Hey guys just put together a 16” Recce build and am going to start developing a load for it. The barrel is a Rainier Ultramatch Mod2 16” .80 gas block intermediate gas barrel with a 1:7.5 twist. I have quite a bit of experience loading for larger caliber bolt guns but this will be my first time loading for a gas gun and for .223. I was wondering wether I should start out with 69g or 75g bullets? I was thinking about going with Sierra match Kong’s over Berger VLD’s due to their lack of jump sensitivity.  Also is there any powders y’all recommend for precision .223? I’m planning on using lake city brass seeing as I would like to load in bulk for it.  Thanks for the info guys and I will post some pictures once I get some loads going.

Gratuitous gun pic:

I'm a big fan of 8208 XBR with the 77gr class of bullet and a shorter (relatively) barrel. You won't win any speed records, but it'll be accurate. Probably somewhere right around 22.4-22.8gr @ 2550-2600.

I have a 1:7.7 16", but doesn't have as much meat on it.

50gr VMAX/25gr H4895/2.250" is hard to beat at 100-200yds. Node could be +/- 0.5gr. About 2900fps.

75gr BTHP/24.0gr Varget/2.250" for 100-600yds. Node could be -0.5gr, and carefully go to +0.5gr.
77gr SMK/23.0gr H4895/2.250". Possibly -0.5gr, carefully +0.5gr. About 2550fps.
These two are moderately compressed and crunch when seating.

I haven't bothered with long range loads yet, but an 80gr SMK or Fullbore at 0.020" off the lands (~0.2" longer than magazine), over 24gr Varget should be close to a node and be above transonic to 800yds. The case capacity is more variable here, but again possibly -0.5gr and carefully go to +0.5gr. About 2550-2600fps.

Ball powders like TAC and CFE223 can give about 100fps more oomph, but need a little more margin below MAX charge for summer temps. I suggest magnum primers for more piece of mind against slamfires - CCI 450 and #41, Remington 7.5, Federal 205MAR. If the firing pin aperture is reasonably snug, I like to use the thinner/softer CCI 400s in initial development to better read when I'm near max.

After extensive testing across numerous rifles I settled on the following load as my all purpose match grade load.

Winchester commercial or Lake City brass
69 grain SMK's seated at 2.250".
24.5 grains of Varget
Rem 7.5 or CCI-BR4 or Russian kvb-5,56m primers

Any idea how it is chambered? If we assume it is similar to the Wylde chamber in a WOA or CLE...

It will probably do great with the above SMK 69 loads.

Also, right in that same vein, try 24.0 RE-15 with the 77 TMK, Rem 7 1/2 primer, at mag length.

The load my 16" WOA SDM likes best, its at listed max so start low and work up to it.

HDY 75 BTHP are working well with the same load at about 2625 fps.

Hopefully going to run a ladder with the 77 TMK later this week, might have to go above max.

LC-02 prepped
CCI 400
77 SMK
23.2 XBR 8208
2.260
2610 fps
10 shots in .7 MOA