I played with Trailboss in .223 some, maybe 10 years ago. Using 4.0 gr. TB with a 50 gr. Nosler ballistic tip gave me an average of 1087 fps. with the highest velocity on the string at 1109 fps.

I backed down to 3.7 gr. and got 1071 fps. average, with the high reading staying under 1100 fps.

These were shot through a 23” T/C Contender.  I didn’t try them in a shorter barrel.  Actually, with the current ammo & reloading supply situation, it might be fun to play with some of the “low volume” guns a bit more. And playing with .223 subs should go right along with that.

As for case fill, people have been using fast pistol powders for decades to make “gallery loads” in rifle cartridges. Stuff like Red Dot and Unique. Hodgdon lists loads using Clays and Titegroup. Trailboss is so much bulkier than any of those, it is bound to be better off, even if you only get a partial case fill. 50% fill is still better than the 10% fill you get with the other fast powders.

Just my limited experience.

Ok, so I loaded some .223 using 3.5 gr. of Trailboss and some 55 gr. Wolf FMJ bullets.  I brought three rifles and the chronograph to the range to try them out.  My plan was to shoot some unsuppressed and then shoot another string suppressed.  It's always a good idea to try a new load without the can in place and verify you are getting stable bullets prior to risking baffle strikes.  Today reminded me why we do this.  

First up was the 23" Contender I had mentioned earlier.  I had previously run some 50 grain bullets through it. Looking at my notes, I had shot these across the length of the basement.  I had a steel plate for a backdrop to shoot into.  As such, I was just shooting for velocity and not for groups.  All of the 55 gr. FMJs I shot today hit the target sideways at 25 yards.  Thinking back, the previously tested 50 gr. bullets could have been doing this as well and I wouldn't have noticed.  The Contender has a slow 1:12" twist barrel.  It shoots supersonic 55s ok, but really shines with 40 and 45 gr. bullets.  I suspect running the heavier bullet at 1/3 the normal velocity was just too much and that slow twist wouldn't stabilize at 1000 fps.  

The good news is that the 3.5 gr. Trailboss load gave an average velocity of 1032 fps. from the long Contender barrel.  

The other two rifles were AR15s.  One with a 16" PSA 1:7 barrel, the other with a 10.5" Aero Precision 1:7 barrel.  I used a couple of very different silencers on them, so probably not a good sound comparison.  I used an old Triple-X Warrior the 10.5" gun. This can has a pretty large overbore, so likely not the most efficient gun for suppressor, even though it is pretty large in size/volume.  At the other end of the spectrum, I had a Bowers MOUT on the 16" gun.  This .223 can is the size of a typical .22LR can at 1"x6".  

Velocity results were:
10.5": 1005 fps.
10.5" w/ Triple-X: 964 fps.
16": 1010 fps.
16" w/ Bowers MOUT: 989 fps.

Just for fun, I also brought a .22LR chamber adapter for the Contender.  I suppose I should have brought a .22 conversion for the AR15s as well.  I shot some 40 gr. CCI Standard Velocity through the Contender, with and without a Liberty Mystic.  While the suppressed guns with the Trailboss loads sounded ok, shooting .22 through the single-shot rifle was just silly quiet.

Velocity results for .22LR:
23" Contender: 960 fps.
23" Contender w/ Mystic: 965 fps.

It seems whenever subsonic .223 comes up, people always advise to just get a .22 conversion.  This really brings home the practicality of that approach.  A .22 conversion will cycle in a semi-auto, where most .223 subs will not (the exception being guns specialy set up with a large gas port and lightened bolt carrier. These are then not so suitable to shoot regular high velocity ammo).   It makes a person wonder if the increase of 15 grains bullet weight (40 gr .22LR vs. 55 gr .223) is really giving much real-world benefit.  

Maybe next time I'll bring out a couple of 16" guns, one with the .223 subs and one with a .22 conversion. I don't have a pair of identical cans, but I could bring the MOUT and a .22LR can which are similar in size.  It might be interesting.  And I think I still have some 60gr. Aguila SSS ammo somewhere that I could compare as well.  I don't think it's anywere close to 1000 fps, though.  But it might still be a more practical option than hand-cycling 55gr .223 subs.  Just something else to think about.

Yes, however...

I went out playing with some more subs today.  Last time I was single-loading the rounds.  Today I fed them from a magazine.  I am reminded that the charging handle is really not in an ideal location for a "bolt action" handle.  You have to break cheek weld and shift position.  If you're prone, you really have to change up your body position.  Not to mention the CH is kind of flimsy.  If one were to operate it over and over with vigor, I can see it getting bent and tying up the action.  

That, and you really don't have much mechanical advantage over the bolt with the CH.  Not like the (relatively) long lever arm you get with a traditional bolt action.  Not an issue until you have a case that's semi- stuck in the chamber and doesn't want to come out.  

I think if I were to set up an AR as a manually operated straight-pull rifle, I would use a side-charging bolt carrier with a big charging knob.  The Brownells copy of the AR180 seems like a good compromise in this regard.  Just thinking out loud.  

So today I tried some 40 gr. VMAX in the Contender.  These all flew straight at both high and low speeds.  Unfortunately, 3.5 gr. of Trailboss appears to be a bit much for the lighter bullet in the 23" barrel, as I was getting an average velocity around 1115 fps, and in the cold weather these were definitely breaking the sound barrier.  Accuracy was ok, giving a 1.25" 10-shot group at 50 yards.  in comparison, a "regular" load pushing these to 3400 fps. gave a 5/8" group at that distance.  Still nothing to get excited about, but not terrible either.  

I shot some of the 55 gr. FMJ / 3.5 gr. Trailboss loads through the 20" 1:7 AR15 today.  These seemed to shoot just as well as the supersonic loads, only with the subs hitting about 5" lower.  What was interesting was that these also appeared to be hitting not so straight.  Not a full keyhole, but definitely not the nice round holes I was getting from the VMAX or from the supersonic 55s.  I just find it odd the group was just as good as (actually slightly better than) the supersonic loads.  I shot 10 unsuppressed, and then shot 10 through a Liberty Mystic.  The suppressed rounds grouped slightly better, and sounded really quiet.  Nothing like shooting subs through a long barrel and a large can.  

I'm still a little put off that the FMJs seem to yaw a bit. I understand having issues with a slow twist barrel, but a 1:7 should run those just fine at any velocity.  I could likely shoot a "better" bullet and see if that helps. But shooting a premium bullet for what is essentially a .22LR seems to be counterproductive.