• jmeineker@hvc.rr.com

Open Bolt Indicator – Trimmer Line

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Open Bolt Indicator – Trimmer Line

Since the Range Committee instituted use of Open Bolt Indicators (Trimmer Line) at our range, several people have expressed some concern regarding proper usage and whether or not there is any possibility of the line melting or leaving residue in the barrel of a hot gun.  I took it upon myself to clear my mind of any preconceptions and honestly research the premise.  I started by verifying the material makeup of the trimmer line that we supplied.  The trimmer line in the tubes at the range is made of polypropylene.  Below you can view the MSDS sheet for polypropylene and also another sheet produced by the manufacturer, Dynalab Corp.  Note that the temperatures listed on the MSDS sheet refer to a flash point of 500 degrees Fahrenheit and self ignition temperature of 735 degrees Farenheit.  The melting point of polypropylene is listed on the Dynalab sheet as 338 degrees Fahrenheit.

Melting Point of Polypropylene Polypropylene MSDS

I also checked the entry for polypropylene on Wikipedia as shown below.

Wiki Polypropylene

I wondered what material is used in the manufacture of official open bolt indicators (OBI) and found that all the ones I could find are also made of polypropylene.  Here are some examples.

Amazon.com_ open bolt indicator

So at this point I now know how much heat the line will take without causing a problem.  Now I needed to know how hot the barrel of a gun gets.  I purchased a thermal imaging device which I could easily point at my barrel to get an accurate reading.  The device has an accuracy of +/- 1 degree Fahrenheit.  I set up my .30-06 rifle on the bench.  The chart below shows the time I began and ended firing a total of 50 rounds, mostly 3 rounds at a time.  I measured the barrel at two places.  I monitored the hottest place on the barrel which was generally about one-third of the way between the breech and the muzzle and I monitored the breech temperature.

Barrel Test Temps

Note here that the chart shows that the entire 50 rounds were fired in 24 minutes.  The temperatures shown do not at any time come anywhere near the melting point.  I decided I needed to see what it would take to melt the trimmer line.  I placed several pieces of the trimmer line in my oven.  I started by heating the oven  and trimmer line to 200 degrees Fahrenheit for 10 minutes then I checked the integrity of the lines.  After that I repeated the procedure for each 25 degree increase until I reached 400 degrees.  Even at that temperature, the line did not melt.

Now I started to wonder what about a semi-auto rifle.  I found the following chart on the internet that shows that firing in full-auto the temperature could actually exceed the melting point.  However, the chart after that shows that there is a real problem with that.  If one is able to bring the temperature up enough to melt the trimmer line, there is an extreme risk of cooking off a round.

barrel temps Cook Off Temp Test

Ultimately, the evidence clearly shows that the melting temperature of polypropylene is high enough that, under normal circumstances, there should never be a time which the barrel gets hot enough to cause the trimmer line to melt.  If, under extreme circumstances, the barrel does get hot enough to melt the trimmer line, there is a distinct possibility of cooking off ammunition which should be a far greater hazard than any residue from the trimmer line.


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