This is my understanding so if sombody noticesI'm wrong on something speak up.

A strain gauge is nothing more than electrodes placed on the barrel that are connected to a computer. They measure the flex of the metal. The problem is that you have to know exactly what alloy your barrel is and you have to place them in a certain place (over the bullet I believe). With muzzleloaders having no case a powder increase/decrease changes the place they have to be and and if they're not placed exactly right can give you false readings. Also I believe you have to calibrate them with a known load. It's just software and the electrode aparatus.

There's also another way to estimate pressure that relies on software and a chronograph. You imput the velocity, bullet etc. and it spits out a pressure graph. It's not that accurate either and can be off by a few thousandPSI if all the variables aren't right.Plus I don't think there's a version for muzzleloaders with all the extra variables we deal with so there's an error factor built in.

The only way to know for sure is a test barrel. And even that will be off because of variances in the bores. It's still by far the best way to do it.