I haven't seen any numbers published yet, so I ran some BBs through my chrono to compare propane to propylene. Here's what I got:
Propane
404.4
397.5
378.5
386.7
390.2
398.8
387.5
375.8
388.5
381.8
Average = 389.0
Propylene
434.4
434.1
421.2
414.2
416.5
407.5
410.6
407.3
399.0
415.8
Average = 416.1
- The test was conducted in 19℃ temperatures, 35% humidity.
- Propane used was Coleman brand. Propylene used was Bernzomatic brand.
- The rifle is an upgraded WE M16A3 with a 500 mm Angry Gun tight bore barrel. The NPAS valve was opened more than normal for consistent gas flow.
- The magazines were new, previously unused WE M16 types of the latest design, well marked to avoid confusion between gas types.
- Each mag was purged, fully filled, purged again, then fully filled for the test.
- The mags remained together for thirty minutes prior to the test, so their contents were at the same temperature.
- I fired one throw away round prior to testing, then fired five rounds with each gas, switching back and forth to keep the magazines from becoming cooled by their respective gases.
- I threw out the highest and lowest velocities for each gas to avoid anomalous reading (although the highest and lowest numbers were not inconsistent with all the other readings).
The real test will come when temperatures drop lower. For now, it can be inferred that propylene delivers slightly higher velocities than propane (about 7%) in warm weather.
Of further note: the type of propylene I used had a scent added to it, just as propane does. If you don't like the smell of propane, you will hate propylene. It has a very sharp and acrid sort of scent.
I'm looking forward to seeing numbers from other testers, especially in cooler temperatures.