Lowering Barrel Friction (Project Polishing Inner Barrel)

Thanks Tomi!
What is interesting about your numbers is that they show that a reduction in friction doesn't pay much off in terms of overall efficiency.

For example, I have heard that stainless steel barrels could have 12-15 times less friction than the stock aluminum barrels but if a stock barrel still only accounts for about 1% of reduction in efficiency/energy loss due to friction, it is not a whole lot of gain you get with a steel barrel.
Also, it will be a struggle keeping the gun from gaining weight with a steel barrel.

So, from a cost-benefit or workload-benefit point of view, there are other modifications that may make more sense.

MarsArtis said:

I think you're mixing up dynamic and static friction.
Your tests were all about static friction, mostly because no external force was involved. Except gravity and friction itself.

Tromic pointed out clearly the relationship with a 30 ATM pressure and the resulting 0.5% efficency gain.

A side tought:
major brands like Mares and Cressi still continue to use classic skirt orings for their pistons.
Obviously this is related to their long testings that resulted in best performance/efficency+security ratio configuration.



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I could have, but I don't think I did...?

You are right in that I tested for static friction and most of the talk has been about that but the post you are referring to talks about my small, unscientific observation of what happened once the piston started moving.

I point out that once the piston actually moves, it does so with a weight on it that is 1/5th of what it took to get it moving (since my hand is no longer pushing down on the scale). Granted, we are talking low speeds, but nonetheless it is moving and as such no longer static(?).

And yes, it was indeed very illuminating to see Tomi's math on how little friction impacts efficiency overall.

MarsArtis said:

A side tought:
major brands like Mares and Cressi still continue to use classic skirt orings for their pistons.
Obviously this is related to their long testings that resulted in best performance/efficency+security ratio configuration.



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You could be right.
But I wonder if it doesn't also have a lot to do with economics. Surely, one could design a slightly more efficient piston 30-40 years since they designed the ones they use now. But it would cost money in R&D and that could very much be the reason they are sticking with a known design. Nothing wrong with that, but sticking with it is not proof that it is the best.

As much as I would like to think the major manufacturers test and test and then test some more before they release their products to us, recent years have shown that may not to be the case.
Mares Cyrano Evo HF (or whatever it is called) has some pretty bad issues with line releases breaking or getting stuck. Sporasub has quietly upgraded pistons and the tail end on their One Air but its line release is still too short. Salvimar PredaTHOR seems to be doing well so far except for reports of sliders breaking. And the Vuoto seals themselves don't really seem to last long if using slider setups. (I got less than 50 shots on one of those seals.)

I can offer another theory - since a lot of these products, if not all, are produced in China, there is a real risk that the production runs of these parts are not made to the standards specified in the contracts. I live in China and a lot of my friends work in production and I hear a lot of these examples.
This is why the biggest brand names producing here have very strict quality control and try to control the supply chain more rigorously than they might have had to do 20 years ago in, say, Italy, Germany or the US.

The handful of issues with the newest designs could be a combination of not enough testing or not enough quality control (which again costs money) but of course, these guns are still fantastic guns when they work, which luckily they do most of the time. No doubt about it.

popgun pete said:

Stainless steel barrels can be polished to a mirror finish inside the bore as that is the final contact surface, but alloy barrels have an oxide surface that has some microtexture over the parent metal surface which itself may be smooth underneath. If you over polish the alloy tubing then you may just be making the ID larger, especially if you remove the original anodized layer. Anodized aluminium can be hard and scratch resistant, unlike the parent metal itself which is easily scratched.

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I agree on all counts.
But the real question now, I guess is how much more overall efficiency can be had from changing to a highly polished SS barrel.

I have seen estimates saying that an SS barrel has about 15 times less friction than an alu one. But if barrel friction, as have been mentioned, above only accounts for about 1% loss of power in the gun, then even lowering that loss by a factor of 15 wont give you any real world advantage. Or am I misunderstanding something?

I am asking because I know of a few "high end" tinkerers who in their quest to build the most powerful airguns go for SS barrels.

One thing that was touched upon and which slipped a bit again was that the measurements should be taken under pressure. While the dynamic friction is substantially lower than the static, in the above example it amounted to so little that it seemed to have no tangible influence on the efficiency of the gun and thus the verdict would be that the modification is not worth the hassle.

But under pressure, the o-rings will be jammed against the wall and thus very likely increase friction a whole lot. So, if the friction reaches a level where it does indeed have an impact on the efficiency, then a 30% decrease in friction would be substantial.

Anyways, that is the what the people in favor of the stainless steel barrels argue and what I personally forgot to take into account.

I might actually order one for a future project. If I do, I will of course as always, keep you updated;-)

The stainless steel barrel I have in mind is from 12x14mm ss tube. It's made by a very good Ukranian spearo, engineer and machinist living in Cyprus. The tube itself is not that much heavier than 13x18mm alu, but he says, the tube is a bit frail if you use it as is for the whole length of the gun. So, he adds a bit of 14x18 alu to the ends. That also helps it work fit in easily in standard guns. Plus beefs it up, especially at the muzzle end.
I can share his contact info for anyone interested. He also makes titanium vacuum muzzles and sliders and have a pretty neat tail end, too. Supposedly, he was actually the one who came up with the system that Salvimar now is selling a ton of... Down to the exact seals, he supposedly, has been doing this muzzle for around a decade.