Ideal barrel length?

My SLR is nearing completion, so I am curious about one point. I know LowGuido (second best blaster channel on the tube tubes) has previously measured the volume of a full cylinder and calculated that a 7.5id barrel should have the ideal length of I think it was 33.5cm. Now currently I have a 40cm barrel glued to the T-piece. Is there any benefits or cons to keeping it 40, or is there a benefit found in shortening it the 6cm?

Id love to hear peoples thoughts.

Thanks all in advance :slight_smile:

My tests with 7.3mm id S/S barrels have shown fps increases all the way up to at lease 400mm with a 75% cylinder so with a 100% cylinder this could be longer.
By the same token anything short like under 250mm doesn’t work well even with lower percentage cylinders. I prefer barrels from 250 to 450mm.
I am in the process of upgrading my son’s RX AK47 and the barrel replacement will be around 430mm.
I personally would not cut it if 400mm suits you.


Thank you sir…

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If you have the 40cm ready to go, I’d say have a game with it and see how you like it.

The Gen 9 has a 33.5cm barrel at a larger diameter than 7.5mm. So the extra length of the barrel at a smaller diameter makes the V/P ratio potentially more favourable. I’m a few 0.1 mm shy of having the correct diameter of a stock air cylinder, but some back of the envelope maths puts your ratio at 1.78, which isn’t too far off a 2.0 ratio.

Stronger springs will help bring you up to a 2.0 ratio. At this point I would ask if the Inner barrel extends past your outer? That would be a good excuse to cut it back, unless you’re graceful enough to not knock the inner on anything.

If you haven’t noticed any drop in performance with the 40cm barrel, call it a win and leave her as is. The more moving parts you’re worrying about the more you’re worrying in general. Hard to enjoy a game if I think I’m at a disadvantage thanks to my handiwork or if the blaster is going to break in my hands.

Please explain how a stronger spring brings the volume ratio up ?

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I am an advocate of getting longer barrels than needed, then cutting them down to size… I hate buying a 29 cm barrel then finding I need a 32cm… or “7mm longer and it WOULD have been perfect” DoH!!

So, I would say, for sprayand pray autos… get 35-40 cm and a small tubing cutter… and get hop ups and things that fit the inner barrel, not the outer… then you can get all the tings like silincers and splash arrestors in place exactly where you want them (even if there is 3cm of inner barrel sticking out… then, once you are satisfird… trim the inner barrel back to exactly where you want it :sunglasses:


You’re right, it doesn’t actually bring the ratio up, my thinking would be the extra oomph from a stronger spring would offset any drop in FPS/distance he might have due to a slightly lower ratio.

My wording was poor, suggesting any change to ratio just from a spring.

Thats my thoughts exactly mate, hence why I got a 40cm one, I have a few pipe cutters, and thats how I did the barrel on my UMP.

I use one of these…

Just start light pressure and tighten cutter slowly… even if it takes 5 minutes. If you try and rush, you will reduce the size of the barrel at the cut line, making like a tight choke on the end of the barrel… if you know what I mean.

You can finish by giving the new end of the inner barrel with a reamer…

If it is an aluminium barrel do everything lightly… if you are just making the trimmed end the same as it used to be, the reamer can be turned backwards…

do everything gently… :+1:


sometimes things can be too long