Chrono-ing three 300 Blackout loads - what a difference...

This is a discussion on Chrono-ing three 300 Blackout loads - what a difference... within the General Firearm Discussion forums, part of the Related Topics category; Some of you may recall my concern about my 20" barrel shooting 276 fps slower than Hornady's spec'd velocity. So, due to some inspiring and ...

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    Chrono-ing three 300 Blackout loads - what a difference...

    Some of you may recall my concern about my 20" barrel shooting 276 fps slower than Hornady's spec'd velocity. So, due to some inspiring and encouraging suggestions I thought it would be interesting to chrono some totally different loads in a different gun and see what happens.

    Wow! What a difference!

    The gun is one I built with a Spikes lower, a JD Machine upper, a BMC bolt and carrier assy, a Wilson Combat 300 AAC Blackout, Tactical Lightweight, 16", 1-8 Twist, Stainless, a Wilson Combat adjustable gas block, and a Midwest Industries SS gen 2 handguard, and a Magpul stock that will soon be replaced with an Ace lightweight fixed stock. The pic below is with a Troy VTAC Alpha handguard instead of the Midwest Industries. I could not get the Alpha to center on the barrel and the MI SS gen 2 is smaller, lighter, and better - I just don't have a pic with that handguard:



    The temperature was 56 and it was about 4:30 in the afternoon, so this time of year, I would be running out of daylight quickly so I only shot 5 rounds each. Well, 6 for the 220 gr load. Here's what it looked like. I've included the manufacturer's spec'd velocity for reference, the average, and standard deviation for each load:

    Barnes Vor-TX PNW 125 gr Later-220 gr
    spec'd vel >> 2350 fps 2180 fps 1015
    1 2339 2195 1042
    2 2348 2156 977
    3 2406 2143 1023
    4 2421 2150 1046
    5 2358 2158 1026
    6 --- --- 976
    average 2374 2160 1015
    std dev 32.8 18.1 28.4

    There are several interesting things about these numbers. One, notice how close all the averages are to the manufacturer's spec'd velocities. The 220 gr is right on, the other two are off by about 20 fps. A higher temperature would make that even closer.

    Second, notice the standard deviation (SD). If you aren't familiar with SD, it's simply a measure of how close the data points lie from the average. It's possible that two sets of data can have exactly the same average, but different SDs. In the above case of bullet velocities, the one with the smaller SD represents a more consistent loading.

    The SD is important because if you just look at the data points, it looks like the 220 gr has a pretty bad variation in velocity, but the SD shows it's better than the Barnes Vor_TX.

    This batch strongly suggests that the manufacturer's spec is a pretty accurate indication of what can be expected for their load under similar conditions of testing. I've also seen this for the Federal .308 168 gr match BTHP.

    These results are a factor of 10 better than what I got for two Hornady .223 loads. That's just too much difference. Part of the closeness of the 300 BO results probably comes from the manufacturer testing in a 16" barrel. I know for a fact that's the barrel length spec'd for the 220 gr.
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    Interesting. It also takes your chronograph off the table as a source of variance I suspect, as discussed in the .223 thread.

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    Quote Originally Posted by Rock and Glock View Post
    Interesting. It also takes your chronograph off the table as a source of variance I suspect, as discussed in the .223 thread.
    It sure seems to. Not the same as 'calibration' per se, but...
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