M4 inner barrel length

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King03sh

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Hey guys and girls, quite a simple question this time, how close to the end of the outer barrel/suppressor does the inner barrel need to terminate? Im looking to upgrade my inner barrel at the same time as updating the front end of my gun with a handrail and suppressor.

I will be fitting a 16.2 in lvoa-c rail and a suppressor of currently undetermined length to my 14.5 m4a1 barrel to get that oh so sexy integrated suppressor look. my standard inner is 370mm, im thinking of using a pdi 6.05 472mm inner and a 107mm suppressor, this should theoretical leave the inner recessed 5mm in to the suppressor, should that be sufficient that bbs exiting the barrel won't catch the inside of the suppressor? Id rather not have to cut and crown a barrel if I can help it. Cheers guys.

 
Use the same length as currently or match the cylinder volume to the new barrel when upgrading.

 
Why would BBs clip the inside of the suppressor? If they deviate far enough from their intended path over the length of the suppressor then you'd have trouble hitting the side of a barn from inside.

The inner barrel can terminate anywhere inside or outside the outer barrel, as long as it isn't unsupported for too great a distance. As above, stick with the stock length unless you want to start changing the cylinder to match. Although there's no harm in having a slightly shorter barrel with the same cylinder of you intend using BBs heavier than 0.2g (and you should...)

 
Thanks for the quick response guys. As far as I understand it a bb traveling down a barrel rides along the top of the barrel, I had assumed that if the inner was to short and the bb had any hop on it it would contact the outer? Obviously my thinking may be flawed but a friend had this issue and it was rectified with a shorter silencer, im hoping to avoid the same as I cant afford to buy parts then have issues with them... see im a little confused with the cylinder voluming, I have a 370mm inner currently on a type 0 cylinder so my understanding is I can run up to around a 500mm barrel? Dont quite know why it has this combination, I guess for joule creep?

 
Hangtight said:
Why would BBs clip the inside of the suppressor? If they deviate far enough from their intended path over the length of the suppressor then you'd have trouble hitting the side of a barn from inside.
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You're assuming that the suppressor entry and exit line up properly! 

 
Hangtight said:
Don't buy stuff that doesn't! ;)
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Thing is though, if you look through a suppressor on its own they'll line up. Put it on the end of a barrel though and you can find that the inner doesn't perfectly line up with the outer barrel or that the thread on the end is a bit pissed...

 
King03sh said:
Thanks for the quick response guys. As far as I understand it a bb traveling down a barrel rides along the top of the barrel, I had assumed that if the inner was to short and the bb had any hop on it it would contact the outer? Obviously my thinking may be flawed but a friend had this issue and it was rectified with a shorter silencer, im hoping to avoid the same as I cant afford to buy parts then have issues with them... see im a little confused with the cylinder voluming, I have a 370mm inner currently on a type 0 cylinder so my understanding is I can run up to around a 500mm barrel? Dont quite know why it has this combination, I guess for joule creep?
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Read this....

http://www.instructables.com/id/Mythbusting-Airsoft-Hopup-and-Barrel-Dynamics/

A 370mm inner barrel with a full cylinder is about right for 0.28-0.32g BBs, particularly if the AOE has been corrected.

Probably what your friend rectified was a wonky suppressor replaced by a not so wonky suppressor. ;)

 
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Lozart said:
Thing is though, if you look through a suppressor on its own they'll line up. Put it on the end of a barrel though and you can find that the inner doesn't perfectly line up with the outer barrel or that the thread on the end is a bit pissed...
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The joys of Airsoft.... And the reason I just bought a lathe!

 
Hangtight said:
Read this....

http://www.instructables.com/id/Mythbusting-Airsoft-Hopup-and-Barrel-Dynamics/

A 370mm inner barrel with a full cylinder is about right for 0.28-0.32g BBs, particularly if the AOE has been corrected.

Probably what your friend rectified was a wonky suppressor replaced by a not so wonky suppressor. ;)
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 That was a fantastic read thanks very much... 

I expect your right about the wonky silencer lol.

I have more confidence now in the parts iv chosen :-)

Out of curiosity you say a 370 barrel is about right for .28 to .32s? I plan on using .28s and my gun is right on 345fps with a little variation, will I likely see a drop in performance with a longer tight bore? I was under the impression 450mm was optimum for accuracy and range provided correct volume ratio?

 
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2.5-2.7 :1 appears to give about the best efficiency with 0.28-0.32g BBs. If you're currently shooting 345fps with a 363mm(ish) barrel and a full cylinder on 0.2g,then you may well find that the muzzle energy goes up with 0.28g, quite possibly to the point where you're shooting the equivalent energy of 0.2g @ 370fps. If your field measures the fps with the weight BB you're using and then uses a chart to calculate the equivalent fps with 0.2g you could find yourself over their limit.

Over around 200mm barrel length makes virtually no difference to accuracy provided the gun is tuned to make the most of it, and it certainly has no effect on range. Range is a function of velocity, BB mass and the amount of backspin (and a few other environmental factors) . Barrel length doesn't come into it. In fact there are arguments against long barrels for accuracy. With a long barrel the BB must have a lower acceleration if it is to be accelerated the whole length of the barrel and still leave the muzzle at the target velocity. This lower acceleration may not generate enough speed over the hop to produce the required backspin.

 
@Hangtight do you happen to know or have available the dimensions for a v2 cylinder for doing calculations? My gun is still with my tech friend atm

 
Bore diameter will be 23.8mm. Full stroke is 58mm,but that usually drops to 54mm once the angle of engagement adjustment is done.

 
Thanks pal, at 6.05 im coming up at 1.88:1 with a 472mm and 1.97:1 with a 450... if my maths is correct. Think 450 is the shortest I can get with a suppressor on (80mm suppressor) for the look I want, recon that should perform ok? I could get that up to 1.89:1 with a 472 at 6.03 or 1.98:1 at 450 with a 6.03 but I didnt wanna go that tight

 
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Go as short as you need to get the ratio needed for the gun to work properly. If the inner barrel finishes half way up the outer , and then you put a suppressor on the end of that it doesn't make any difference. The outer barrel length is purely for aesthetics.

Why are you wary of 6.03 barrels?

 
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Old pic, but the gun on the right has a 300mm inner barrel, and the outer barrel is only 200mm long. The rest is in the suppressor and over half the suppressor is inside the rail. The gun on the left has a 260mm barrel, but has a full carbine length outer barrel, plus the bloody great suppressor. Both have the internals tuned to match the individual barrel length.

 
Cool cool, so I could stick with a 370 and still add a suppressor with no negative effects? Im not wary of 6.03s per se, just read a lot of reviews that the 6.05 is more accurate at range than 6.03, most of my engagements are at range as I play woodland and have dodgy knees lol

 
Do whatever you like with the aesthetics. As long as everything lines up you'll be fine.

Bore quality has far more influence on accuracy than bore diameter. I use Prometheus EG 6.03mm barrels for my nicer builds, and my 350fps M4 is good out to 65m, shooting 0.28g ASG Devils. The R hop helps. My son's gun uses a 300mm ZCI 6.02 and is as accurate. My bullpup AK has a standard brass 6.08, 363mm barrel and isn't quite as accurate, but still sends 0.28g BBs out to the same distance. It mostly comes from the hop up and the BB quality.

 
Thats exactly the advice I was after thank you :-).

So by my calculation the ideal ratio for 0.28g is 2.14:1 (based on 7.5(0.28^2)+1.55)

Cylinder volume = 24023.57mm3

There for barrel volume should = 11236.47mm3

Working backward:

11236.47/pi =3576.68

3576.68/(3.015^2)=393.46

I calculate that to be 393.46mm inner barrel length with a 6.03 bore. Can you confirm if my workings are correct?

 
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