220grn MK’s louder then 180grn SST’s.

[Skulls]

I loaded up some rounds using 11.2grns of 1680 with 220grn MK’s and 180grn SST’s. I seated the 220 MK’s to 2.089 OAL, which I read is the recommended OAL listed for the 300 Blk sub rounds. I then took the ogive length and used that to seat the 180’s.

Both ran fine in my 8.5” AR with adjustable gas block and Gemtech Sandstorm, but the 220’s were a lot louder then the 180’s. With the 180’s the bolt was the dominant noise and with the 220’s it sounded like they were close to being supersonic. I tried them at night and during the day so the temp ranged between low 60’s – mid 70’s. I also tried all three settings on the gas block and it didn’t make much of a difference on the 220’s.

When I pulled the 220’s that I had left over I noticed that the powder was packed solid in the case with an indention of the bullet in the middle. I had to use a wooden dowel to break up the powder before it would all dump from the case. I’m not sure if dropping a few .1’s of a grain would help make room in the case to prevent the powder from being packed when seating the 220’s. I know I can make the AOL longer to make room in the case, but wasn’t sure where to start so I went with the 300 Blk OAL.

Any recommendations?

[Hoser]

Run them over a chrono.

[robrob]

The Quarterbore 300 Whisper loads section lists 10.3 gr of AA1680 with the 220 SMK gives 1023 velocity so your 11.2 gr of AA1680 is probably pushing them too fast.

[Skulls]

Run them over a chrono.

Yeah. I just bought one, but haven't had a chance to use it yet. It's a 3 hour drive to where I was shooting the first batch and the closest range to me is indoor, which is useless

robrob:

The Sierra manual recommended 11.7grns for 1000 FPS and 11.2grns is recommended for the 300 Blk. I went with the 11.2 since they offer a 9” barrel and mine is 8.5”. Everything else I could find was for a 16” barrel. I will give the 10.3grns a try and see what happens. Thanks.

[i8asquirrel]

I use 10.5gr 1680 under 180 gr sst with very good luck.....

[robrob]

Yea sorry, I was quoting the load for a 16" barrel. I just ordered a chronograph from Midway so I can fine tune the listed loads for my rifle. Let us know what you end up with for the 220 SMK, it's my preferred bullet and I'll probably end up with a 10" 300 Blk sometime in the future.

Rob