Air ratio question for a DMR

[Lozart]

I'm building a DMR from an M14. It has a 500mm long, 6.03 mm diameter inner barrel. It's a full cylinder with standard gears and no short stroking. By my estimate, that should give me an air volume ratio of about 1.68:1 (cylinder is 65 mm internally to the back of the cylinder head, less 8mm thickness for the cylinder head at full stroke).

Is that going to be OK for heavier BBs (0.4 ish), or do I need to get the ratio up (probably by using a shorter barrel)?

 

[Lozart]

Initial tests using an M130 spring and 0.28's I'm seeing around 1.4J. to get up to around 1.6 I'd be looking at least an M140 but anything more than that and I'd be worried about the gearbox shitting itself.


And no, I'm not going to HPA it.

 

[Impulse]

1.68:1 ratio is very low, honestly. I typically refer to the old 1tonne forum posts on the airsoft sniper forum when he was testing different barrel lengths with VSRs to work out the best ratio. It's old knowledge, but I don't see it being any different these days. I also don't see how it'd be much different between VSRs and AEGs, since at the end of the day it's a piston compressing air and forcing it through a tube to propel a spherical projectile

https://www.airsoftsniperforum.com/threads/advanced-vsr-sniper-building-guide.6075/

For .4s he found that the best ratio was 2.67:1, which is significantly higher than 1.68:1. And I found his ratios were about right for my SRS as well.

And don't HPA it. I did that with one of my m14s and it was annoying.

 

[ButcherBill]

Don't you still need to keep it under 1.3j ?

 

[Nick G]

Don't you still need to keep it under 1.3j ?

Click to shrink...

Presumably it's a DMR semi locked , so will be a higher limit .

 

[ButcherBill]

As a rif capable of full auto (even previously) isn't it required to be under 1.3j tho?

 

[Colin Allen]

As a rif capable of full auto (even previously) isn't it required to be under 1.3j tho?

Click to shrink...

(4)The permitted kinetic energy level is—

(a)in the case of a weapon which is capable of discharging two or more missiles successively without repeated pressure on the trigger, 1.3 joules;

(b)in any other case, 2.5 joules.”

Probably not; as with most laws the wording is deliberately vague. However, it would would currently be a reasonable interpretation of the law to state that 2.5J would be the limit for a gun that is mechanically locked in a manner that cannot be changed in the field or without serious disassembly.

If a legal case ever occurs, which would be very unlikely, we will have a better idea of what is allowed.

The key word is "capable"; is it referring to capability at the moment or capability if we make changes to the gun? If it is the latter, how extensive do those changes have to be?

 

[Lozart]

Yes, it's a DMR. My regular site allows DMRs to be electronically locked and can be up to 1.65j (425fps on a 0.2).

Is that a legal grey area because it's relatively simple to make it fire full auto? Yes. Is it ever likely to be an issue? No. I don't wave my RIFs about in public, so 5-0 has no reason to get involved. If I should ever become a test case, then by all means revive this thread and post "I told you so" in the largest possible font.

Anyway, back on topic

1.68:1 ratio is very low, honestly. I typically refer to the old 1tonne forum posts on the airsoft sniper forum when he was testing different barrel lengths with VSRs to work out the best ratio. It's old knowledge, but I don't see it being any different these days. I also don't see how it'd be much different between VSRs and AEGs, since at the end of the day it's a piston compressing air and forcing it through a tube to propel a spherical projectile

https://www.airsoftsniperforum.com/threads/advanced-vsr-sniper-building-guide.6075/

For .4s he found that the best ratio was 2.67:1, which is significantly higher than 1.68:1. And I found his ratios were about right for my SRS as well.

And don't HPA it. I did that with one of my m14s and it was annoying.

Click to shrink...

I did find references to the old 1tonne stuff which is why I started to look into it. There's some very conflicting info on what the ratio should or shouldn't be, but given that the cylinder volume is as high as it can ever get (unless someone can actually point me in the direction of a V7 bore up kit that will actually fit in a Cyma box) then it looks like the only option will be to shorten the barrel.

 

[Lozart]

I think, realistically, I need to be looking at using something more like a 0.32 or 0.36. The M14 has nowhere near enough swept volume in the cylinder to get to 2.67:1 without dropping to such a short barrel it becomes silly.

 

[Skara]

Drop the weight down to 0.32, according to maths you should "only" need a 420mm barrel but keep in mind that air volume isn't everything, there are factors like piston mass and initial/final (piston) velocity and initial pressure spike to take into account..
Stuff i can't explain but it's there.

 

[Adolf Hamster]

I suspect if e locking isn't good enough then every solenoid based hpa dmr is naughty as they can be changed to fa via fcu settings.

I remember one of my battle buddies contemplating doing the inverse to run my hpa'd mg42 in cqb, purely for the hilarity of running the literal worst cqb gun ever.

Tbh i've never found aeg's to be tremendously sensitive to voluming in the way hpa is. Hpa's getting your dwell dialled in for a specific barrel length/weight is critical but aeg's let you get away with some pretty egregious sins.

End of the day if the gun shoots good it shoots good.

 

[Impulse]

Ratio is still a factor in AEGs, but it only really becomes an issue in DMR builds honestly. Any 1.14J gun running sensible weight ammo will be fine with the air volume in an AEG, but DMRs tend to have longer barrels and want heavier BBs, which is a really bad combination for proper air voluming.

But all my DMRs are GBBRs (and two HPA guns I never use), so I don't have to worry about that