Cold weather affect AA1680?

[RobRat]

Just go my new YHM Suppressor, and went to the range today to try it out on my 300/221 upper, and test some loads. Conditions - COLD (around 34F), windy, snow flurries. And because its 2 days before PA Buck season, crowded. No down time to set up the Chrono. What the heck, I'll just listen for the supersonic crack to go away as I switch loads to lighter powder loads. Shot a few loads, made sure there were no keyholing going on, then screwed the suppressor on.

Pfft...Whump! :grin::grin::grin: "Dang that thing is quiet!" the guy on the next bench said. :grin:

I wanted to try out the 300 AAC BLACKOUT 220 gr loading, and was dropping in .3 gr increments to get there. Unfortunately, somewhere along the line, the load notes blew out of my ammo box, so I'm not sure what loading was that is just under the speed of sound. I do know that I was probably around 12gr, and no-where near the 11.2gr Blackout duplication load, Heck, I was seeing the bullets arcing to the 100 yard target on some of the loads 3 steps up from 11.2gr. With those notes gone, I'm back to square one, and will be pulling some bullets to reload the unknowns. :mad:

How much does temp affect the velocity of AA 1680?

Is there a chart or program out there that shows the speed of sound if you feed in temp, altitude and humidity?

[rsilvers]

I don't think the cold is the issue.

What is your barrel length?

What type of barrel blank?

What OAL did you seat to?

What primer did you use?

[robrob]

The cold is the only thing that affects the speed of sound. 1000~1050fps is a good target to ensure your bullets don't go transonic in the cold and start to make noise. It has to get really cold for the speed of sound to drop into the low 1000s.

[RobRat]

Fed 205 primers, AA1680, SMK 220's, seated to 2.09 (Per AAC's data).

The rifle is an AR upper with a McGowen 1:8 barrel, 17" long, TP555 gasblock in the pistol position.

With my notes gone, just guestimating, but I'm thinking the loads that got me subsonic were in the 11.7-12.5 range. (Im really torqued I lost my loadsheet--next time my loadsheets go in a notebook, and in the computer)

I shot 3 rounds of the loads below the ones that cycled nice and shot nice, and they seemed like they didn't cycle the rifle very hard, and I had 6" of verticle at 100. I'm thinking the AAC load of 11.2gr wouldn't have cycled the rifle. I was actually seeing the bullets go downrange and hit the target through the scope, which was kind of neat. (how slow is that?)

I had a lot of verticle this summer also. I also noticed that one of my cases developed a crack just behind the shoulder from a previous test session. I'm thinking I may be bumping the shoulder back too far, which may be causing these issues. I'll try just necksizing this batch of brass, and see what happens. If they don't chamber in the AR, I'll shoot them in my Savage bolt.

[Alleycat]

I don't know about your gun with AA1680, but here are the notes from temperature testing my 308.

Re 15 44 gr
Nosler brass
Fed 210
168 A-max
Cyclone suppressor
20" Savage

85 degrees 2675 fps
70 degrees 2670 fps
40 degrees 2640 fps

I need a few more data points, but you get the idea.

[rsilvers]

As for cycling your rifle, that depends on your gas port size, type of spring, and type of buffer.

As for needing more powder - maybe because the 30-221 chamber is shorter and so has higher startup pressure? Or maybe bore cross-sectional area?

But just use more powder to get to 1000 fps if that is what it takes.

[rsilvers]

For shoulder cracks - probably due to not annealing the case. Remington 300 AAC BLACKOUT brass is all annealed in that area to avoid cracking.