The military Modular Sleep System: layers, ratings and spotting a fake

Plenty of civilian campers, hunters and preparedness-minded households sleep in surplus military bags, and the one most of them end up with is the Modular Sleep System, or MSS. It became standard issue for American soldiers around the turn of the century, and used and new-old-stock sets have been sold through surplus stores and online auctions ever since. Its appeal is simple: one kit of snap-together layers covers nearly every climate a person is likely to camp in, and it is built to take years of rough handling.

Build quality

The sets were made in the United States, chiefly by Tennier Industries under military contract. The construction shows it. Zippers are large and run without snagging, and they open from the inside quickly, which matters when someone needs to get out of a bag fast. The shells are ripstop nylon, light but not flimsy, and the straps, snaps and stitching are heavier than on most retail bags. Outdoor retailers tend to compete on weight; the MSS was designed to survive being thrown in the back of a truck every day, and it does.

The components

The original woodland-pattern MSS has four parts:

• A waterproof, breathable Gore-Tex bivy cover. A bivy is a fitted outer shell that keeps rain, wind and snow off the bag, and in many conditions it replaces a tent.
• A green patrol sleeping bag, the lightweight layer, rated roughly 30 to 50°F.
• A black intermediate cold weather bag, the heavier layer, rated roughly -10 to 30°F.
• A compression stuff sack with multiple straps that squeezes the whole system down to about one cubic foot.

Each layer can be used on its own or snapped to the others. Put the patrol bag inside the intermediate bag, both inside the bivy, and the combination is rated to about -30°F.

The later ACU-pattern set has five parts: the same idea with a foliage-colored patrol bag, a gray intermediate cold weather bag, an ACU-camouflage bivy, and two compression sacks, one large and one small.

Putting it together for the night

Choose layers by the lowest temperature expected, not the evening temperature.

• Above about 30°F: patrol bag inside the bivy.
• From about -10 to 30°F: intermediate cold weather bag inside the bivy.
• Below about -10°F, down to about -30°F: patrol bag inside the intermediate bag, both inside the bivy.

Snaps and ties along the edges join the bags so they behave as one bag rather than a pile of loose layers. That stops the tangle that happens when someone rolls over in two separate bags, and it means the whole system unzips as a unit.

Those ratings assume the sleeper is lying on an insulated sleeping pad and wearing dry base layers. Bare frozen ground pulls heat out of the body far faster than cold air, and no bag makes up for a missing pad. Moisture is the other enemy: on long cold trips, sweat and breath condense inside synthetic insulation, so air the bags out in the daytime whenever possible.

Ratings and testing

Military sleeping gear is developed and tested by the Army’s research labs at Natick, Massachusetts, and the MSS ratings come from that work rather than from a marketing department. They are widely considered more honest than the comfort claims on many retail bags. Still, soldiers are expected to sleep in issued clothing, and people vary a lot in how warm they sleep, so treat any rating as a guide and test the system on a cold night close to home before relying on it in the backcountry.

The drawbacks

Weight is the big one. A complete system comes in at 11 pounds or more, several times the weight of a good down winter bag. The trade-off is durability, and for anyone who leaves the tent at home and sleeps in the bivy, some of that weight is recovered.

Bulk comes next. Compressed, the kit is still about a cubic foot, too big for the inside of most backpacks. It straps on well to the bottom or outside of a pack frame.

The camouflage is a minor point. Woodland and ACU patterns blend in well in some places and poorly in others, but a bag sits still in camp, and it is easy to tuck under cover.

Care and storage

Synthetic insulation survives damp far better than down, which is one reason the military chose it, but it still loses loft if it is crushed for months. Keep the bags loose at home, hung in a closet or stuffed into a large cotton or mesh sack, and use the compression sack only for travel and for a bug-out kit that gets unpacked and aired every season.

Wash the bags rarely, in a front-loading machine without an agitator, on a gentle cycle with a mild soap made for synthetic sleeping bags, then tumble dry on low heat until fully dry. A bag that goes into storage even slightly damp will smell and can mildew. Wipe the bivy clean and let it dry before packing it. When water stops beading on its outer fabric, a wash-in or spray-on water repellent restores it; the waterproof membrane inside is usually still sound.

Check the snaps and zipper pulls before a trip. Surplus sets sometimes arrive with a missing snap or a torn stuff-sack strap, and both are easy to fix at home with a snap kit or a needle and heavy thread.

Prices and where to buy

In 2010, brand-new ACU sets were listed for as much as $600, while used surplus sets sold for about $120 to $160 at army-navy stores and on eBay. Surplus prices have moved since, but the pattern holds: a lightly used set costs a fraction of a new one and, because the bags are so tough, is usually in very good condition.

For anyone willing to pay more for less weight, commercial modular systems such as those from Wiggy’s and Snugpak work on the same layered principle and pack smaller.

How to identify an authentic set

A lot of cheap bags are sold as “GI sleep systems” or “military spec” for well under the going surplus price. Many are imitations with thinner insulation and weaker zippers. The easy check is the National Stock Number (NSN), which NATO countries call the NATO Stock Number. It is printed on a sewn-in label on each bag, bivy and compression sack.

Woodland pattern set, NSN 8465-01-445-6274

The first version of the updated system, made by Tennier Industries. Four parts: black compression stuff sack, black intermediate cold weather bag, green patrol bag and woodland-camouflage bivy.

ACU pattern set, NSN 8465-01-547-2757

The later release, also by Tennier Industries. Five parts: two foliage-colored compression sacks (large and small), gray intermediate cold weather bag, foliage-colored patrol bag and ACU-camouflage bivy.

Check that the labels are present on every piece, that the NSNs match the set being advertised, and that the zippers and snaps are the heavy military type. A seller who will not show the labels close up is best avoided.