M4 AEG to DMR

[Jmason98]

Good morning all, I need some advice, I am currently running a Max Hopp Unit, with maple leaf bucking in my M4. I have ordered the kit to HPA it this weekend. However, whilst apart, I thought it would be best to change barrel, now with the intention of it being a dmr, what length inner and outer barrel will I need please? If anyone can offer any advice, that would be great!

 

PS I am aware that barre length makes very little difference, but I would like it for aesthetic purposes

[The_Lord_Poncho]

I think there are a couple of thoughts here - firstly, as you mention, inner barrel length is generally regarded as not being overly critical - up to a point. I believe that people have established that circa 300mm or so is about optimal in order to stabilise the bounce of the BB as it moves along the top of the inner barrel post hop unit. But the other consideration is the volume of your inner barrel - I'm not sure if that is usually an issue with HPA, nor indeed GBB (but it very much is for AEGs). However you mention that you have ordered an HPA 'kit' - some of the mechanical only HPA engines - such as the Mancraft PDIK require a minimum PSI in order to operate effectively - you might find that with a very long inner barrel, that minimum PSI would end up pushing you over your sites Joules limit for DMRs. I believe they have a table on their website indicating the power vs different barrel lengths - might be worth a read. Generally, with outer barrels, as long as it is sufficient to cover your inner barrel, you can choose whatever length you like the look of.

[Fatboy40]

2 hours ago, Jmason98 said:

Max Hopp Unit, with maple leaf bucking in my M4. I have ordered the kit to HPA it this weekend. However, whilst apart, I thought it would be best to change barrel, now with the intention of it being a dmr

 

Which model of hop unit is it? For example is is the M4A-PRO that comes with the solid metal concave nub?

 

An option for you, if you wish to lift heavy BB's, is to go with both a Maple Leaf inner barrel and the MR bucking as the ML inner barrel is intentionally designed to support the unique features of the MR bucking. In combination with the appropriate nub for your hop unit you'll be able to lift very heavy BB's so that's one thing ticked off of the list of DMR tasks.

[Pseudotectonic]

for aesthetics I wouldn't go less than 14.5" for a "DMR", a 16" would be even better

 

inner barrel the longer the better (as long as the volume is matched)

[Badgerlicious]

1 hour ago, Pseudotectonic said:

inner barrel the longer the better

Not true. If anything, the shorter the better for lifting heavy BBs. 

 

Jmason, you should give this thread on the airsoft sniper forums a read. Despite being nearly 15 years old, the principles are still very much the same https://www.airsoftsniperforum.com/threads/the-holy-grail-of-building-your-dmr-aeg.892/ Basically eliminating movement, vibrations, and air leaks to create the most consistent performance you can. 

 

And to be a bit helpful in a succinct way, you probably want a barrel of 300-400mm with a full cylinder for a good air volume ratio. You can work out your own cylinder's volume exactly with this calculator and ruler. http://airsofttech.dk/Calculator.cshtml

Edited April 24 by Badgerlicious

[Pseudotectonic]

i think you are confused, heavier BBs can be accelerated with a shorter barrel than a lighter BB, but it doesn't mean a shorter barrel is more efficient than a longer barrel for the same heavy weight BB which you should be using anyway with a DMR

 

if shorter barrel is better than longer barrel we'd be all sniping with .48s with a pistol

[Badgerlicious]

6 minutes ago, Pseudotectonic said:

i think you are confused, heavier BBs can be accelerated with a shorter barrel than a lighter BB, but it doesn't mean a shorter barrel is more efficient than a longer barrel for the same heavy weight BB which you should be using anyway with a DMR

 

if shorter barrel is better than longer barrel we'd be all sniping with .48s with a pistol

Definitely not more efficient, it's about backspin. A shorter barrel needs a higher pressure (heavier spring) to get the same muzzle energy, this means the BB is pushed over the hop with increased speed, hence more backspin, hence you can hop heavier BBs. In fact there are people who run 0.4g+ BBs out of 1j pistols. Usually MK23s since they have easily available TDC kits and don't require high volume fire. Most pistols also have the downside of lubricant being sprayed down the barrel with every shot. 

Edited April 24 by Badgerlicious

[Pseudotectonic]

yeah but are they sniping with it?

[Impulse]

Typically AEGs will find longer barrels more difficult because of air volume issues. The gearbox only has so much air to push through the barrel, so if you have more barrel volume than cylinder volume your BB will stop accelerating and start decelerating while it's still in the barrel. However, if you have more cylinder volume than barrel volume you will end up wasting a lot of air as there is more air to push after the BB has left. This makes it a bit louder as you get a more noticeable pop as the BB escapes the barrel and the air expands, but it can also have some issues that I can't remember. All I remember is discussing it with a tech and he told me what happens but I can't remember exactly what he said, I just remember it sounded like a valid issue that I wouldn't want.

 

For HPA it isn't such a problem because you can typically adjust your dwell depending on how long your barrel is, allowing more air to released per trigger pull in order to properly fill the barrel. A properly volumed HPA setup will joule creep like a gas gun.

 

For gas guns, the problem with longer barrels is joule creep. With a gas gun the BB will constantly accelerate through the barrel as the gas constantly expands through it, much in the same way a bullet will accelerate more through a longer barrel than a shorter one. So when you have a heavy BB, which starts and moves slower than a lighter BB at the same energy, it will have far longer in the barrel to accelerate and will leave with a far greater muzzle energy than a short barrel due to the relative increase in energy imparted through the longer acceleration.

 

All that said, you don't need a long barrel to have a "DMR" as it's just a designation in airsoft of a rifle that is semi-auto locked and typically has a higher energy limit and an MED. However, I'm a bit of a purist and am of the belief that a DMR should at least look like a DMR, so a 14.5" or 16"+ outer barrel is a must in my opinion, and then I'd just get an inner barrel that will go all the way through it. If you're going HPA, the only thing I would watch out for is joule creep, however you can easily fine tune the power output by tweaking the psi on the regulator to make sure you're within your site's limits.

[Pseudotectonic]

At the end of the barrel you just want the heaviest BB you can afford to be travelling at the field limit FPS and at the correct hop amount which is neither overhopping nor underhopping

 

And if you compare two systems tuned to deliver the exactly same exit projectile of above (an x gram BB moving at y FPS spinning at z RPM), you will find the system with the longer barrel will be more accurate and more consistent than the one with a shorter barrel, simply because all angular jitters of the exit vector are contained at a smaller angle of degree, plus the longer shape allows you to have more precise manual control over where it is aiming at biomechanically speaking, that is why I say for sniping you want a longer barrel to do the same job rather than a shorter barrel

[Rogerborg]

10 hours ago, Pseudotectonic said:

you will find the system with the longer barrel will be more accurate and more consistent than the one with a shorter barrel

 

Should be.

 

But given the QC of airsoft, you could also find that you've bought 4" of extra wonkiness at end of what might have been the perfect barrel if the cut had fallen in a different spot.

[Fatboy40]

15 hours ago, Pseudotectonic said:

yeah but are they sniping with it?

 

Well... some people actually can and do. There are some who with an AAP-01, along with the Loki + appropriate bucking + inner barrel to support an appropriate bucking's patch + 0.40g or heavier BB's, can be effective at DMR / sniper ranges.

 

If I had the money my AAP-01 would be setup like this, shooting 0.40g, but I've not yet won the lottery  

[Lozart]

15 hours ago, Pseudotectonic said:

yeah but are they sniping with it?

 

17 minutes ago, Fatboy40 said:

 

Well... some people actually can and do.

 

 

Thing is though, it doesn't make it a sniper rifle, does it?

 

While yes, I agree that to many it's just semantics the whole "is an M4 a DMR just because it has higher FPS and scope slapped on it" is a valid argument. Given that the whole basis for the need of an exemption under the VCRA is that we want our toys to be "realistic", running a 400FPS pistol with a 1J boltie sidearm just seems a bit disingenuous.