Realistic velocity with 20" AR

[ReddyToGo]

I just finished my 6ARC 20" AR build and was really wondering what realistic velocities should I expect?

55 gr Nosler BT - IMR 8208. I have shot it with 31 grains and it shoots really well. Did not do any load development, just based it of Hornady max load for the 58 Vmax and it shoots consistently 1" or less at 100yds with 2 different shooters. Myself and the other, my grandson who is a first time rifle shooter.  I plan to run this as my coyote load if the speed is adequate. I suspect it is as I have killed two pigs with it already. Figured I would try it and it worked really good on pigs at 100yds.... Don't plan to use it for pigs however.

100 Sierra SBT - Using Lever. I have put OCW together and running 27.9 grains it was shooting 2484 fps and 28.2gr was 2525 fps. Little slower than I expected but the barrel is brand new and it was about 50°f. Lever is temp sensitive as I have done temp testing using my 6.5 Grendel and it was 1.4 fps for every degree of temp. 

As far as velocity I was hoping to get about 2650?  I can go up another .2 grains according to Hornady's data for a 100gr projectile. I'm running 2.14 COAL, which is about 20 thousands off the lands.  My grandson shot a 1.5" group at 100 yards so I was happy with that but the velocity was not as good as I was expecting. For velocity comparisons, out my 18" AR 6.5 Grendel I could push a 100 grain AMAX over 2700 fps.

[alwie]

I get 2600 fps for the 105 gr BTHP out of a 18” AR. So, 2650 seems very reasonable velocity for a 20” AR.

[grayfox]

20" AR with lever and 100 gr, like the sierra... 2680 should be a good spot for you and achievable.
Watch out for the nose getting tinked by the feed ramps.

[StoneHendge]

Every bullet has its own internal ballistics and the Sierra 100 SBT has a lot of bearing surface. More bearing surface = more friction. More friction = a slower start down the barrel. Which changes all of the pressure dynamics as the powder burns vs what may be published for a bullet with less bearing surface. Essentially, you end up with an earlier pressure peak and aren't able to attain the velocities achieved with bullets with less bearing surface. If you're married to the bullet, try a slower powder.

[Rickhem]

I have a Howa Mini, 20" barrel, so not a direct comparison. But for numbers, my hunting load of CFE223 and the Sierra 85 gets me an average of 2954, and groups about 0.75 MOA. I can run it faster, but the group opens up, so I don't.
Also worked up a very good shooting load with the 70TNT and Varget, averages 2876, and shoots about 0.5 MOA. I'm sure that I can push that bullet much, much faster with different powders, but not sure I want to chase something better than I'm already getting.

StoneHendge has good advice. I'd add that a good group trumps velocity in my opinion.

[grayfox]

The 100 Sie SBT is not my favorite 100 gr pill in 6Arc, when also considering the 100 gr hornady btsp, and the 100 gr Tgk. But I've had them all to 2680 +/-10 in my various 20" barrels with lever.
The gameking's drawback is that it is a tad stubbier, so must be seated deeper than the other 2, which takes away some powder room. But they can all get there and have for me.
I save my 100-lead tips for my 243 btw, but if that's what you've got, then ladder them up and see how they do.
Also agree that accuracy trumps raw speed. Lever is a forgiving powder for the 6Arc, although a bit slower as the temps go down (~16 Deg F here right now, ugh!)

[ReddyToGo]

I appreciate what everyone is saying about the 100 SBT's but they are what I have the most of. Being a brand new caliber for me, I wanted to try them, for sight in, fire forming new brass and to just get some rounds thru the barrel with the hopes of possibly using them as a potential hog bullet. They seem to shoot descent. Still in the try what I have state for the 6ARC to see what it does.

I have been running in the 6.5 Grendel world so long, most of the powders I have on hand have burn rates between Xterm / H335 and Lever. Lever is about the slowest powder I have used.

I do have several 6.5 creedmoor compatible powers, H380, Varget and H4350 but not sure if anyone has tried any of these with the 6 ARC.

[StoneHendge]

Save the H4350 for your CM. I use Varget in a practical match load. It's extremely accurate with low velocity variation. And slow in the minds of everyone on this forum, running a 105 RDF at 2450 out of a 16" Lilja near max. But temp stability is far more important to me than another 100-150 fps . A slow hit is a point - a fast miss isn't. Case capacity can become an issue - i use a drop tube to fit enough powder in the case. I'm not going to say how much that is because my current keg runs 80 fps slower than my prior keg at equal charges.

[300BLK]

I use 29gr WW748 under Sierra 100gr BTs in my Savage boltgun - excellent load, but maybe a bit much for a gas gun.

[300BLK]

I ran some Hornady Black 105gr through my 20" Faxon barrel today. It was about 40F and 10 shot average was 2668fps across a Garmin. My gas block is reversed to cover the port, action operated manually via sidepull bolt, so maybe a bit higher than a true gas gun.

[rickt300]

I was not able to get the 100 gr. Speer SPBT to 2600 fps with two powders. BLC2 got me to 2575 fps. I easily got the 105 gr. Hornady BTHP into the mid 2600 fps area using BLC2, Imr 4895 and LVR. Sectional density comes into play trying to match 100 grain bullet velocities with the Grendel. An 80-85 grain 6MM is more of a match to a 100 gr. bullet in the Grendel. 100-105 grain bullets are more akin to 120-123 grain 6.5 bullets. You still get slightly higher velocities but not much.

[CZ527 Guy]

Not sure what distances you are considering, though accepting a slightly slower velocity with the 100 gr sierra should not be seen as much of a set back.

Hodgdon Load data for the 100 grain nosler partition should be extremely close for you allowing roughly 30 fps per inch less for your barrel. So subtract roughly 120 FPS due to your barrel being 4" shorter & that should get you rather close to your actual velocity.

With your 100 GR Sierra I would guess LVR or CFE 223 may be your optimum powder for your intended purpose. As always Start low & work up. Focus on safety 1st, accuracy 2nd & velocity 3rd.