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It really depends how far you want to go, but for a DMR you need to achieve 3 things: 1) consistency of power, 2) range, 3) accuracy. Consistency because to get the most range you can, you need to push your FPS to close to your site's limit, so you need to ensure that when you chrono you don't have every 3rd shot going over the limit, or whatever. Plus consistency guarantees accuracy in elevation, which is particularly important when you are firing at crouching targets closer than your sight is zeroed for (because you need to know exactly how much to aim down or the BB will sail over your target's head) and also at whatever becomes extreme range for your gun, where hits become a bit doubtful anyway, but people often stand out of cover assuming they are out of range - you have to get 'em with 1 shot, because if you miss they will take cover and you'll not get a hit on half a head. Range and accuracy speak for themselves, but there are a few things worth noting - ultimately both range and accuracy are a function of how stable the backspin of the BB is in flight, because if the BB yaws wildly then even with a lot of power behind it the lift available from backspin is squandered by not acting directly against gravity, but conversely if the backspin is very stable then more FPS will translate into even more range than it does from a worse set up with a stiffer spring. So let's assume a perfect hop unit, as the BB travels down the barrel the spin around a perfectly horizontal axis can only get knocked off axis, it can't get improved, and how much imperfection is introduced is down to how well polished the interior surface of the inner barrel is - thus the better the barrel you can afford, the better results you will get from your hop. PDI is the Rolls Royce, or their Raven next best, from x-fire.org - they often have free postage events which will save you about 1600 Yen, but Prometheus also have a good reputation. Madbull Black Python are initially probably better than the likes of Lonex and Deep Fire stainless steel, but you will also need to keep the inside very clean and the Madbull has a surface coating which is not as resilient as steel. I've had good results from a Guarder chromed brass 6.02mm barrel too, but this is in an AKS-74U and I don't clean it that often... still chrome is very hard, so gentle cleaning ought not to damage the layer of it. I also have a chromed 6.03x455mm Falcon barrel and that is pretty impressive too, although the rubber it comes with struggles to lift 0.25g BB's a little so I get better range from 0.2's, which makes accuracy at extreme range suffer. To get max stability in applied spin you need either a rubber with a split bump, such as a stock CYMA, a PDI W-hold, or a Falcon Dual Point (see above); or a shaped nub such as a RATech hourglass shaped hop cushion (also available from ASPUK), Element H-nub (fleabay), or Madbull Fishbone Spacer (in that order of effectiveness); you can't combine split bump and shaped nub, it's one or the other. You also want a sticky rubber, because although the softer the rubber the faster it will wear out, especially at higher FPS, the less the rubber is applied to the BB to get the same amount of backspin, the less FPS is lost to its breaking effect, which makes power consistency less temperature dependent. Also it is likely that the harder the BB is squeezed through the hop unit, the more any tiny imperfections in the design will translate into off axis spin. Prometheus Purple (soft) is my fav so far, but Maple Leaf 60 degree has a good rep also. When you fit a TBB you will instantly get a boost in FPS because less air will escape around the BB in the barrel, so you may well have to change spring, but before that you can do other things to maximise compression, which also increase consistency, and will affect your choice of spring. If you fit the hop rubber so that the bump fits as close to the rear of the barrel as you can get it, the lips of the rubber will make better contact with the air seal nozzle. If you then wrap a few turns of PTFE tape around the forward end of the rubber, so it doesn't cover the bump/barrel window end and use some grease on the outside to make this fatter assembly slide into the hop chamber, you will minimise the amount of air which can escape between the rubber and barrel. Make sure that your air seal nozzle has an O-ring inside it at the base. There are some with 2 O-rings but i'm not convinced they do actually do anything which a smear of grease around the base of the cylinder head spout wouldn't do better. This grease should be applied carefully so that there is enough to make a good seal against the nozzle O-ring but not enough so that it is forward enough to get blasted down the barrel off the interior of the nozzle (check the travel of your tappet plate to determine how far up the cylinder head spout the grease can go). SHS make good aluminium air seal nozzles with O-rings (Bullseye Country Sport). If you replace your piston head with a Double O-Ring design you will get 20-30 more FPS from your spring and more consistency. To get maximum benefit also thoroughly grease it and the inside of the cylinder with CT-2 Teflon grease (fleabay). Magic Box make the piston head, but they seem to be rare - there are other manufacturers such as Dream Army (khmountain_uk fleabay). Also coat the rails of the piston and spring in CT-2 so that their travel is consistent. A shiny steel spring guide may also help consistency and it can't hurt (SHS - Bullseye CS). Lastly choose your spring. Predicting what power you need is something of a black art, because the more powerful the compression inside the cylinder, the more any failures of air seal will affect power (and this is not necessarily so simple as more pressure = more leaks because there will be a point at which each component gives way and that will be affected by temperature and humidity, thus it is best to try to achieve whatever FPS is your target with the least powerful spring and best compression, hence O-rings, Teflon grease and the tightest TBB you feel comfortable with for your ammo / or choose your ammo to work with the best 6.01mm TBB you can afford). Guarder springs have a good rep for consistency over time, but SHS are also good. Element springs are not as hard wearing over time, but they do make many half sizes, like M115 rather than 110 or 120 which is the usual convention for manufacturers. Obviously you will have the spring the gun came with, so if you chrono your set up with that, you will know how much more or less you need. 10 on the spring scale is meant to be 10m/s or approx 30FPS, but bear in mind that those figures are based on averagely decent compression with normal diameter barrels. The longer a TBB is, the more extra FPS you will get from any given spring power increase and also, the better compression, the more that will make the TBB increase power. So basically if you followed all my advice above and are chronoing at say 450FPS and your site limit for semi-auto locked DMR's is 420, then dropping spring power by 10 is not likely to be enough, you would be better to drop by 15. If this then proves a smidge too low, you could use something like a stack of washers slotted over the spring guide to pre-tension the spring a little - this will give a small boost in power. You could also clip a few turns off a spring of 10 less power, but this can be dodgy because the spring needs to be under enough tension when as fully extended as it gets to prevent the piston bouncing off the cylinder head and causing problems with your sector gear engaging it. If you did do that, be sure to bend the end to make it as flat as possible, like the end as supplied, otherwise the power will be off axis making the spring rub against the spring guide and inside of the piston. Hope that helps