I wanted to share this N120 subsonic load data for 220gr bullets that I got from Bob Blaine at VihtaVuori:
This load data uses a COAL of 2.230" and was tested using a 16" barrel, but it still looks a little conservative/low to me... My particular application is an 8.3" BA Hanson barrel with a Dead Air Nomad-30 suppressor. I see a lot of guys recommending subsonic N120 loads somewhere in the range of 10-10.5gr for 220gr bullets, and given the velocity drop with a shorter barrel, I'm thinking that I could probably load to somewhere in that higher range with my suppressed SBR while still maintaining subsonic velocities (~1050 fps) and providing more gas pressure for reliable cycling of my AR bolt. I'm assuming a higher fill ratio will also aid with consistency and reliability in the finished rounds.
I've replied to Bob and we'll see what he says. I just wanted to share this data because I don't see a whole lot of verified N120 load data for 220gr subs straight from the source.
You might ask for data for max pressure, or for supers.
The data given, like most subsonic data is misleading, the max load is limited to velocity, 1050 fps. Max by pressure will be somewhere around 11+ grains, maybe as high as 1300 fps.
Sierra and Hornady both have had data for 220+ grain bullets and N120.
You won’t lose a significant amount of velocity between 16 and 8”, but you have plenty of room to get back what you do lose.
dellet wrote: ↑Fri May 12, 2023 10:41 pm
You might ask for data for max pressure, or for supers.
The data given, like most subsonic data is misleading, the max load is limited to velocity, 1050 fps. Max by pressure will be somewhere around 11+ grains, maybe as high as 1300 fps.
Good point, and I do have some super data from Bob, as well (though no pressures are listed):
Based on this data, I should be able to start out somewhere around 11gr and work my way down until I have something that's sub velocity and reliably cycles in my setup. Does that sound about right?
I think between 10.5-11.5 you can find a pretty nice load. It will depend on other factors. Bullet used, seated depth velocity desired.
On your cycling issue, check for leaks, look for carbon tracks from under the gas block, around the tube at the gas key on the carrier. Make sure everything is well lubed. If yo pull the gas block, there should be a perfect carbon circle centered around the port. Check port size while you have it off.
Yeah, I have to get the SBR cycling properly before I can even think about working up a load anyway.
As far as gas leaks are concerned, everything looks fine to me. No carbon tracks, barrel is clean as a whistle, gas tube and key look fine. I'm going to try out the white JP SCS spring before I unpin my gas block, so I'm not looking at the port just yet. However, I have absolutely no doubt that the block is properly positioned over the port -- I'm incredibly OCD when it comes to my builds, and I spend an abnormally long amount of time ensuring that my gas blocks are straight. I also don't think there are any obstructions in the gas tube. I can plug the chamber and blow down the barrel and get a nice steady stream of air through the tube, so I doubt that's the issue... Given that the rifle would cycle properly half the time, I'm guessing it's right on the edge of being in the sweet spot. I imagine it's short-stroking just slightly, and once I lighten the spring, it should throw the bolt back far enough to cycle reliably and finally lock the bolt back on an empty mag. If not, I'm sure Ballistic Advantage would be willing to help me out with a gas port enlargement to get some more pressure in there.
