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Posted: 5/25/2014 8:24:29 AM EDT
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1) Caliber - 300 BLK 2) Case manufacturer - converted 5.56 3) Case length 4) Bullet manufacturer Sierra 5) Bullet weight 220 6) Bullet type matchking 7) Powder manufacturer Hogdon 8) Powder type H110 9) Powder charge - 9.5,9.9,10.2 10) Primer used - wolf 11) OAL 2.26 12) Firearm tested in (with barrel length) 8.2 Leonidas 13) Velocity - 992, 1082, 1130 Altimeter was 30.11 at 75F. First subsonic loads I've made. The 10.2 were pushing it. The 9.9 were all sub given the conditions (1133 fps based on temp), but right on the line. 9.5 were a good amount below. My questions is what do most folks load to ensure the round is always subsonic in different temp/alt conditions? I am thinking the 9.5 would be good, but giving up a little velocity; maybe 9.7 grains. The relationship of powder charge versus velocity was linear at 9.7 would give my about 1020. Would 1020fps always be sub (@ 0F 1050 is Mach, but would about bullet velocity change with temp & P)? All rounds functioned (bolt stayed open) and had good groups. |
| It's good to give yourself a little buffer room when loading for subsonic. When you get to about Mach 0.8 (80% the speed of sound) you get into the trans-sonic region. In this region, you can have localized supersonic flow of air over the bullet and you will get the ballistic crack of a supersonic round. I've chronographed the factory Remington 220 grain bullets and handloaded my own to match that velocity with AA 1680. |
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Quoted:
It's good to give yourself a little buffer room when loading for subsonic. When you get to about Mach 0.8 (80% the speed of sound) you get into the trans-sonic region. In this region, you can have localized supersonic flow of air over the bullet and you will get the ballistic crack of a supersonic round. I've chronographed the factory Remington 220 grain bullets and handloaded my own to match that velocity with AA 1680. Mcrit should depend on bullet profile, not sure on the Sierra MK. I ended up at 1038 today, no cracks could be heard. I had some in the low 1100s that I could hear a crack; so maybe the MK does well at keeping Mcrit high. |
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