Captchee -
Once again, thank you for the kind words. I'm not too sure I understand the question about breech design, but I'll give it a try. Most of the guns I have seen have a very definite shoulder at the bottom of the threads in the barrels, and the plugs snug up at two places, one at the bottom of the threads, and the other at the outside end of the barrels, where there is a shoulder on the breechplugs. Sometimes the shoulder inside the barrels is not very wide, and may have disappeared due to burn-out in the breech area, but (so far as I know) it is always there. Several years ago, one of the modern makers of double guns(can't remember who) made their doubles with breech plugs that were completely screwed into the barrels, with no plug visible outside of the ends of the barrels, similar to a hook breech Hawken. The nipple holes were drilled and threaded right through the barrel into the plug. It was a pretty clunky looking design, and one that must have had old Joe Manton spinning in his grave.
The browning method I am using now is a process developed by Dr. Oscar Gaddy, a university professor from Illinois. Briefly, it works as follows:
1 - Polish the barrels to 400 grit or finer (400 is usually enough). If you have tested the barrels with acid, this area will have to be polished bright again, before proceeding.
2 - Apply a diluted solution of Laurel Mountain Forge Browner (see below).
3 - Hang barrels in a warm, humid location until a coating of rust forms (may take 4 hours to overnight, depending on temperature and humidity).
4 - Dip rusted barrel in a 15% solution of Radio Shack circuit board etchant in water (I keep a capped piece of 3" PVC pipe as a dipping tank). Dip for no longer than 5 seconds (important)!
5 - Immediately after dipping, flush barrels clean with a garden hose and lots of water. I do this outside my shop in the yard. It is important to stop the etchant from working any longer than 5 or 10 seconds.
6. - Card (scrub down barrels) with medium steel wool under water. This will make the pattern begin to appear.
7 - Repeat steps 2 through 6 until the barrels are dark enough to suit you. This may take a lot of passes, maybe as many as 8 or 10.
In step 2, dilute the LMF with 2 parts water for the 1st pass only. The next couple of passes can be 5 parts water to 1 part LMF. After that, the remaining passes can be 8 - 10 parts water to 1 part LMF. I no longer use the browner diluted any more than this - the dip in Radio Shack etchant serves to remove the rust from the white striations without affecting the brown striations.
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Hope this helps, Bill