I have a barrel that is 15-1/2" long. It was cut to 16" but I didn't take extension at chamber end into consideration, so now it is under sized. [:(]
My question is, can I silver solder a extension tube on the barrel to bring it back to legal length?  Thanks.
( I cut it off to remove the welded on comp. and I'm going to build a pick-up truck/beater rifle with it.)

I would leave it.  I am hard pressed to figure out who would ever care or know.(assuming you are not going to use in in a bank robbery or something)

Send it to Kurt Wala and have him add a Phantom or Vortex flash suppressor if it's preban or one of his brakes if it's a postban.  Ignore markm's advice.  He will not have to do your jail time, or lose his right to own guns if you are caught with an illegal SBR.

I would have a gunsmith permately attach a brake to reach the legal length.

I don't recall the exact name of a case (Weavers?!?) in which the shotgun was .1 inch below the legal length limit and the BATF when after that family.  To make the long story short, the poor guy lost his wife, his eldest son and other family members.

Always stay legal.  My 2 cents.

I appreciate everyone's concern on my keeping it legal. I have no intention of taking markm's advice. I ignore illegal advice. I know the law says minimum length of 16"; that it why I asked if silver soldering an extension tube on it would bring it back into compliance so I could use it to build a beat around rifle. I cut off a mini-y-comp brake, so I don't want to put another brake back on it. Can anyone here answer my question??? Thanks.

You would have to either weld it in place or use high temp silver solder to make it a permanent part of the barrel.  You also need to have the crown recut.  Even a beater rifle needs a clean crown to be reasonably accurate beyond rock-throwing range.  Kurt's brakes look very much like a standard A2 flash suppressor and would make your barrel legal again.

Thanks Dave.
I'll be using a silver solder "wire" w/ flux paste that they used years ago at work to put fittings on hydraulic lines. I don't know what the actual temp. is but I attached a MB to my R1 FAL with it and had to heat it to a dull red to get the solder to melt, so I assume it is high enough. I have all ready trued and put a 11 degree target crown on the muzzle w/ Brownells hand crowning tool, so that should be good to go. I just needed to know if the "tube" extension would be acceptable.
  I believe I have seen the brakes you mentioned at the Indy gun show. They look like a bird cage suppressor except they are only slotted in the top half. I imagine installing one of those will require turning down and re-threading the muzzle.(?) I prefer doing my own work if at all possible and want to build this rifle as cheap as possible. [:D]

Kilowatter:  Yes, any barrel extension, when permanently affixed, contributes to the overall measured length of the barrel.

If you're just going to stick an extension on, threaded or otherwise, you're OK so long as you weld or silver solder it.

If you put a muzzle device other than a solid barrel extension back on, just make sure it's post-ban legal if you've got a post-ban rifle.

I think you need to pin AND silver solder it to make it legal! It needs to be made so it cannot by any means be removed!! Don't ever, ever, ever risk a federal violation, its not worth your freedom!!!!!