The comment on using tape is the correct answer. I designed and manufactured strain gage load cells, and can suggest the following steps. I gaged several barrels for the Pressure Trace.
1) clean area to be gaged with solvent, wipe with clean cloth to dry, do not touch the area, wear vinyl gloves.
2) scribe a line with a ball point pen where you want the gage. For a Pressure trace, this line should be perpendicular to the bore. The reason for the ball point is that it burnishes the surface but doesn't scratch it.
3) clean the area again
4) If the gage is in a plastic folder, leave it there, don't touch the gage with your bare hands.
5) lay the gage folder on a CLEAN piece of glass (mirror, plastic, metal, formica) with the top of the gage up. (the side that will get glued is then down) Work the terminal end of the gage out of the folder about 1/4 inch. Use a set of tweezers.
6) Lay a piece of masking tape across the end of the gage, covering the gage end by approximately 1/8 inch, and extending out to either side by a half inch or more. If there are no leads on the gage, have the tape cover the solder pads ( terminals) on the gage.
7) with an x-acto knife, cut the masking tape off each end about 1/4 inch from the gage. Then cut off the tape parallel to the end of the gage at about 1/4 inch from the end of the gage, don't cut the leads.
8) Pick up the gage/tape by lifting the masking tape with the tip of the xacto knife. The knife then contacts the tape, but not the gage.
9) using the indicator points on the gage, align it with the scribed line, and lower it into place so the tape sticks to the barrel. Gently burnish down the tape to secure the gage in place.
10) With the tweezers in your left hand, lift the untaped end of the gage, put a drop of instant glue (or VERY thin epoxy) on the back of the gage, then with a piece of Teflon tape ( or saran wrap) between the gage and your right hand finger, roll the gage down onto the barrel, from the taped end towards the open end. Hold in place until instant glue sets. If using epoxy, place a small piece of foam plastic on gage, then a small piece of wood or metal, and tape down securely.
11) After glue is cured, remove the masking tape by brushing on alcohol or lacquer thinner, which will make the tape adhesive soft. Clean up area with same solvent ( if you have leads, test the solvent on the leads before starting install) don't try to clean off the stuck-on saran wrap, just trim away what isn't stuck to the barrel.
If the gage had leads already soldered on then you are good to go., if you have to solder leads onto dots on the gages follow the next procedure:
1) Clean the terminal area with an eraser on the back of a pencil. (remember you wee supposed to not get glue on this area. If you did get glue here, you may need to use fine sandpaper to start)
2) With a small ( 15 to 25 watt soldering iron) first place the solder wire directly on the cleaned terminal, then bring the iron STRAIGHT down to touch the solder and terminal area for about 2 seconds, then lift iron STRAIGHT up. This should result in a small solder ball on the terminal.
3) do same thing to the wire end
4) with a small piece of masking tape, tape the wire down with the tinned end resting on the solder ball.
5) Wipe the end of the iron with a paper towel, apply a small amount of solder to the tip ( about 1/8 inch worth of solder wire), then immediately bring the tip STRAIGHT down on top of the wire/solder ball joint for about 1 second, then lift the iron straight up. Clean area with alcohol.
This soldering technique is what is used in manufacturing transducers, and will benefit from a bit of practice.
After cleaning up, cover the gage with some RTV ( caulk) to prevent accidental damage.
Notes on firing:
Do it slowly. Remember, your gage is directly over the chamber, the hottest point in the rifle, and you do not want to get the gage too hot. Excess heat will cause the glue to fail, and the instant glue fails earlier than epoxy. I eventually melted the glue on the gages on several of the rifles I was testing.
To locate the gage, I placed an empty case in the chamber, then put a cleaning rod down the muzzle and marked the rod with a wrap of tape. Then placed the case on the end of the rod, and aligned the tape at the end of the barrel, and marked the position of the gage on the case and barrel. I then cut the case at that mark, and measured the case wall thickness, and the barrel diameter for the strain equations.
You may have to cut the barrel nut on an AR to get to the correct location for the gage
