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I did watch out for that while testing, but there was no notable difference in consistency between them. I don't think you're likely to lose any power/suffer consistency issues to such air loss unless the cylinder is undervolumed for the spring strength & barrel in use. Edit to add: All of the O-rings I tested here do seal fully against the cylinder without relying on pressure ahead of the piston to form a seal, so there shouldn't have been any losses that way... the only piston head O-ring that's loose-fitting enough to not seal outright is in my P90RD & I'm not going to take that apart and mess about with it/invite problems while it's running well

A lot of attention is paid towards various components when tuning performance in an AEG, but it seems piston head O-rings are often overlooked beyond merely testing that they seal against the cylinder. A typical piston head O-ring measures about 24mm outer diameter, like these ones: https://www.ak2m4.co.uk/internal-parts/pistons-heads/piston-head-o-ring-19.5mm https://www.ak2m4.co.uk/internal-parts/pistons-heads/piston-head-o-ring-viton-19mm However for some time now I've been wondering if it's actually better to use slightly smaller O-rings, somewhere around 23.7-23.8mm, or perhaps even a bit smaller than that (as close as possible to or just a tiny bit smaller than the diameter of the cylinder; if it's a tiny bit smaller the pressure ahead of the piston & air flow through the ports in the piston head should expand it to seal completely once it's moving at speed). The idea being to reduce losses due to friction and allow the piston to gain slightly more energy before compression begins. This is because a few years ago after performing maintenance on my TM P90RD and replacing the piston head O-ring that I thought was worn out with a new "good" one, I noticed it had lost a significant amount of power. Putting the "bad" O-ring back in put the power back up to where it was supposed to be... I subsequently discovered that while the "bad" O-ring didn't seal when tested the usual way (which is what made me initially conclude that it was bad), it would actually seal just fine when the piston was moving fast enough. I've since noticed this effect with my other guns too, although none of those have managed to reach the performance of my P90RD (which just about manages 340fps on an SP90 spring while the others do a little over 330). I recently decided to test this, so I bought some O-rings that are supposed to be 23.74mm outer diameter and have now tested them alongside some other piston head O-rings. The test gun is a TM P90TR with: ZCI 6.02mm barrel (247mm) Prometheus purple hop rubber & bucking (hop is not properly set but remained unchanged throughout the tests) Guarder air seal nozzle Guarder enhanced stainless steel cylinder head Guarder enhanced cylinder, 3/4 type Deep Fire aluminium piston head Tokyo Marui stock piston Guarder SP90 spring Deep Fire (I think) bearing spring guide Guarder steel bushings IRF2804 MOSFET Blade fuse Deans connector Each O-ring tested was given a very light surface coating of silicone oil before being coated in TechT Gun Sav and installed on to the piston head. Before testing the gearbox was cycled a few times without the upper receiver/barrel assembly to clear any excess grease out of the nozzle & prevent any finding its way into the barrel and risk affecting the results. BioSphere 0.2g BBs were used for testing, 10 shots were fired for each test and the average value used. These are the results (O-ring size measurements are approximate as they're never perfectly round): Lees Precision Engineering piston head O-ring 24mm (19mm inner diameter/2.5mm cross-section): 322 fps (min 319.6, max 323.5) This is a standard size good quality O-ring that comes with LPE piston heads, with the result being about what I expected. Deep Fire 'super' O-ring (measured ~23.8mm OD, 2.6mm cross-section): 332.6 fps (min 330.9, max 334.2) This is what came with the Deep Fire aluminium piston head that's currently in the P90TR. It is a brown colour and feels softer than most other O-rings in addition to being slightly smaller, and there is much less resistance when moving the piston through the cylinder. With this O-ring the gun has an extra ~10fps over the LPE O-ring. Uxcell "23.74mm (18.5mm/2.62mm)" O-ring #1 (actually ~23.9mm-ish): 322.9 fps (min 320.4, max 324.7) This is one of the O-rings I got recently specifically for these tests. I was very surprised by this result, so I checked its size and found that it was larger than specified - around 23.9mm OD. I then measured the rest of the O-rings in the pack and found that all of them measured closer to 23.9mm. I tried again with the smallest one I could find. Uxcell "23.74mm (18.5mm/2.62mm)" O-ring #2 (actually ~23.85mm-ish): 325.6 fps (min 323.1, max 328.3) This time there was a more notable improvement over the standard O-ring, but still not close to the performance of the Deep Fire O-ring. I next went through the O-rings in my spares box to find ones measuring towards the smaller end, and did manage to find a couple that were closer to the size I was looking for: Unknown origin O-ring #1 ~23.8mm (2.4mm CS): 333.3 fps (min 330.3, max 334.7) Unknown origin O-ring #2 ~23.8mm (2.4mm CS): 333.1 fps (min 331.5, max 334.7) These both are about the same size as the Deep Fire O-ring that performed very well, and their performance was about the same too... I was unsure how much of an effect O-ring size would have with a full cylinder & long barrel, so I then fitted a longer barrel to the P90TR to see if that makes any difference in the results, still using the Guarder enhanced 3/4 cylinder to begin with. The barrel is a 509mm unknown brand Chinese non-tightbore barrel (IIRC it originally came from my ~15 year old A&K M249). Ideally I would have used a ZCI tightbore to better compare against the ZCI barrel used in the earlier tests but I don't have one of those so I had to make do with this. LPE standard O-ring + 509mm barrel: 309.5 fps Deep Fire 'super' O-ring + 509mm barrel: 320.1 fps As with the earlier tests the standard O-ring is around 10fps lower than the smaller O-ring. The barrel being a cheap non-tightbore barrel probably accounts for most of the power loss seen here compared with the shorter barrel tests. Finally I put in a Guarder enhanced full cylinder to see how the O-rings compare in a long barrel + full cylinder setup: LPE standard O-ring + 509mm barrel + full cylinder: 321.7 fps Deep Fire 'super' O-ring + 509mm barrel + full cylinder: 318.1 fps Unknown origin O-ring #1 + 509mm barrel + full cylinder: 320.6 fps Interestingly, this time the Deep Fire O-ring actually performed slightly worse than the LPE O-ring. I was expecting that with the full cylinder all O-rings would perform about the same, maybe with the smaller O-rings having a slight advantage. I tried one of the unknown O-rings that was the best performer in the initial tests and while that was better it still had slightly lower performance with this configuration than the LPE O-ring. I'm not sure why this is, as they all form a seal right from the start... it may be that the smaller O-rings have more room to inflate against the cylinder and that with the lack of any significant acceleration phase prior to compression starting this costs some energy that otherwise would be used to propel the BB. Conclusions In builds using full cylinders the normal 24mm AEG piston O-rings would seem to be slightly better, although the difference doesn't seem to be that big. However in builds using ported cylinders it looks like it's better to use smaller O-rings, as was the case in both sets of results using the 3/4 cylinder. It can be difficult to find such O-rings though since most AEG piston O-rings are about 24mm and generic O-rings are typically only supplied in a standard set of sizes that are either slightly too big or slightly too small, and even if you can find some that are supposed to be ~23.7-23.8mm they're not likely to be made with such a high degree of precision, as I found out with the ones I bought. Anyway this might be something to keep in mind if you want to gain/lose a little power, or if you're just looking to maximise efficiency - it may not be a huge boost, but a potential 10+fps isn't insignificant and if done in conjunction with other small things that can be done to help boost fps this might be enough to allow the use of a weaker spring to achieve a given power level, which in turn will reduce stress on the rest of the system/improve reliability as well as slightly improve ROF and trigger response etc.

Some brands do this & I'm not sure why, I don't like it because it's an easy way for crap to get in and mix with the BBs if they're not kept in a clean environment. I'm not surprised, Nuprol stuff seems to be very hit & miss in general (trending more towards "miss"). Personally I'm going to stick with Geoffs and maybe 6mm Ammo... although at this point anything I use will be checked over before it reaches any of my guns regardless of brand, as even decent brands have the occasional bad batches. According to the packaging G&G claims their bio BBs will take a few years to decompose if left lying on the ground at 10+°C, or up to 4 years in river/lake/ocean conditions at 4+°C, other manufacturers make different claims about their own bio BBs and Wikipedia claims differently... so there's clearly some bullshitting going on. Still, it should at least be a bit less environmentally harmful than conventional plastics though.

It's quite possible that this could just be a bad batch, as I've only tested the one bag / single weight - and the tracer versions are fine. It's a shame as they do look pretty good otherwise. If anyone else gets any 6mm Ammo BBs I'd recommend spending some time checking them over before using any just in case... actually, I'd recommend doing this regardless of brand (and before opening the pack if the packaging is clear or has a window).

I bought some 6mm Ammo BBs to test. Shipping was extremely quick; I placed my order late on Friday evening, received the shipping notice email on Saturday & they were delivered on Monday - this was with free delivery option too! These are the 0.30g version, in both the regular and tracer versions. The regular BBs are supplied in a black coloured bag, while the tracers are supplied in a transparent bag. Both bags are completely sealed & resealable - there are no air holes that might allow in dust or any other contaminants from the environment. 6mm Ammo 0.30g Bio BBs Just like my other tests the very first thing I did was sift through the BBs and inspect them for any obvious defects. Unfortunately I found a number of BBs in the pack that had significant surface defects, ranging from minor dimples to larger dents/scratches (and even a protrusion on one of them) - the latter potentially being a big problem as significant defects could damage the hop rubber an/or cause a jam in addition to compromising the performance of those particular BBs. Here are some of the worst ones I found (best photos I could get; my phone's camera isn't very good): In addition to this, despite the pack having been sealed there were a few bits of dust/debris in the pack that had stuck to some of the BBs. I only noticed due to paying such close attention to them (and it's not as bad as I've seen in some other brands that don't come in fully sealed bags) so it's likely nothing to be concerned about but I think it's worth noting. I measured only BBs that didn't have any obviously visible defects. Weight: All but two were slightly overweight at 0.31g (my scales wavered a little between 0.30 and 0.31 on a few of them before settling on on 0.31) Size range: 5.91 - 5.95mm; 4 measured within the 5.95 +/- 0.01mm tolerance. 1 was 5.91mm minimum, 2 were 5.92mm minimum, 13 were 5.93mm minimum, 4 were 5.94mm minimum. Roundness: 1 measured 0mm variation all around, 15 measured with max variation up to 0.01mm, 3 with 0.02mm, 1 with 0.03mm Defective BBs aside these are among the better BBs for consistency and roundness, with 17 being 5.93mm+ and only one having more than 0.02mm variation, although they lean towards the heavier side. These are harder and more brittle than the Geoffs 0.30s, breaking apart cleanly. I did not find any air bubbles in the ones I broke apart. 6mm Ammo 0.30g Bio BBs (tracer) I did not find any BBs with any visible defects in this pack. I did see some kind of minor dust/debris in the bag though, again probably not significant enough to worry about but worth noting anyway. Weight: 15 measured 0.30g, 5 measured 0.31g. My scales wavered between 0.30/0.31 before settling for most of the ones that weighed 0.30g so these all seem to lean towards the heavier side (though not to the same extent as the regular version) Size range: 5.93 - 5.94mm; 2 measured within the 5.95 +/- 0.01mm tolerance. 18 were 5.93mm minimum, 2 were 5.94mm minimum. Roundness: 7 measured 0mm variation all around, 13 measured with max variation up to 0.01mm. These seem to have slightly better sizing and consistency than the regular version, with all measuring 5.93-5.94mm and only up to 0.01mm variation. They are a bit softer and much less brittle than the regular version. I did not find any air bubbles in these either.

That's very interesting. I guess it's not that surprising that most BBs perform about the same, as from what I've seen with my testing they tend to have similar size ranges/variation & as they mentioned in the video many brands are probably made in the same factory anyway! It would also be interesting to see how various BBs perform after being fully loaded into midcap mags (especially ones with particularly strong springs like the EPM1s) as some BBs are much softer than others and could end up being deformed & suffer reduced performance as a result. The results for the Ares BBs are a little surprising as the ones I got were unimpressive, with the bag I got containing a bunch of badly deformed BBs (and the ones that looked ok didn't seem all that special either). Although the ones I looked at were the 0.25g version & it wouldn't surprise me if some weights are just more consistent/better made than others for whatever reason, as I've seen with other brands. It's also possible that I just got some from a bad batch but then I would've thought that any that were so visibly dodgy as the ones I found would've been picked up by QC, especially with them being visible through the packaging... It's a shame they didn't test any BioSphere BBs as those are the closest to perfect BBs I've found so far in terms of size & roundness, but every one I broke apart had an air bubble; it would be interesting to see how much of an effect that has on their performance. I'll definitely have to take a look at the 6mm Ammo BBs though, those sound very promising.

More Geoffs bio BBs, this time in 0.30g. Only white + tracer versions this time. Geoffs 'Super Natural Precision' 0.30g Bio BBs Weight: All measured 0.30g Size range: 5.91 - 5.95mm; 0 measured within the 5.95 +/- 0.01mm tolerance. 1 was 5.91mm minimum, 7 were 5.92mm minimum, 12 were 5.93mm minimum. Roundness: 12 measured 0.01mm variation all around, 7 measured with max variation up to 0.02mm, 1 with 0.03mm These trend slightly more towards the smaller side like the other versions - they're especially similar to the 0.25g version. None measured within tolerance, but many were at least 5.93mm minimum which could be borderline/within measurement error due to the limitations of my calipers. Like most of the other Geoffs BBs I've tested they do seem to have good consistency though. These are very similar in softness to the 0.28g version, flattening out rather than fragmenting & needing to be twisted apart. I found air bubbles in one of the BBs. Geoffs 'Natural Precision' 0.30g Bio BBs (tracer) Weight: All measured 0.30g except for one weighing 0.31g. Size range: 5.90 - 5.95mm; 2 measured within the 5.95 +/- 0.01mm tolerance. 1 was 5.90mm minimum, 1 was 5.91mm minimum, 8 were 5.92mm minimum, 8 were 5.93mm minimum, 2 were 5.94mm minimum Roundness: 3 measured 0.01mm variation all around, 15 measured with max variation up to 0.02mm, 1 with 0.03mm, 1 with 0.05mm These seem much better than the 0.28g version, being closer to the specified tolerances and having greater consistency (although still not quite as good as the white versions, but at least these aren't too far off). These felt a bit harder to break apart than the 0.28g version but were slightly more brittle. I found air bubbles in one of them. Forgot to reply to this before - they are available from https://outdoorandtactical.co.uk

The regular white versions do seem to be pretty decent (in both 0.25g and 0.28g at least) and I've not had any issues shooting with them so far & intend to continue buying them. It's just the black and tracer versions that seem to be be a bit dodgy for whatever reason - perhaps the addition of black dye/GITD material compromises the strength of the BB making them softer/more brittle, maybe there's poorer QC standards applied on those specific kinds of BB, or perhaps I was just unlucky & got sent some from bad batches. Given how far out of spec the tracer BBs were I'm pretty shocked that those made it through QC though, given that they're supposed to be 5.95 +/- 0.01mm.

Most bio BBs don't swell up, or at least modern ones don't (although their "biodegradability" is questionable). In any case, don't use poor quality BBs, bio or not...

I bought some more Geoffs bio BBs, this time in 0.28g. These are 'Super Natural Precision' in white and black and 'Natural Precision' tracer BBs (they don't do tracer versions of the former - I'm not entirely sure what the difference is supposed to be between 'Super Natural Precision' and 'Natural Precision' as it doesn't seem to be stated anywhere; both are supposed to be 5.95 +/- 0.01mm). The packaging for these is just like the 0.25g BBs I bought before - self-seal bags with air holes punched into them near the top (I only bought small bags of the black & tracer version). They all looked fine except for a single tracer BB that had some kind of scuff mark on it. Of these three, only the white ones seem decent enough - about on par with the 0.25g version I tested before. The black and tracer versions seem like they're best avoided. Geoffs 'Super Natural Precision' 0.28g Bio BBs: Weight: All measured 0.28g except for one measuring 0.29g. Like the 0.25g version these seem to lean towards the heavier side. Size range: 5.93 - 5.96mm; 4 measured within the 5.95 +/- 0.01mm tolerance, the rest were 5.93mm minimum Roundness: 1 measured 0mm variation all around, 9 measured with max variation up to 0.01mm, 8 with 0.02mm, 2 with 0.03mm These are pretty similar to the 0.25g version, generally not being within the specified 5.95 +/- 0.01mm range (trending towards the smaller side). They're a little softer than their 0.25g counterparts but more difficult to break apart as they had a tendency to flatten out & I had to twist them apart after initially splitting them. I didn't see any signs of air bubbles. Geoffs 'Super Natural Precision' 0.28g Bio BBs (black): Weight: All measured 0.28g, again seeming to trend more towards the heavier side. Size range: 5.92 - 5.97mm; 15 measured within the 5.95 +/- 0.01mm tolerance. 3 were 5.93mm minimum, 1 was 5.92mm minimum, 1 was 5.96mm minimum (and that one had an upper measurement of 5.97mm) Roundness: 13 measured 0.01mm variation all around, 6 measured with max variation up to 0.02mm, 1 with 0.03mm These have similar consistency to the white BBs but with a larger size range (although this could just be down to the small sample sizes). However they are also much softer and easier to break apart than the white version so it would probably be best not to use these in midcap mags or GBBRs, as their softness will make them prone to getting deformed/flattened or chewed up. In addition to this I found an air bubble in one of the 20 BBs I broke apart. Geoffs 'Natural Precision' 0.28g Bio BBs (tracer): Weight: All measured 0.28g except for one weighing 0.29g. Size range: 5.87 - 5.95mm; None measured within the 5.95 +/- 0.01mm tolerance. 1 was 5.87mm minimum, 3 were 5.88mm minimum, 4 were 5.89mm minimum, 6 were 5.90mm minimum, 5 were 5.91mm minimum, 1 was 5.92mm minimum Roundness: 6 measured 0.02mm variation all around, 7 measured with max variation up to 0.03mm, 4 with 0.04mm, 3 with 0.05mm Surprisingly these are far less consistent than the others, and a fair bit undersized. Despite their smaller size they're still around 0.28g though, even leaning towards the heavier side. These are about as soft as the black ones but a lot more brittle; it didn't take much force to crack them open and they readily split into multiple fragments. I really wouldn't recommend trying to use these in midcap mags or GBBRs (or at all; loading a few into my HK416A5 AEG resulted in some double feeds/misfeeds and pre-shattered rounds exiting the barrel on firing). On the plus side I didn't find any air bubbles in the ones I broke apart.

I used to use one of those vests with my P90... it's a bit of a squeeze, but the big pouches at the bottom of the vest are *just* big enough to fit two P90 mags each (although it may make the vest stick out a bit at the sides due to how long P90 mags are).

In my experience the cheap pot metal has a tendency to become brittle over time (and in some cases expand & cause lockups). I bought some Chinese guns pre-VCRA and while they were all fine to begin with they all suffered from this issue over time and needed replacement parts after a few years or so regardless of use. OTOH all of the Tokyo Marui guns I have from that time are still fine, with the oldest being close to 20 years old now.

That's why I decided to get some BBs of various brands to examine and come to my own conclusions. AFAIK there are only a handful of OEM BB manufacturers (all seemingly based in Taiwan - I can't think of any BBs I've used that weren't made in Taiwan) that the various Airsoft brands buy from, with the only real difference being the material spec/tolerances/cost.

Ares BBs seem to be the floor sweepings: I tested some bio BBs a while back and the 0.25g Nuprols actually seemed pretty reasonable, however the 0.20g Nuprols I tested were a quite soft & easy to deform. I guess how good/bad they are probably varies by weight/composition/whether they're bio or not as well as from batch to batch etc... The G&G 0.25s seemed ok but I found air bubbles in some of them. Geoffs seem to be the best overall out of all of the BBs I tested (I did notice they were a little on the softer side, but didn't think they were soft enough to be concerned about... that may be an issue for GBBRs though).

It's not a Brexit thing or just our government; the EU has been planning to do this for some time and would have introduced it at the same time as us, but they delayed implementing them until July due to the coronavirus: https://ec.europa.eu/taxation_customs/business/vat/modernising-vat-cross-border-ecommerce_en