I would lean towards swaging, as described above, before using a sealer like silicone. You need a little "tougher" barrier to the gases coming back around the primer than silicone, IMO. As my Grandaddy used to put it, "That's a lot of sugar for a dime". You're going to a lot of trouble to save a very little money, unless there are other considerations in Mexico, such as scarcity of 209 primers, or maybe a "lot" is something like thousands.
If you really want to pursue it, make up a little swage like Kutter described, load one really light load with a 57, put it in an old beater single shot and shoot it with a string. Then evaluate for gas leakage. All is pure speculation until you do.
It does not take nearly as much to prevent gas leakage as we think, sometimes. I had a dear old friend who was raised very poor in Texas. They had access to a single barrel 12 ga. as kids, but no access to 12 ga. shells. But, they had access to plenty of 16 ga. shells for some strange reason. He said they would wrap tape around the shell head until it was a snug fit in the chamber, then happily kill game along the Red River. Did this for years with no apparent damage to the gun.
SRH
