Shortest 300 blk barrel

[PNW17]

Yes, mystic x

[2manyToys]

I sure like my Mystic-X, though I don't think you get as much back pressure with a 9mm can, it will be interesting to see if it will cycle. Also, have you considered that Liberty lists a minimum length for 300K as 5"?

[PNW17]

5” limit with supers. These are all subs with the energy of a 9mm/.40sw. I would not shoot supers through this.

[dellet]

I try to bring this up with short barrels all the time, the only number you really need to be concerned with is muzzle pressure. That is what the suppressor has to absorb. If you can get a manufacturer to give you a number it makes it a lot easier, most won't.

Based on common loads and barrel lengths, it seems around 20,000 psi is a good number to work with. Quickload will give estimates.

Example would be your #5 load with the 180 Ranier. By the time you get that to 1000 fps. your muzzle pressure will likely be around 13-15,000 psi. or 2-3 times a normal sub load in an 8" barrel. Change to 220 grain bullet and you may be over 20,000 with that powder.

Then there is the whole heat issue of the gasses being so much hotter under rapid fire, you can easily over heat one.

Just food for thought.

[PNW17]

Did ladder tests and settled on 7.0 grs of Accurate #5 which averaged 970 FPS. It ran great. Accurate #2 didn’t cycle well at 5 Grs. When cleaning after, it is getting a lot of copper fouling through the gas port and into the receiver and the expansion chamber of the bolt. Some chunks are quite large. We’ll see if that fades after those sharp edges of the newly drilled gas port round off or if that’s just the nature of a gas port at an inch forward of the chamber. These are plated bullets.

I want to get a lightweight bolt, reduced power buffer spring and turn down the gas to get recoil to be nonexistent.

[PNW17]

I try to bring this up with short barrels all the time, the only number you really need to be concerned with is muzzle pressure. That is what the suppressor has to absorb. If you can get a manufacturer to give you a number it makes it a lot easier, most won't.

Based on common loads and barrel lengths, it seems around 20,000 psi is a good number to work with. Quickload will give estimates.

Example would be your #5 load with the 180 Ranier. By the time you get that to 1000 fps. your muzzle pressure will likely be around 13-15,000 psi. or 2-3 times a normal sub load in an 8" barrel. Change to 220 grain bullet and you may be over 20,000 with that powder.

Then there is the whole heat issue of the gasses being so much hotter under rapid fire, you can easily over heat one.

Just food for thought.

Can your calculator take into account the gas port bleed off? This is basically the same load as a .40sw 180gr through a 4-5” pistol barrel so this shouldn’t be over pressuring the suppressor right?

[2manyToys]

That's great that it cycles with subs! How did it sound?

[PNW17]

Sounds great, for a semi-auto gas op gun. It’s no single shot contender though

[dellet]

Here’s some quick and dirty numbers, if you can post some complete load data the numbers will be closer but this works for an idea.

You can’t really compensate for the gas port or at least I can’t. The comparison to 40 S&W will get you in trouble. You have the same amount of gas in a cylinder that is 30% larger. When you are talking about barrel length. Peak pressure will be close but not muzzle.

For the Blackout load I used a 180 grain cast bullet, 7 grain powder charge and a seated depth that gave reasonably close velocity.

4.25” barrel
7 grn #5
Peak pressure 25,500 psi
Muzzle pressure 10,000 psi
Velocity 920 fps

11 grn N120
Peak pressure 11,500 psi
Muzzle pressure 7,800 psi
Velocity 680 fps

8” barrel
11 grn N120
Peak pressure 11,500 psi
Muzzle pressure 4743 psi
Velocity 900 fps

40 S&W 4.25” barrel 175 grn cast, tried for same peak pressure by adjusting seated depth.
7 grn #5
Peak pressure 25,500
Muzzle pressure 4573
Velocity 1030 fps

308 Winchester. 180 grn cast

47.5 grn 2520
Peak pressure 58
Barrel length/muzzle pressure/velocity
20”/9700/2600
18/11,000/2550
16”/12,800/2450
14”/15,100/2370
12”/18,200/2250

300 blackout 125 sst 20 grn 296
Peak pressure 53,000
16”/5230/2270
8”/12,000/1950
6”/16,500/1800
5”/21,000/1700

[PNW17]

Here’s some quick and dirty numbers, if you can post some complete load data the numbers will be closer but this works for an idea.

You can’t really compensate for the gas port or at least I can’t. The comparison to 40 S&W will get you in trouble. You have the same amount of gas in a cylinder that is 30% larger. When you are talking about barrel length. Peak pressure will be close but not muzzle.

For the Blackout load I used a 180 grain cast bullet, 7 grain powder charge and a seated depth that gave reasonably close velocity.

4.25” barrel
7 grn #5
Peak pressure 25,500 psi
Muzzle pressure 10,000 psi
Velocity 920 fps

11 grn N120
Peak pressure 11,500 psi
Muzzle pressure 7,800 psi
Velocity 680 fps

8” barrel
11 grn N120
Peak pressure 11,500 psi
Muzzle pressure 4743 psi
Velocity 900 fps

40 S&W 4.25” barrel 175 grn cast, tried for same peak pressure by adjusting seated depth.
7 grn #5
Peak pressure 25,500
Muzzle pressure 4573
Velocity 1030 fps

308 Winchester. 180 grn cast

47.5 grn 2520
Peak pressure 58
Barrel length/muzzle pressure/velocity
20”/9700/2600
18/11,000/2550
16”/12,800/2450
14”/15,100/2370
12”/18,200/2250

300 blackout 125 sst 20 grn 296
Peak pressure 53,000
16”/5230/2270
8”/12,000/1950
6”/16,500/1800
5”/21,000/1700

So I am good at this load right? It’s reasonably similar to a 9mm with peak at 35k, a gas port close in that will bleed pressure quickly and a 4.4” barrel (I measured it accurately finally). Considering the can is 300 win mag rated it should be fine


4.25” barrel
7 grn #5
Peak pressure 25,500 psi
Muzzle pressure 10,000 psi
Velocity 920 fps