Incorrect temperature reading

Hi
I don't think 3d printer is showing the correct hotend temperature.
When I try to print PLA at 200ºC it seems too cold and the filament skips on the extruder.
I have a temperature gun and it reads 160ºC at the nozzle when the hotend is set to 200ºC.
When I set the temperature to 240ºC the prints come out well and the temp gun reads 205ºC.
I was wondering if there is a way to fix this and further troubleshoot the issue?
I tried switching thermistors and the issue stayed.
I have a smoothieboard and E3d V6 with a cartridge thermistor.My old thermistor was a bead style thermistor.
Thanks
Tanner

Temperature readings will vary from one printer to another and one hot-end to another. There are all sorts of errors that can creep in. The thermistor reports the temperature at the thermistor, not inside the nozzle. If you need to set the temperature higher at the thermistor to get the temperature inside the nozzle higher, do it.

You can look up data on the thermistor and calculate Steinhart-Hart coefficients to model it, and it may be a little better.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Ok. The only reason I as asking is that it used to read perfectly, and about a month ago it started having print fails due to temperature.
While troubleshooting it I found that the temperature was not hot enough, and adjusted print temperatures accordingly.
Should I be concerned with this change, or is it something normal?
Thanks
Tanner