U.S. Military M65 Trouser Liner

The M65 pant liner is a sub-$10 hidden gem for cold weather camping or backpacking. The key is using them as an overpant as they're sized large enough and easily unbutton to quickly don or doff without having to remove boots or existing layers.

Perfect for those static situations like sitting around at camp or belaying or glassing while hunting. Would be way too warm for active use unless well below 0° IMO while wearing a base layer and hiking pant/shell.  

The m65 pant liner was designed to be buttoned into the m65 pant so they don't exactly have a good waist closure system, just a button with 2 slots of adjustment.  I'm going to mod mine with some 550 cord.

Regardless, 10ozs, very warm, lots of room to layer under and I paid around $10 shipped for an unused pair.

Cool added bonus is that since the sides have 5" gaps between the buttons, you can access your pockets without having to remove or unbutton the pants.  Also the crotch is open for quick and easy access.  Even with the gaps, the pants still add a ton of warmth.

Be sure to order the long version as the pant liners are cut short to not interfere with tall combat boots. Even with the long they will be a bit short but still work.  

Pics below are me wearing a size large long (18 degrees F, very comfortable).  I'm 6'2" 190lbs with a 36" inseam for context.

I used these in the late Cold War Army—wonderful when worn alone but sometimes not warm when buttoned in the cold-weather fatigue pants. It's the same construction as the wonderful poncho liners which thankfully the U.S. military still provides. In addition to Amazon, etc, you can get these at Army-Navy surplus stores, although Amazon etc tends to be cheaper.

Just concurring with other reviewers—it's a great piece of military surplus that is cheap, warm, and dries fast, although when buttoned into the Army cold-weather fatigue pants while on patrol in subfreezing weather on the Korean DMZ or in Germany, if you sweated in them, you got pretty cold. But those were always forced situations and not the same as much more leisurely backpacking or winter sports, or even search and rescue. They're a good bargain and work well as described.

The problem with buying any kind of military surplus online is you don't know what you'll get and it's such a hassle to send back. It's always best to go in person to a military surplus store so you can touch, examine, and pick out what you want. Most surplus stores carry the M65 liners.

If you have to go online, then still go to a military surplus store. One of the best is Old Grouch's. They have the liner for $3!  (oldgrouch dot com)