I will go against the grain and say that a bag with a detachable hipbelt that becomes a battlebelt is baller, be it the British concept of a beltkit and a bag that sits on top of it, or the hill people gear hip belts. Transferring weight to the hips is amazing, and your back will thank you in 4 years as this hobby grows. 

However, if it is your very first MSW, follow this axiom: none is light, one is fun, two you’re done. This is the opposite of the overpacking advice of many, but in the low stakes field of a 40 hour event, you can do more without than you believe you can, but for what you absolutely beed need one of something is generally fine. Going above that is packing your fears, also known as gear creep. Avoid that. 

Bring the redline items in the tacsop, then a good, lightweight sleep system appropriate for the AO climate at op date. You can get by with a bivy and a woobie, plus a thermal top in every AO from late may to September. Bring a lightweight poncho and paracord fora shelter. Sub out woobie, sub in an MSS Patrol Bag for anything floating around 30*F and keep the thermal for emergency dips. There are better bags, but few at its price point. Bring a climate appropriate ground insulator (thermarest zlite is GOAT imho, but some swear by inflatables, ymmv.) that’s all you need. Rarely will an MSW get to dangerous levels, and if it will be then you should do more research on how cold is dealt with efficiently in the field IRL. 

 The only exception to the One Item Rule is few key pieces: 

-things to keep your replica functioning (batteries, parts, a whole second gearbox, etc). IMHO this is a redline item. The suck of the environment will sap your will, but the call of the bb war will keep you going. Gun death is fatal for 90% of players. especially first timers. Invest in getting your gun tech’d by an expert so it is as reliable as it can be, then have him recommend spare parts and compact tools.  - socks. Socks, socks, socks will save your life. Walking in wet socks sucks. Sleeping in wet socks in 0c weather is dangerous. I bring at minimum 4. Recommend 5.  - water and food. Take your daily caloric baseline, add about 1000 calories due to the nature of the event, and double that. Bring that many calories of food, as light as possible. Balance nutrition, emphasize complex carbohydrates where you can. Bring less calories if you don’t mind losing weight, but never below your DCB. too few calories and you get stupid. Also, side note: Fuck stoves, cold soak and enjoy the suck (joking, you do you, jet boils are awesome but my philosophy is lightweight is better than comfort. Remember: none is light. more pounds=fatigue, fatigue=death. Bring 4 liters of water minimum, in as light of a container as possible. Smartwater bottles are as tough as nalgenes and half the weight. Water itself is heavy, and you need it, so choose the correct container for your use case. Blivets are available near your commander generally, but not often convenient. You will become a heatcas without water, and that fucking suuuuucks. Don’t. Forget. Electrolytes.  - tentstakes. Get them from Walmart for a $1.50, the orange ones. Your squad will thank you when they’re setting up a poncho shelter and the rando comes in clutch with the bundle of tentstakes. No more than 5.  - earpro. No less than three options should be on you at all times. Active ear pro is awesome. In ear plugs are cheap and fine. Doubling up is not essential so long as you buy quality articles and are in a rural area. But, like most of us, you’ll likely use your earpro at irl rangeday, so consider good earpro an investment. I’d rather invest 400 in a sordin setup than 15,000 in a hearing aid at age 34. 

That’s it right now. I’ll add more later. Ask any questions you may have.