Modern Versus “Vintage” Down Bags.

The quick answer as copied from a down site –

"What Is 'Fill Power'?
Down’s insulating capability is described as ‘fill power.’ Fill power is measured by how much a certain type of down can fluff up (loft) in a given space. Typically, fill power is measured as the number of cubic inches (in3) one ounce of down can fill—or, more simply, cubic inches per ounce (in3/oz). High fill-power downs (800-900 in3/oz) will insulate far better than low fill-power downs (around 300 in3/oz) of the same weight."

That should pretty much explain it…

I haven't been able to find any data regarding the M-1949, but its fill power is clearly lower than modern bags.

My guess would be much lower…and I would guess plenty of feathers in the mix.

Just a guess.., the weight of lighter nylon shell fabrics used today compress the down fill less. That, combined with higher quality down used in premium bags (fill power/expansion) could account for lower temperature rating as compared to weight ratio Some old bags use heavy metal tooth zippers also.

All joking aside, aren't feathers–feathers?

Here is some info on that bag.
http://www.vietnamgear.com/kit.aspx?kit=677

As you can see it was made with a mix of down and feathers.
Technically down is a type of feather however for simplicity sake we refer to the cotton ball kind of thing as down and the ones with some spine as feathers.
This link http://www.biosci.ohio-state.edu/~jcondit/current/feathers/feathers_main.htm
lists the six types of feather and has a picture of the three most commonly found inside a sleeping bag. Yours will have all three types, a WM will have a max of 5% of the "semiplume" , in practice provably less.

The finer type "down feather" also can vary a great deal, hence the loft rating between 500 and 900 used by the better brands.
WM rate theirs at around 800.
That simply means a 900 fill will weigh almost half as a 500 for the same loft and (close enogh) warmth.
BTW another site lists a filling of 2 lbs and 8 oz for that M1949 ,about the same as the -40f rated WM Bison .

Just had another look.
The total weight on the second site is listed at 6 lbs 2 oz something , so I guess it is for the "inner" only of the two part bag.
That means the shell alone of the "inner" is about 32 0z
Franco

Franco,

I should have mentioned that I removed the water-resistant shell on mine, as it is about 2 pounds. I plan on finding a modern GI version in Gore-Tex because I have a decent source for those.

So I guess the short answer is that feathers aren't all created equal, and the military was clearly using a lowest bidder. Because this bag was free, I have decided that 6 pounds in weight isn't quite worth $450 yet.

You can get a 20dF campmor bag that weighs about 2#-4 for about $130.

I just got my Versalite. It is a lovely bag. The loft is the best I've ever owned.

We had some of those old bags when I was a kid. The really old ones were a tan color; the newer ones were olive drab.

They definitely were not the best bags. Despite being heavier and bulkier than some of our other bags, they were not as warm. I still have the Seattle Quilters (long since out of business I'm sure) ca. 1959 down bag that was dad's and my old down bag from the 70's (got 'em when dad passed), but those old USGI bags are long gone. They just weren't good bags.

Look for sales or at Campmor. If you're near an REI they have fantastic used gear sales. Save the old USGI bag for sleepovers for the kids or maybe for car camping. Save your bones and get a real bag for BP'ing.