Okay, here i go....
For those of you not wanting to read this wall of text, there is only one modification that i feel like the platform needs, regardless of what you want out of a gun and that is the Green Gas Mod. It allows the rifle to get 2.5 - 4 mags per gas fill, which makes the system extremely practical for skirmishes. Last Saturday i played a woodland game with a completely standard, box fresh MWS (internally.. it had a fancy rail) and the only time i re-gassed my 5 mags was at dinner time. This consists of removing the magazine baseplate, held on by two hex keys, being very careful not to shoot the follower spring and the little plastic retainer across your room. From there you remove the valve with either a proper valve tool or a small screwdriver if you're careful. You'll be met with a brass valve with a long zinc tube. At first people removed these, but i don't for two reasons.. One because its a gigantic pain in the ass and two because I've found cutting them to be the best compromise between gas efficiency and shot consistency. If you look inside the tube you'll see the brass valve. You want to cut the tube so that its level with (within 5mm or so) the brass. With the mod done, reassemble, remembering to swear profusely at the spring that doesn't want to stay in the hole. All mods after this are not needed but can enhance aspects of performance. I would however suggest you run the gun on propane. It works better than green gas, it's cheaper and it doesn't coat your bucking in silicone oil after a while. Avoid ASG ultrair like the plague. It's 10% butane content increases cooldown and lessens gas efficiency.
Hop nub
The mod i recommend next when people ask for more range and accuracy is to increase the BB weight. Heavier BB's retain their energy for longer periods and are less effected by wind, but the MWS struggles to hop much more than a .28-30 max. This is due to the hollow rubber hop nub, which compresses substantially towards the end of the hop travel. There are multiple options to fix this, from filling the original nub, to a cut up biro pen, to the million 'upgrade' nubs that litter the airsoft market. Personally i find the SixG brass super nub to be the best nub for conventional mound buckings. It's machined specifically for the MWS chamber, so it's about as perfect of a design as you can get. This will allow you to hop virtually any BB weight. If you don't plan on using BB's above a .28, don't bother with any mods other than the green gas mod, as you'll gain very little.
Outside of the conventional size nubs, there are elongated ones that are designed for R hops or maple leaf/ TNT/ Flamingo buckings. This is where things can get a little technical.. A lot of the large hop up nubs like the prometheus bridge for example wont fit inside the chamber and index with the hop arm. To fix this problem you have two options.. Cut the nub shorter to fit in the hole, or the ideal solution is to extend the chamber window a few MM towards the front. I've done this on multiple builds with good success. As a side note, the prometheus MWS chamber should be the best option for these longer nubs because it applies pressure vertically almost like a TDC. I have not used this chamber though so it's merely speculation.
Hop Buckings
The standard TM bucking really is all you'll ever need for 99% of builds. It's soft enough to get a great seal on the nozzle and hop the BB under cold GBBR temps. The only other standard mound style bucking i would recommend is the Modify Tan. I discovered them years ago and they've been put into about 7 of my GBBR builds from mws', WE G36's and a VFC SR-25. They are probably the cleanest moulded hop rubber I've ever seen and they have a narrower side locking tab than something like the blocky maple leaf one which can cause Issues that I'll get on later in the barrel section. I have heard of people having wear issues with them, but I've personally never experienced this.
Large contact patch buckings or 'drop in R hops' like maple leaf, TNT, Flamingo or the new modify stuff have the advantage of a larger surface area to apply hop to the BB. This is good for two reasons. One because it needs less downward pressure to apply the same level of hop, meaning you're not being robbed of FPS or blocking the barrel in extreme cases, and two because it gives a more repeatable hop effect BB after BB. The drawback of this is not all barrels are compatible. Small window barrels like the PDI are not compatible with these buckings. I currently have the flamingo bucking in my MK12 Mod0 DMR and it's performing very nicely using .43's (400 fps on a .20). A build for a friend is running a TNT TR hop at 340fps and is performing really well too. Maple leaf buckings I have tried with mixed results. They perform amazingly in MK23's and a multitude of gbb pistols and WE gbbr's, but i ran into an issue with the mws. It started off really well, but performance started to drop and on inspection the tip of the contact patch was being damaged by the lip of the barrel window. This may not be a problem with some barrels like the crazy jet, but i moved on to other buckings. One thing these bucking's tend to have is a Degree rating (50,60,70,75,80). Due to the cold running of GBBR's I'd highly recommend you use a 50 bucking for 280-375 fps and a 60 for anything above 375.
Inner Barrel
Stock inner barrel on the mws, CQBR and MTR is a 250mm, brass barrel that performs very well from the box. You will gain very little by changing the barrel in terms of accuracy, but you can get a nice bump in FPS by either increasing the length or decreasing the bore diameter. As a lot of batches of MWS differ when it comes to stock fps, it's very hard to say "X barrel boosts your fps by X fps". If you stick to a well known barrel you should be golden. If you want to stay at 250mm I would probably recommend a crazy jet or a TNT. For longer barrels you've got 370mm Prometheus barrels in 6.03 and 6.00, crazyjet and TNT. I've never used one in an MWS, but i found the 6.01 stainless RA-tech barrels to be really good in gbbrs. All will serve you well, so i would choose the one that will get you closest to your desired FPS limit.
DMR's
For specialist roles like designated marksman, the MWS really shines. It's already great range can be improved upon much more simply than an AEG. Nothing in the lower needs to be changed unless the stock trigger pull bothers you. Once you've decided on a max FPS or Joule depending on how your site works, you can now begin the build. Your absolute cheapest option is semi auto conversion (free), 370-420mm barrel depending on your desired fps and an upgrade/modded hop nub. A 370-410 should get you in the area of 400 fps with a 6.03-5 barrel. If you're looking for 450 fps and above, then a 6.01 or even 6.00 at 410mm+ will get you closer. Just bare in mind GBBR's run dirtier, so good quality BB's (I only ever run Geoffs) and regular cleaning is paramount. Heavy BB's are essential for DMR's! Absolute minimum of .32 to get any real range benefit. Personally i prefer .36-.46 depending on my dmr setup. It's hard to advise exactly because everyone want's different BB trajectory characteristics or 'flight times'.
If i think of anything else or anyone has any specific questions I'll edit the post and add it in. Once i feel like it answer's everything, I'll try and move it to the very first post.
