Fair forewarning: I haven't attended a MSW game. Not a big fan of the whole milsim airsoft thing. I don't play outdoors.
That being said, I have spent a few moments here and there rocking a ruck, and I did spend the night at a holiday inn once. So, my advice is as follows:
It totally depends on what you are looking to carry and how big your budget is.
If you are planning on carrying a second primary, or carrying a spare DMR / bolt gun in addition to a rifle, AND you have a pretty decent budget, check out Eberlestock. Some (but not all) of their packs are set up right proper for packing a rifle. Within limits.
Packs to go for:
Eberlestock Gunslinger / Gunslinger II: Slightly larger than an assault pack. Great for carrying most AR-pattern rifles (without a magazine, or without optics. Your choice) or any long gun up to and including a Barrett. Kinda small as a primary pack for a weekend camping. Find a smaller pack that you like (if you don't have one already) that has MOLLE on it and get a set of 5.11 Tier straps and you can expand the volume enough to get by. 5.11 MOAB 10 works great (and comes with the Tier straps). Phantom is similar payload, but set up more for long range shooters.
http://www.eberlestock.com/G2%20Gunslinger%20II.htmG4 Operator: About the goldilocks size for a weekend pack and a rifle. 4700 cubic inch pack, nice wide scabbard.
http://www.eberlestock.com/G4%20Operator.htmJ107M Dragonfly: Note that this is the "M" variant. This is IMPORTANT. The "M" variant has a wider scabbard than the standard J107 hunting pack, so you can carry an AR-pattern rifle (with either optics OR a mag). This pack is pretty dang brilliant. It expands from an assault pack all the way up to a 7900 cubic inch monster. It's basically a couple of pockets built around a cloth bucket. I'd recommend picking up a couple of the Spike's Camp Duffels (good) or the waterproof duffels (brilliant). They zip and clip onto the pack for expansion. If you pick up one or more (waterproof!!) duffels, you just shove your camp stuff into them and pull them out or off the pack when you get to camp, and then fold the bag back up into an assault pack. If you are looking for something that you can do more than just a weekend out of, but is still manageable for shorter outings, it's a really good choice.
http://www.eberlestock.com/J107M%20Dragonfly.htmEberlestock packs to AVOID:
X3 LoDrag: It's a small assault pack with the ability to also carry a bolt gun. Emphasis on small.
Skycrane II: At first glance, it seems just perfect. The G1 Little Brother works great as an assault pack, and docks neatly into the load bay of the main pack. There's an optional scabbard that you can buy that mounts to MOLLE loops in either the main pack or the little brother pack. Just don't. Save yourself the heartbreak. The optional scabbard blows goats. For quarters. In Medford. It has 6 columns (left to right) of MOLLE (sort of) that you can use to attach to the seven (yes, an odd number) columns of MOLLE on the main pack or five (yes, an odd number) columns of MOLLE in the little brother pack. Except, instead of giving you MALICE clips, or attaching MOLLE straps to the scabbard, it comes with TWO old-school ALICE clips. Nothing you can do easily (to include buying your own MALICE clips) will make the scabbard mount tightly to the pack. Especially since the scabbard has all of like two rows (vertical) of MOLLE loops, and they are right freaking next to each other. Unlike the packs with dedicated scabbards, it's guaranteed to go all floppy on you. The design of the scabbard itself is worse than half-assed (think pancake holster for your long gun). And since it sits inside the load bay, it means that the awesome Little Brother pack doesn't zip into the load bay properly with it installed.
Other packs I've had good luck with:
5.11 Rush 72: Big ole bucket, really. Has a spot between the main pack pocket and the front organizer that you can shove a jacket or helmet into. Has MOLLE on the sides so you can Tier an assault pack to it.
Blackhawk Special Operations Medical Bag: Lots and lots of organizer spots. Designed to carry all sorts of medic goodies. Repurposes really well for being able to keep everything organized. If you look around, you can find them lightly used for screaming deals.
Tactical Tailor Extended Range Operator pack: A little smaller than most of the others. Think of it as a large assault pack that you can clip a smaller assault pack to. High quality, good internal organization.
Options on a budget:
USGI rucksacks WITH FRAMES. Always, always, always get one with a frame. If you get it used (surplus store) check the frame CAREFULLY to make sure it isn't jacked up.
Regular civvy-style camping backpack (i.e., NOT a daypack. Something with a frame preferably) with an aftermarket camouflage rain cover. Eberlestock sells some. There are bunches of them available on amazon / ebay. As to the pack itself, check local camping shops, sometimes you can find good deals for last year's model or whatnot.
Like anything else, you get what you pay for in terms of quality, comfort, and durability. If you treat your gear gingerly and pack light, you can get away with spending not much. If you are like me, you end up spending a bit more. And next thing you know, you have an entire garage full of tactical man-purses.