SUBSONIC 300 BlACKOUT 1:8 TWIST RATE

What bullet failed you? A 1:8 should stabilize a 220 Sierra pro hunter or match king well

I use 208-220 in multiple rifles, including an sbr, with no problem.

1-8 is the go to twist rate for bullets up to 220 grains.
How heavy do you want to go?

Agreed with above, my 1:8 does just fine up to 220 Nosler Custom comps. What projectiles did you buy?

My 10.5 inch 1:8 twist slings 220 just fine

Not sure what a grain is other than wheat or corn etc.....are you talking bullet weight? Silly semantics.

The others are right. A 1-8 twist should stabilize up to about 240gr pointed boat tail bullets. If you're having trouble check the actual twist rate of your barrel. You'd be surprised how many 1-8 marked factory barrels are 1-10 or slower. I've had more than a few customers bring in rifles that were way off. The remington 700 police versions were especially bad.

Take a cleaning rod with a patch that fits tight in the bore. Push it in to the breech end a bit and mark the shaft with a sharpie or marker. Make a line at least as long as the twist rate you're expecting to see plus a bit. Push the rod in until the line comes back around and then mark it with a line around. Pull the rod out and measure the length of the distance between your marks. My guess is your twist is slower than stated.

The solution to the problem is replace the barrel with the correct twist rate barrel or shoot lighter bullets.

Frank

As was said, an actual 8 twist should do just fine. 220 smks and 208 amax and 250ish cast stuff has run fine in the few ive shot fwiw.

Are you getting keyholes, poor accuracy etc? If yes, as was said, id wonder if its barreled right as well.

My buddy has a shorty dd, like 7 inches maybe? Shoots cast 250s at 600 fps with cfe blk no issues. 220 smk at 900 no prob either. Id think it has a similar barrel?

Smilies said:

Trying to do some calculations to get a starting point. Everything in Bergers calculator comes out "Unstable".

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Seems like you need to learn to work the calculator, here is the heaviest one from your list

I've had no stability issues using a 220 grain .308 bullet out of a 16" Faxon R5 1-8 twist barrel. Granted I'm not shooting 300 blackout, instead shooting 7.62x39 subsonic. I've shot 245, 255 and 265gr bullets subsonic out of this barrel with no stability issues. Its odd that Faxon uses a .308 groove diameter instead of .310/.311, but I guess I get the best of both worlds. Results posted on a different thread:

As everyone else has replied, a 1-8" will stabilize a 220. I just started playing with subs a couple months back and my 10.5" 1-8" will stabilize a 208 HPBT or a 220 SMK subsonic very nicely. A 190 SubX is just as nice.

DHDeal said:

As everyone else has replied, a 1-8" will stabilize a 220. I just started playing with subs a couple months back and my 10.5" 1-8" will stabilize a 208 HPBT or a 220 SMK subsonic very nicely. A 190 SubX is just as nice.

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I've just started working up some 220 SMK. What powder you using and COAL? THANKS

I use H110 for everything 300 Blackout. As far as OAL, I'm at 2.180" with the 220 SMK (010" off jam). The leade is not "out there" on this barrel and I'm pretty happy about that. The 208 HPBT I load to mag length as it's a different critter.

Somewhere around 9.5 grains is where I'm at with the 220. I do remember 9.7 shot real well but I had a few out of 20 go supersonic so I knocked a couple tenths off. I have other powders to play with but H110 is great with the 110-125 grain bullets and also runs clean and accurate with the subs. I quit looking.

I use H110 as well for everything subsonic in 7.63x39.

Because subsonic bullets do not have to push against the pressure wave that supersonic bullets generate, the spin required is much less than what most think. Also, most stability calculators assume supersonic drag so they tend to under estimate the real stability factor.