Posted: 3/3/2017 7:42:40 PM EDT
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Does having a clean cold bore affect the muzzle velocity? For you chrono users, have you experienced a difference in MV on your first shot after the rifle has been cleaned compared to later shots?
Just curious how much a difference this causes in MV if any? My .270WSM was averaging 3400fps the other day. Was wondering how much a difference I would see on a clean cold bore shot? |
| It may, but if it does it will generally be negligible; a clean vs. fouled barrel will affect your "cold bore" zero shift much more. If your barrel is new, you will generally see a muzzle velocity shift after it has been broken in (i.e. Our new M24's shoot slow for the first 50-200 rounds, then gain an average of 50-70 fps once broken in). |
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So I was shooting a week ago and averaging 3400 FPS with a box of ammo that a guy gave me that was supposed to be 270 WSM 140 grain Accubond CT Winchester Supreme ammo (according to opened box). I did not clean the gun all week since shooting. I came back and was shooting what I thought was a newer version of the same ammo...the box looks different and is labeled Winchester 140 grain Accubond CT Big Game Expedition. I thought this was the same ammo with a new box design but it may not be. It was also much windier and colder today( Will wind affect Muzzle Velocity?...I know it will affect down range velocity. I'm assuming it's negligible on MV). I was shooting mostly with the wind.
The different factors I had were the gun hadn't been cleaned for a week(last week I didn't start chrono until 4th shot and shot around 20 rounds), the position of the radar chronograph may have been slightly different , more wind, colder, and possibly different ammo. This time I was averaging 3176. Last time I was averaging 3400 fps. I did find one round on the ground which I think was from last time...it shot faster than any other ammo today, but slower than any ammo last time. It was at 3344 fps. This is only 56 fps slower than last time but 168 fps faster than this time...I know this bullet didn't come from any ammo today so it had to be from last time. I also tried moving the position of the radar for one shot more similar to how I had it last week and it was was only 21fps slower than the days average, but for this test I was using 130gr and the days average was 3292 for the 130gr. I shot about 40 rounds on the day...(a box of each weight) What do you think caused my difference in MV...can you eliminate any of these as possibilities? I'm guessing it's the ammo but wanted to see what someone more knowledgeable thinks. I'm thinking about testing the gun one more time after cleaning to see what MVs I get. Thanks |
| It's the ammo and I'd be willing to bet it's the bullet weight as well as the powder volume. As a rule of thumb, if I don't know exactly where a bullet in a previously opened box came from, I don't really trust it for anything other than grouping drills because of this exact issue. Temp and/or wind will not affect your MV to the extent you saw from lot to lot. As for shooting in the wind, if you have a crosswind within 70-ish meters it will give you the appearance of a MV increase or decrease due to crosswind jump, but it takes a lot of wind to produce the results you recorded. Shooting into a headwind or with the wind technically does affect your trajectory but not to the extent you can compensate for it so don't even worry about it. |