.

Semi auto lockout - 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! You can also use computerised MOSFETs to do this…

MOSFETs - MOSFETs are electronically-controlled triggers that allow the electric current to go straight to the motor, and not through the trigger contacts. This improves electrical efficiency and consistency, and protects the trigger contacts. If you use LiPo batteries you’ll definitely want one. You should get one even if you don’t!

Quick word of warning - some shops sell plug and play fire control units, calling them 'MOSFETs'. These don't protect your trigger contacts. If it's plug and play it's no good.

Active braking (AB) MOSFETs use a reversed shot of electricity to stop the motor quicker between shots. This will cause a little extra wear on the motor and gears, but will prevent overspin and stop the motor in a more consistent place. The original guide mentioned the ability to remove the anti-reversal latch with an AB FET - personally I wouldn’t risk it as if the gearbox does overspin there’s nothing to stop it reversing itself and snapping the tappet plate, or worse!

Computerised MOSFETs can lock the gun to semi-auto electronically, allowing removal of the semi auto cut off lever. This will prevent gearbox semi auto lockups! Some can also pre-cock the gearbox. This is great for trigger response, but bad for wear. Use at your own risk! You can also modify gearboxes mechanically to do this - look online for guides if you want to. Or get a PSG-1 :)

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 a new rubber, nub and F'ing long barrel in and away to go.

There are a few methods for improving the standard hop design, and currently the r-hop is the best modification. Second to r-hop, the easier but less effective flat-hop mod is easier to do (you can even buy flat pre-modded hop rubbers).

The original guide has a short description of a flat-hop modification, but even that is a little outdated now and there are better guides if you search around online.

In short - r-hop your barrel. It’s worth the time and effort, especially in a DMR using heavy BBs. Just do it!

The hop unit itself has many variations, but going for something one-piece and solid is a good bet. Popular M4 units are Lonex and ProWin. The ProWin is beefy and uses a rotating collar similar to the awesome TM P90 design.

Shimming the hop arm helps if there’s any play. It enables you to fine tune your hop and it also holds the arm as stable as it can be. You can make shims from thin brass sheet, thin plastic card or anything that’s thin and flat….

The barrel

From the original guide: most noobs and a surprising 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 AEG can be accurate with a barrel as short as 363mm.

While we are considering barrels it’s 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?

Most of that is still current thinking. There’s a lot of controversy over bore diameter, but in short it’s the QUALITY of the inner surface that matters most. A longer barrel will allow you to reach a high fps without putting too much stress on the gearbox components, but a shorter barrel may be more accurate. It’s impossible to fairly test as there are so many variables that can’t be controlled. Optimal seems to be 380-509mm.

Also on the market today are widebore barrels, up to 6.23mm bore. These are primarily used for HPA/gas guns that can provide a huge volume of air to make up for the leakage around the BB. The theory is the BB has a cushion of air around it which keeps it stable, but as ever this is hard to test and prove. In any case, an AEG can’t really provide the air volume necessary if it’s a long DMR-length barrel with heavy BBs. Try one out if you’ve gone HPA, but I wouldn’t bother with an AEG…

The most important thing to do is polish it, even if its new, extremely well.

Now we have the barrel and hop assembly ready to install we need to make it a snug fit into the outer barrel. Usually a single layer of Teflon tape or electrical tape is enough, and its best to ‘Barber’s pole’ it to provide even support along its length and smear it with silicon grease so it doesn’t get stuck.

The final, and most important, part is you must do everything you can do to support and try to remove vibration from the barrel and hop unit. A spring on the front of the hop unit, over the barrel or on the front, or o-rings around the barrel can force the hop unit back against the gearbox, helping maintain airseal and consistent position.

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 damp the vibration the better.

The Stock

Everyone knows to fill the stock with something to absorb 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 seem 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 overdo it as there’s an art to getting the balance right. The secret is to place weight in it, reassemble, and test for even distribution of weight. Then repeat until you’re comfortable with it.

Batteries

In the airsoft DM world trigger response is king. The original guide talks of NiMH but these days there’s no reason not to go with a lithium ion battery - they’re pretty much the same price and perform so much better.

There’s two main chemistries - LiPos have a slightly higher capacity and voltage but are more susceptible to overheating and catching fire, and LiFe(PO4) are a little safer as they don’t suffer thermal runaway but slightly lower capacity.

There’s a little more to caring for them, particularly during charging, but it’s worth it for the performance boost. Read into this separately and be safe!

LiPos come in 7.4V and 11.1V flavours, and LiFe 9.9V. The 7.4V probably won’t be enough for a high fps build. Get something with at least 1500mAh capacity and a discharge rate of at least 20C.

If you do go for NiMH, make sure it’s at least 9.6V and 30A discharge.

As covered above, you should fit a MOSFET to protect trigger contacts and improve electrical efficiency.

Sound Proofing

Anything you can do to decrease to muzzle report and mechanical 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 fourth shot, if you space your shots far enough apart, from the same position.

HPA guns will be the quietest to fire by far, as they have no major moving parts - just the compressed air being released.

Sorbo on the piston face, dyno mat / acoustic foam packed around the gearbox case and foam filled suppressors (if you are allowed to use them), all help.