Aahhhhh!!! Chopping off lengths of shotgun barrel is anathema to us Brits. With the shock horror out of the way I have performed this operation on a gun owned by local game keeper for the same reasons you describe, Using a gun in thickets and dense woodland. I did put up all the arguments with a suggestion that he purchased a 25 inch barrel set for his Webley and Scott 700. Unfortunately, I was not persuasive enough though and after the suggestion that he would take a hacksaw to the gun which I am sure he would know I would relent and take the barrels down to 25 inches. I must add here that it was in the early 1960s when Webley and Scott shotguns where a reasonable cost and another barrel set would not have broken the bank.
My method was first to measure the length of finished barrel required then add an eighth of an inch to allow for trimming and cleaning up, then cut making sure the cut is square and straight. Next file up a three eights in length fillet of mild steel shaped to take up the major part of the space between the bottom and top ribs and each of the barrels the closer the fit the better. Then prepare the barrels for soldering and the fillet, firstly at about an inch from the muzzle wrap wire around the barrels and between the ribs and the wire slide in some metal wedges (I used Aluminium wedges a good heat conductor) doing this is all to keep the soldering heat away from the ribs and melting the solder holding them and keeping everything tight. Next flux up the inside of the ribs and barrels also flux the steel fillet and to improve the odds of first time success I did tin the metal fillet before fitting (you MUST use a non-corrosive based flux) now the solder I recommend is electrical grade of 60:40 Tin Lead because it has a lower melting point of the common available solders.
I held the barrels in a vice fitted with wooden cheeks at an angle of about 20 degrees with a wet cloth wrapped around the barrels from the wire wrap down towards the breach (do not fall in to the trap of mounting the barrel vertically muzzle up) because there is a good chance the solder will empty out of the joint caused by gravity, and unless you have some form of stop on the fillet it has the potential to drop out of sight when things expand due to the heat. WORK QUICKLEY!!!! And try to keep the heat to just enough to get the solder to flow. When everything has cooled down use a flat fine file to trim the fillet taking care to keep everything true and at right angles. Even now I have not changed my mind, purchase a low price gun with short barrels and leave the said gun as designed and manufactured.
