300 Blackout: How slow cam you go?

[bellavite1]

So, I want to try to get to the lowest velocity that will reliably cycle my AR pistol.

Sort of getting 45ACP ballistics out of a 300 BLK.
Why, you ask...
I have noticed quite a difference in sound going from 1050 to 970 fps, shooting suppressed.
It obviously makes sense, less powder, less bang, no sonic crack on either one...
I'd like to try to lower it as much as 850fps with 220gr and see how quiet would it get.

The real question is:
Would the bullet (220 gr Hornady ELD-X) be stabilized at that velocity or would I risk a baffle strike?
I know a bullet will loose stability going transonic in flight, but is there a muzzle velocity below which the bullet does not stabilize?

The specs:
10.5" upper, 1:8 twist.
Suppressor is a Griffin Optimus 9 (not an actual .308, but rather a multi-cal .357).
Bullets are 220 gr Hornady ELD-X
Powder is Accurate 1680, load TBD.

 

[GregP42]

I would shoot them first without the suppressor into a stiff backer behind the target and look to make sure the holes are nice and round. There are formulas that will figure out if the bullet will be stable or not based on speed, twist rate and length of bullet. This is how I did loads out of my bolt gun.

Greg

 

[Snuby642]

To go that slow I figure a 1/7 twist would be needed.

Also may want to check into hornady rnfb interlocks in 220g for slow loads. (12.1g cfeblk) 1015 fps movie quiet.

 

[bellavite1]

I would shoot them first without the suppressor into a stiff backer behind the target and look to make sure the holes are nice and round. There are formulas that will figure out if the bullet will be stable or not based on speed, twist rate and length of bullet. This is how I did loads out of my bolt gun.

Greg

How far would you set that backer to get meaningful readings?

 

[GregP42]

How far would you set that backer to get meaningful readings?

I sat mine at 10 yards and looked there first, if they looked good there I would retest at 25 yards, to make sure it hadn't started to tumble due to the low speeds.

 

[redbullitt]

Ive personally seen 650fps 240g cast fly and function from a 6 or 7ish inch barreled Daniel defense lol. Cfeblk is no joke with getting guns to cycle. Its pretty impressive, but it is always noisy. No free lunch.

In a bolt gun, 16 barrel, had some 700 fps cast 240g actually do really well. VERY quiet with fast powders and plenty accurate inside 100 yards.

Over 1000fps is notably noisy compared to 900ish fps with respect to bullet flight. Hollowpoints with big cavities make funny noises too in flight, so for quiet they are not the best.

Definitely test stability before adding your can or muzzle device etc. Nice to shoot through a cardboard box up close and check 25 and 50 yard targets for nice round holes and decent groups. Be mindful... conditions may change your stabilty if youre right on the edge .

Using the 9mm can will give you a little extra wiggle room in there lol, definitely a good idea when doing iffy shit like subsonic limbo lol.

I cannot comment on your stability for sure, but i will say the ones I've messed with had 7or 8 twists could do 900 fps no prob with 220smk and slightly shorter cast stuff. Test it and see. Thats the fun part anyways.

180g and 220g round nose bullets tend to do very well if you can get your rifle to feed them. Ive have some that would, others did not. The 175 smk may be a worth a shot too with a nice gassy powder like 1680 or cfeblk to keep it cycling.

Dropping a lighter buffer setup and or bcg may help too if you want to run slower speeds, light bullets, or quieter powders. Just dont start slamming supers through it that way lol.

ALL that said... if you want quiet, go manual cycle and use a lighter bullet, lighter than 125g with a fast powder. Cast is best, but I've had really good luck with the little 30cal xtp bullets. 220s etc smack things, but like you said, need more powder. Light bullet going 800 or 900 fps is really fun!

A big gas port, subs only gun with adjustable block would be just the ticket for some Unique powered semi auto goodness.

 

[impactaddict]

Not sure if it works into your equation or not but bullet expansion will be the problem when going slow. Lehigh Defense Max Expansion bullets do a good job at slower velocities - its their thing. The 194gn expands as slow as 750fps. That will be very quiet and expand nicely.

If you're just going for movie quiet blasting ammo, then get some cheap 220s from everglades ammo and call it good.. I've used them, they're fine.

 

[redbullitt]

Good call Impact, forgot about the lehigh stuff! They are expensive, but worth it if you need subsonic expansion.

Seen a few deer taken with them at about 50 yards, lung/heart hits, 900 fps muzzle velocity.

 

[Greenmoustache]

Ive personally seen 650fps 240g cast fly and function from a 6 or 7ish inch barreled Daniel defense lol. Cfeblk is no joke with getting guns to cycle. Its pretty impressive, but it is always noisy. No free lunch.

In a bolt gun, 16 barrel, had some 700 fps cast 240g actually do really well. VERY quiet with fast powders and plenty accurate inside 100 yards.

Over 1000fps is notably noisy compared to 900ish fps with respect to bullet flight. Hollowpoints with big cavities make funny noises too in flight, so for quiet they are not the best.

Definitely test stability before adding your can or muzzle device etc. Nice to shoot through a cardboard box up close and check 25 and 50 yard targets for nice round holes and decent groups. Be mindful... conditions may change your stabilty if youre right on the edge .

Using the 9mm can will give you a little extra wiggle room in there lol, definitely a good idea when doing iffy shit like subsonic limbo lol.

I cannot comment on your stability for sure, but i will say the ones I've messed with had 7or 8 twists could do 900 fps no prob with 220smk and slightly shorter cast stuff. Test it and see. Thats the fun part anyways.

180g and 220g round nose bullets tend to do very well if you can get your rifle to feed them. Ive have some that would, others did not. The 175 smk may be a worth a shot too with a nice gassy powder like 1680 or cfeblk to keep it cycling.

Dropping a lighter buffer setup and or bcg may help too if you want to run slower speeds, light bullets, or quieter powders. Just dont start slamming supers through it that way lol.

ALL that said... if you want quiet, go manual cycle and use a lighter bullet, lighter than 125g with a fast powder. Cast is best, but I've had really good luck with the little 30cal xtp bullets. 220s etc smack things, but like you said, need more powder. Light bullet going 800 or 900 fps is really fun!

A big gas port, subs only gun with adjustable block would be just the ticket for some Unique powered semi auto goodness.

I’ve played around a lot with different subsonic 300blk loads just to see the absolute quietest I could get it. So far 110gr sierras and ~4 gr TrailBoss is hands down the quietest. Decently accurate out of my 16” CZ 527 too. Absolutely nowhere close to cycling an AR but quiet as hell.

 

[bellavite1]

Well, Lyman shows 220gr HPBT clocking 1123 fps with 11.5gr of 1680 from a 16".
I am going to start at 8.5 and work my way up to where it cycles the action firing them at 25 yds on cardboard to check for nice round holes...
No can, no muzzle devices, out of my 10.5".
Once I get there I will probably add 0.5 gr to get some extra safety against baffle strikes (sounds like a good idea to me, what do you guys think?).
Wish me luck...

 

[GregP42]

Well, Lyman shows 220gr HPBT clocking 1123 fps with 11.5gr of 1680 from a 16".
I am going to start at 8.5 and work my way up to where it cycles the action firing them at 25 yds on cardboard to check for nice round holes...
No can, no muzzle devices, out of my 10.5".
Once I get there I will probably add 0.5 gr to get some extra safety against baffle strikes (sounds like a good idea to me, what do you guys think?).
Wish me luck...

I always start high and work down, that way I don't get a bullet stuck in the barrel, believe me that is no fun to get out.

 

[bellavite1]

I always start high and work down, that way I don't get a bullet stuck in the barrel, believe me that is no fun to get out

Been there, done that...
Might be a good call.
I think I can skip the 11.5 gr load, 1123 fps sounds kind of fast for a "subsonic" load.
Probably start around 10 gr and work my way down until it doesn't lock open anymore or looses stability.
I wonder if it even possible that a load high enough to cycle would be too slow to stabilize the bullet at the muzzle...
With a 220gr in a 1:8, that is.

 

[Snuby642]

Your minimum powder charge to cycle will vary considerably can on / off.

220g rnfb @ 1015 fps has cycled every thing we have tried it in. It is quite and the slow speed has allowed for use on a 100 degree day without cracking.

I have tested things to 1050 fps without temperature pushing it over the limmit.

I have a wag that rnfb bullets can be pushed faster than pointed or hollow point and not crack.

Fwiw

 

[Old Man with Gun]

The general school of thought in load development for subsonic is start high and work down to subsonic. My understanding is there are significant risks with getting a powder charge too low, like squibs.

From ammo testing I have seen on 9mm, slower does not always equate to quieter.

I think you may find Powder type, suppressor choice and firearm setup has far more to do with overall performance than slowing down the bullet.

The quietest load I have come up with is:
Berry's 220g Spire Point
10.5g Accurate 5744
CCI 450
2.100 COAL
Minimum crimp with Lee taper crimp

Shot from 8-10.5" barrels getting 1030-1050fps.

 

[Gustav7]

This is in my 357 lever gun so take that for what its worth, but I've found the 900-950fps mark to be a nice sweet spot for me. Anything over 1000 was still subsonic but noticeably louder. Anything under ~800-850 was atrocious on accuracy, as I'm assuming it started getting less stabilized at that point.

This is 125gr coated cast with Titegroup in 38spl loads.

 

[84bthomas]

Hmm, I bet my 1/5 barrel could go pretty low.

 

[Snuby642]

With enough twist maybe it would moonwalk all the way to the target.

 

[Ledzep]

If you download & run 4DoF you can use the table function on it to get the SG. The solver in 4 DOF is going to be more accurate than most any stability calculator on the net.

A lot of times bullets actually get more stable in the subsonic realm the slower you go. Less pressure on the nose. Obviously the ultimate test is just to poke them through paper and make sure they're straight.

 

[bellavite1]

Well, suggested load was 11.5 gr of 1680.
I started with 11gr and worked my way down to 8.5.
So far so good, round holes, bolt locks back, cycling the action just fine.
So tomorrow i'll slap the can back on and chrono it.
I should probably stop here, while I'm ahead...

 

[bellavite1]

Tested with the can on, 8.5 gr of 1680 clocked at 767 fps. (10 rnds avg).
It failed to lock open once, but it cycled fine.
I think I may bump it up 1 gr to bring it to 850-ish and and call it a day (at least until I find a bolt action, that is...).

 

[roostercogburn98]

I run 220s in the 875 range iirc

 

[Snuby642]

If you have an agb turn it all the way in.

Now you have a bolt gun.