Having my new Garmin chronograph, I'm starting to test my chosen factory ammo in the gun(s) I intend them for. In this test, I wanted to see how the Remington .357 Mag 180gr SJHP ammo performed out of my Marlin Cowboy rifle. The Marlin Cowboy has a 24" barrel. My hope was that I would be getting some great performance and screaming velocity. My only knowledge up until now with this video, where the guy tested this ammo in a 16" Rossi rifle. He got 1,563 fps from a single shot, so I was hoping I was going to see higher velocity from the 24" barrel. Of course, that might not be true if the powder completely burned inside the barrel.
Here are the results from five shots:
While these results are great, it does tell me that my concerns were realized about the powder completely burning in the barrel. None of my shots got to the number the guy in the video got out of his 16" barrel. I realize there are some caveats, such as different chronos, different conditions (I was indoors), and different guns. However, I think it's pretty clear that the powder has burned completely inside the barrel before the bullet has exited, so the bullet is actually slowing down because of friction. I need to develop a handload with a slower powder so I can take advantage of the longer barrel.
Here are the results from five shots:
- 1,498.8 fps
- 1,548.3 fps
- 1,549.1 fps
- 1,512.4 fps
- 1,523.1 fps
- 1,525 fps average, with an energy of 930 foot pounds
While these results are great, it does tell me that my concerns were realized about the powder completely burning in the barrel. None of my shots got to the number the guy in the video got out of his 16" barrel. I realize there are some caveats, such as different chronos, different conditions (I was indoors), and different guns. However, I think it's pretty clear that the powder has burned completely inside the barrel before the bullet has exited, so the bullet is actually slowing down because of friction. I need to develop a handload with a slower powder so I can take advantage of the longer barrel.

