300 blk twist rate for cast boolits

[Nine Guy]

Hi All,

I am wondering if a 1:7 twist is too fast for a cast boolit. Am planning to powder coat them. Doing it for 9mm now with good results.

Would 1 in 8 be better? Does it even matter? I do plan on shooting lots of subs through a can.

Thanks!

[Jupiter7]

Got both twist rates. Difference is pretty much nil on paper.

[Boosted98gsx]

If you're running subs and want to hunt (ie expansion) you may want a slower twist rate to CAUSE tumbling so that you can get a larger permanent cavity. There really isn't any good way (that I've found) to increase expansion at low velocities, and I haven't seen any HP molds for such boolits.

[45 2.1]

Try looking here:
http://castboolits.gunloads.com/showthread.php?208311-MiHec-311410-Cramer

[Boosted98gsx]

Try looking here:
http://castboolits.gunloads.com/showthread.php?208311-MiHec-311410-Cramer

That's only a 130gr round. Far from the mass needed to be sub (200+ gr).

[Jumbopanda]

1/7 spins the bullet VERY fast, and it becomes easy to exceed the RPM threshold for cast boolits. I can't get any kind of accuracy with my cast boolits in 300 Blackout if they are going much faster than 1100 fps or so. Harder alloys seem to do a bit better than soft ones. Although I have heard of other people having better luck than me.

It's generally thought that the RPM threshold is 120,000-140,000. If you shoot a cast bullet at 1200 fps out of a 1/7 barrel, it's already exceeded 120,000 RPM. As a comparison, a big bore caliber like .444 Marlin with a slow twist rate of 1/20 is only spinning at 72,000 RPM if you push the bullet to 2000 fps.

I think going with a 1/8 barrel might give you a little extra leeway in regards to how fast you can push your boolits.

[45 2.1]

That's only a 130gr round. Far from the mass needed to be sub (200+ gr).

You or the OP never mentioned boolit weight, just sub sound barrier. That is velocity dependent... NOT WEIGHT.

[Boosted98gsx]

Yes, I am aware it's velocity dependent, but usually it means a 200+ gr round in 300 blk for cycling energy.

[45 2.1]

Yes, I am aware it's velocity dependent, but usually it means a 200+ gr round in 300 blk for cycling energy.

Several of us cycle semi-autos at low velocities with 22 & 6mm caliber boolits. It's not that hard. A AR15 is over-gassed usually. I'm not with the crowd that needs or wants a boolit to tumble. Here is what you posted:

There really isn't any good way (that I've found) to increase expansion at low velocities, and I haven't seen any HP molds for such boolits.

I simply linked you to a thread where people were doing what you wanted to do...............

[blikseme300]

If you're running subs and want to hunt (ie expansion) you may want a slower twist rate to CAUSE tumbling so that you can get a larger permanent cavity. There really isn't any good way (that I've found) to increase expansion at low velocities, and I haven't seen any HP molds for such boolits.

There is no need for expansion or hollow points when subsonic. 247gn boolits smite hogs like the hammer of Thor.

[blikseme300]

Yes, I am aware it's velocity dependent, but usually it means a 200+ gr round in 300 blk for cycling energy.

Some AR's in 300blk have had problems reported with cycling but both of mine cycle fine with 155gn boolits and 125gn j-words. A common problem seems to be too small a gas port or the use of M-16 BCG's. I had problems cycling 60gn boolits in my 5.56 AR-15 but switching out the M-16 BCG solved that.

[Boosted98gsx]

I am still awaiting my Form 1 back from the ATF, so I haven't been able to play with mine at all. :(

[Jupiter7]

Some AR's in 300blk have had problems reported with cycling but both of mine cycle fine with 155gn boolits and 125gn j-words. A common problem seems to be too small a gas port or the use of M-16 BCG's. I had problems cycling 60gn boolits in my 5.56 AR-15 but switching out the M-16 BCG solved that.

Just so anyone reading is clear on the specs, the 300BLK was designed to run in a normal M4 with a simple barrel change. If the gun doesn't run, it's outta spec. For reloaders, a lot of that is powder choice. My three run with normal m16 style BCG, carbine and pistol length gas. I've only had issues using low charges of h110 & lil gun. A1680 cycles all of them sub or super. Out of spec gas ports and shotty builds from newbs count for a fair amount of issues along with new reloaders. I can build a house out of legos, that don't make me an architect.

[Jumbopanda]

Just so anyone reading is clear on the specs, the 300BLK was designed to run in a normal M4 with a simple barrel change. If the gun doesn't run, it's outta spec. For reloaders, a lot of that is powder choice. My three run with normal m16 style BCG, carbine and pistol length gas. I've only had issues using low charges of h110 & lil gun. A1680 cycles all of them sub or super. Out of spec gas ports and shotty builds from newbs count for a fair amount of issues along with new reloaders. I can build a house out of legos, that don't make me an architect.

+100

I'm really tired of hearing people say that one should enlarge their gas ports to get a certain load to function. It's one of the worst pieces of gun-related advice I've ever heard.

[Boosted98gsx]

+100

I'm really tired of hearing people say that one should enlarge their gas ports to get a certain load to function. It's one of the worst pieces of gun-related advice I've ever heard.

I am glad to hear that. I've read so much about people enlarging gas ports, and it had me worried about my new barrel. I don't like post-processing parts.

[Jupiter7]

I am glad to hear that. I've read so much about people enlarging gas ports, and it had me worried about my new barrel. I don't like post-processing parts.
The moral was that your barrel has to have an in spec gas port. Not sure what actual spec is, but .095 is about the smallest I've seen function without issue. Plenty of quality barrels are .10 and up. My friends factory built DPMS upper wouldn't cycle factory Remington 220gr OTM ammo, pulled barrel, gas port was .88, he drilled to .11, cycles and functions factory subs now. I was lucky I guess, all mine are built by me and all barrels were .095 to .120.