interesting subsonic situation

B

brmfan

New member
So for over a year I have been using 11.0 grains of Trailboss, CCI BR2, 175 SMK, and Lapua brass for my .308 subsonic loads out of a 21" Remington 5R (1:11.25) w/ a YHM Phantom. It's always been very accurate and consistently stays subsonic between 20-80 degrees @ 550 ft above sea level.

I recently had another Remmy 700 PSS (1:12) cut down to 16.5" (better handling while deer hunting) and for kicks I tried the same load, but out of this rifle it went supersonic! I realize I need to work down a new load for this rifle but I'm very curious as to why it gained velocity with the shorter barrel. :confused:

All I can think of is that the longer 21" barrel actually put the bullet on the downward slope and was slowing it down after all the powder had burned off. But with the shorter barrel in the other rifle the powder was still burning and the bullet was still gaining speed?

Thoughts???
 
Check the throat lengths of both rifles with a comparator and the bullet you're using.

If the bullet is on, or very near the lands in the 16.5, and well off the lands on the longer rifle, you could have higher chamber pressures in the 16.5 because the bullet won't have a "running start".
 
Ok, I'm tracking what you're saying. Makes perfect sense! I tried some old Black Hills 180 subs and they worked fine, and off hand I'm pretty sure my 175 loads were seated slightly longer. I'll measure and compare for sure though.
 
All I can think of is that the longer 21" barrel actually put the bullet on the downward slope and was slowing it down after all the powder had burned off. But with the shorter barrel in the other rifle the powder was still burning and the bullet was still gaining speed?
Click to expand...

That is one possibility. Another possibility is that the shorter barrel is "slicker" than the longer barrel and the bullet loses less energy to friction. Another is that you had a really high pressure/humidity low temperature day and the bullet went supersonic in that specific condition.

Already mentioned is differences in initial pressure spikes due to chamber dimension differences.

No telling what is actually happening without more data.

Jimro
 
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