I came across these eight-point irreversible Temperature indicators.
There small adhesive decals that will record peak temperature from 170-240F (10F increments).  Once a temp has been reached, the cooresponding dot turns black (permanently).
They make different temp ranges also.  

For a fun experiment, I was going to adhere one to a AR15 barrel.
Im not sure what the ideal temp range of such a indicator would be if one wanted to monitor the temp of a AR15 barrel during rapid fire. Im sure the C3 guys would want a higher temp. range

Cat # TL-E-170
www.omega.com

get ready to have them all turn black in a hurry, it'll be a short experiment

Quoted: get ready to have them all turn black in a hurry, it'll be a short experiment

I second that. Of course we could ask the questions of where you live, Semi or full auto, Shots per minute.