A good point is made above: the silver braze needs the proper flux.
There are different melt temp silver brazing alloys(also called hard silver solder), some melt and flow at 800 deg F, and others run between 1000 deg to 1500 deg. Each one needs a specific flux.
The hard solder mentioned above is very strong, and would hold a lug on a guard with no problem. If you solder two steel bars together, you can try to break the joint, but the bars will twist all to heck before the solder gives way.
The least complicated solution is to buy the matching solder/flux set from Brownell's. They have high quality proven goods, and 'how to' instructions that come with their products.
To get more efficiency out of a torch, one can build a little 'reflector oven' out of two or three firebricks. Set the work up on a brick, with another behind the work. These bricks heat up and reflect the heat back on the work, making the solder flow better. A charcoal block is really nice to solder on for delicate work, the charcoal helps the work from oxidizing. Obtainable from Rio Grande and other jewelry suppliers.
The 'silver bearing solder' is actually quite good for many repairs. It is convenient to use, and very strong. If soldering a lug on a guard, for example, I'd like to have the lug screwed to the guard, and soldered. I would not trust it as a permanent repair for such mechanical strain situations. For soldering on front sights and barrel lugs, it's great.