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?