Guns should of course be used with shells which meet the criteria for which they were designed. This predominately though has to do with factors other than the minimum thickness of the barrel wall at some point down the barrel.
"Low" pressure shells is a term we have applied to shells having a lower Max "Chamber" pressure. This does not automatically equate to a lower pressure down the rest of the barrel. If two different loads push the same amount of shot out the barrel at the same velocity with one having say 10 K PSI & the other 8K PSI then those 2K PSI's have to be made up somewhere. They would thus be expected to be spread out over the forward portion of the bore thus would show a very slight "Increase" in pressure once about 4" from the breech has been passed. This is not to be construed as saying these low pressure loads are unsafe in a barrel having this wall down the barrel, but it also does not make them particularly safer on that account.
Total pressure can essentially be lowered by only one method & that is to lower the overall ballistics of the load, IE lighter shot charge, lower velocity or both. Max pressure can be manipulated by powder burn rate & a few other minor factors but as stated what you gain in one area you lose in another.
