Twist rate and powder burn

[levidmiller22]

I’m in the process of building a 300 blk “pdw”

My question, if anyone can help, is regarding twist rate and powder burn. I’m by no means an expert but have done some research and hoping someone can shed some light

I’m planning to run suppressed with subs as often as possible. But certainly may use suppers and unsuppressed here and there

From what I can tell the “optimal” barrel length is 9” so the bullet can burn all powder. Does this change based on a suppressor being added in? Just for an extreme example would a 4.5” barrel running an 8” can still be far over that 9” optimal length? Or does that more so relate specially to the barrel exclusively. Got me thinking because the “typical” 300 blk guns like honey badger and Daniel defense pdw both have a 7” barrel.

My next question is twist rate. Again my minimal research has shown ideally the bullet should complete at least a full twist before leaving the barrel. Honey badger uses a 1:5 twist for their 7” barrel. Is faster twist preferable for short barrel unsuppressed or is there no real difference.

I found a barrel from lothar Walther 7.25” that uses a 1:8. Is this odd considering the bullet doesn’t complete an entire twist before leaving the barrel?

[dellet]

I’m in the process of building a 300 blk “pdw”

My question, if anyone can help, is regarding twist rate and powder burn. I’m by no means an expert but have done some research and hoping someone can shed some light

I’m planning to run suppressed with subs as often as possible. But certainly may use suppers and unsuppressed here and there

From what I can tell the “optimal” barrel length is 9” so the bullet can burn all powder. Does this change based on a suppressor being added in? Just for an extreme example would a 4.5” barrel running an 8” can still be far over that 9” optimal length? Or does that more so relate specially to the barrel exclusively. Got me thinking because the “typical” 300 blk guns like honey badger and Daniel defense pdw both have a 7” barrel.

My next question is twist rate. Again my minimal research has shown ideally the bullet should complete at least a full twist before leaving the barrel. Honey badger uses a 1:5 twist for their 7” barrel. Is faster twist preferable for short barrel unsuppressed or is there no real difference.

I found a barrel from lothar Walther 7.25” that uses a 1:8. Is this odd considering the bullet doesn’t complete an entire twist before leaving the barrel?

If we start with the basics, “optimal barrel length” is complete bullshit. The length needs to fit your needs. 9” is a balance between reasonable velocity with supers, and a short package.

There is no commercially available ammo that will burn completely in a 9” barrel. Very few handloads that will burn completely and cycle an AR, 95% is more realistic with the fastest powder and custom loads. Those choices narrow without a suppressor.

Smokeless powder needs pressure to burn, when the bullet leaves the muzzle, pressure drops to almost a relative zero inside the the suppressor, so the burning of powder is greatly reduced. Generally there is an increase in velocity, but most of this is from muzzle pressure being directed towards the bullet, vs burning powder. It’s like shooting with a 500 mile an hour tail wind for the length of the suppressor. It does not speed the bullet up, it slows the deceleration.

Barrels shorter than 7” can be very hard on suppressors shooting supers due to very high muzzle pressure and heat. They are more of a gimmick than practical for most shooters. They have their place, but are typically louder, dirtier, less efficient for supers and generally harder on the whole system.

Twist rate won’t matter until you start trying for sub 1/2 MOA accuracy, as long as it’s fast enough to stabilize the bullet. Keep in mind that if you want a full twist, the barrel needs to be 1 3/4” longer than the twist, that’s where the rifling starts. There are plenty of very accurate 6” pistols with a slower than 12 twist.

The big advantage to fast twist is rotational damage in tissue. There is a certain amount of evidence that indicates given the same forward velocity, a faster spin causes the bullet to open or come apart faster in impact, and stop sooner. Dumping more energy, faster and preventing more pass throughs and creating more tissue damage. This would be very dependent on bullet design.

Don’t know if that answers the questions directly, but should help with understanding and helps make a choosing easier.

[levidmiller22]

If we start with the basics, “optimal barrel length” is complete bullshit. The length needs to fit your needs. 9” is a balance between reasonable velocity with supers, and a short package.

There is no commercially available ammo that will burn completely in a 9” barrel. Very few handloads that will burn completely and cycle an AR, 95% is more realistic with the fastest powder and custom loads. Those choices narrow without a suppressor.

Smokeless powder needs pressure to burn, when the bullet leaves the muzzle, pressure drops to almost a relative zero inside the the suppressor, so the burning of powder is greatly reduced. Generally there is an increase in velocity, but most of this is from muzzle pressure being directed towards the bullet, vs burning powder. It’s like shooting with a 500 mile an hour tail wind for the length of the suppressor. It does not speed the bullet up, it slows the deceleration.

Barrels shorter than 7” can be very hard on suppressors shooting supers due to very high muzzle pressure and heat. They are more of a gimmick than practical for most shooters. They have their place, but are typically louder, dirtier, less efficient for supers and generally harder on the whole system.

Twist rate won’t matter until you start trying for sub 1/2 MOA accuracy, as long as it’s fast enough to stabilize the bullet. Keep in mind that if you want a full twist, the barrel needs to be 1 3/4” longer than the twist, that’s where the rifling starts. There are plenty of very accurate 6” pistols with a slower than 12 twist.

The big advantage to fast twist is rotational damage in tissue. There is a certain amount of evidence that indicates given the same forward velocity, a faster spin causes the bullet to open or come apart faster in impact, and stop sooner. Dumping more energy, faster and preventing more pass throughs and creating more tissue damage. This would be very dependent on bullet design.

Don’t know if that answers the questions directly, but should help with understanding and helps make a choosing easier.

Very very helpful. It seems like a lot of the info I have found is one of those things where, sure it’s helpful, but also wayyyyy over technicalalized and not as necessary as people might make it seem. Thanks for your response.

[popper]

Is this odd considering the bullet doesn’t complete an entire twist before leaving the barrel?
Nope, not odd. It's a matter of rotational momentum. Bullet spin is pretty much the same from muzzle to target. As long as it spins enough to remain stable all is fine. That spin momentum is determined by the mass and rotating speed (caused by the bullet fps and twist). So slow heavy bullet needs more twist to get the momentum up. 7 - 8 twist is fine in short barrel for subs. Works fine for supers in my 10" pistol too. CFE pistol burns completely but 2400 doesn't in my short barrel.