UMC Blackout

[carcass]

rsilvers,

Great job on getting these to market. I purchased a box of primed brass from Midway and have been experimenting. I have a 7.5" barrel suppressed upper that I chambered and built myself, and have a few questions.

First some info:
I am currently using a recipe I worked up for the 300-221 using:
Handmade brass (from lake city 5.56mm)
Sierra Game King BTSP #2165 (200 grain Boat Tail Soft Point)
9.2 grains Lil'Gun
OAL 2.20"

QuickLoad software predicts about 1050 fps out of my 7.5" barrel and is pretty darn close to reality. It also shows that at the position of the gas tap, the bore pressure is around 8200 psig. This is just above the minimum that I think is enough to run the bolt on an AR system.

I have had some luck with this load (semi and full auto) in my M16 using a light carbine buffer, but when I load a magazine full, the rounds tend to bind up--and we all know why: the ogive of the bullet is tangent to the magazine rib at a point on the bullet that is too large in diameter.

So, I modified my load, using QuickLoad as a simulator, to the following, to get the bullet ogive to meet the mag rib at a point where the bullet diameter is equal to that of the contact point of a regular 5.56x45mm cartridge. The load is as follows:

Handmade brass (from lake city 5.56mm)
Sierra Game King BTSP #2165 (200 grain Boat Tail Soft Point)
8.8 grains Lil'Gun
OAL 2.05"

The rounds stack nicely in the magazine and the gun works fine in semi or full auto with about 10 rounds in the magazine, but short-cycles when more ammo is in the mag. QuickLoad estimates bore pressure at the gas tap as about 7900 psig. I suspect the volume and pressure of gas is not sufficient to drive the bolt carrier under the friction load of a full magazine in this case.

I have played around in QuickLoad with some other powders (including AA1680, which I see you have recommended in the past) and found few that will get me to sufficient pressure at the gas tap. Also, most of those powders use a lower charge weight, which (by the ideal gas law) means lower volume at a given pressure, and therefore, less mass to drive the bolt system.

Do you have any recommendations for a powder for a subsonic load with a short barrel (7.5") with a gas tap at 4.7"? Should I bite the bullet and chamber a new barrel that is longer, giving more time for the gas to act on the bolt carrier? Should I be using a heavier bullet? Any hints would be appreciated.

[pud-knocker]

Just something to think about....

A regular mag stack of 55gr 5.56 bullets is 1650gr, a mag stack of 200gr gamekings is 6000gr.

The bolt may be over riding the stack from the additional weight causing the stack to come up slow.

The 10 rounds of 200 gamekings weighs more than 30 of 55gr 5.56.

Maybe try different mags or sacrifice one and stretch a spring a little to try.

[carcass]

Originally Posted by pud-knocker

Just something to think about....

A regular mag stack of 55gr 5.56 bullets is 1650gr, a mag stack of 200gr gamekings is 6000gr.

The bolt may be over riding the stack from the additional weight causing the stack to come up slow.

The 10 rounds of 200 gamekings weighs more than 30 of 55gr 5.56.

Maybe try different mags or sacrifice one and stretch a spring a little to try.

That is a good observation, which I hadn't considered. I have tried several different types of magazines and had similar problems with all of them. Interestingly, I don't know whether the bolt is cycling back all the way and the mag spring isn't getting the rounds up fast enough, or whether the pressure on the bolt carrier from below is keeping it from cycling back far enough. Maybe if I do some slow-motion video of the action, the problem will reveal itself. Thanks!

[carcass]

Of course, if the weight of the rounds in the magazines is really the problem, it doesn't bode well for the practicality of the 300BLK as a fieldable full-auto cartridge, then, does it?

[pud-knocker]

I only bring it up as your setup sounds very similar to mine.

I have an 8" barrel, with 175 m118s I use 8.5gr of lil gun, and 8.8 with 200 gamekings and it runs great semi and full with the can installed.

It's a pistol length system, I don't remember the exact port size but I think it was a little under .090"

Not very scientific, but I did stretch the mag springs a little in my case.

[carcass]

Originally Posted by pud-knocker

I only bring it up as your setup sounds very similar to mine.

I have an 8" barrel, with 175 m118s I use 8.5gr of lil gun, and 8.8 with 200 gamekings and it runs great semi and full with the can installed.

It's a pistol length system, I don't remember the exact port size but I think it was a little under .090"

Not very scientific, but I did stretch the mag springs a little in my case.

Thanks for the info. I made the gas port pretty large (my shop drawing says 0.120") so I should be getting at least as much gas into the tube as yours is. I can't imagine the extra 0.5" on your barrel makes much of a difference. What kind of magazines are you using? Have you looked into extra power mag springs?

BTW, what is the OAL of your loads?

Thanks again for the interesting information.

[pud-knocker]

Again, I could be off on the port size, I've got a couple different length barrels but .090" comes to mind. I'm doing this from my phone and away from home.

I set the ogive of both loads a little over the .260" mark, lined up with the rib, to get a little extra length. I'm using old generation PMAGS, the rib is kinda triangle shaped with the peak a little farther forward than the radiused rib of metal mags, that allows to load out a little farther as well without binding.

I have not tried any extra power springs.

[LouBoyd]

My first suggestion would be to use even heavier bullets. It takes more pressure to drive a 220 SMK than a 200 SMK to the same velocity. There are no doubt even heavier bullets no longer than the 220 SMK though with a lower BC. Remember the impulse to drive the bolt carrier only comes from the time the bullet passes the gas port until it exits the barrel. With a 7.5" barrel and a 4.7" gas port that's only 2.8" of barrel where the gas can exit the port. At 1000 fps that's only 233 microseconds that the propellant can feed into gas system. That's damn little. You could shorten the position of the gas port by and inch or more but keep the gas tube length long by folding or wrapping it around the barrel. That's easier (at least cheaper) than buying a new barrel.

The gas pickoff for a Desert Eagle is only about 1/8" from the case mouth but the "gas tube" runs the length of the barrel to drive a piston at the front. It's a requirement that enough ENERGY is put into the gas system to operate the mechanism. Energy is pressure multiplied by time. If you don't have enough energy to drive the bolt carrrier you have to either increase the pressure or time the of the gas that's applied through the bolt carrier gas key. Opening the gas port diameter will help but you get rapidly diminishing improvement once that diameter is equal to the inside diameter of the gas tube.

I don't buy the argument that the cartridges are too heavy in the magazine stack. That's just silly. I've shot 50 Bewoulf with 600 grain bullets with no problem in standard AR magazines. A test is to shoot just one cartridge. If the bolt doesn't lock open you don't have an adequate gas impulse. It could still be inadequate even if it passes that test.

[carcass]

Originally Posted by LouBoyd

My first suggestion would be to use even heavier bullets. It takes more pressure to drive a 220 SMK than a 200 SMK to the same velocity. There are no doubt even heavier bullets no longer than the 220 SMK though with a lower BC. Remember the impulse to drive the bolt carrier only comes from the time the bullet passes the gas port until it exits the barrel. With a 7.5" barrel and a 4.7" gas port that's only 2.8" of barrel where the gas can exit the port. At 1000 fps that's only 233 microseconds that the propellant can feed into gas system. That's damn little. You could shorten the position of the gas port by and inch or more but keep the gas tube length long by folding or wrapping it around the barrel. That's easier (at least cheaper) than buying a new barrel.

The gas pickoff for a Desert Eagle is only about 1/8" from the case mouth but the "gas tube" runs the length of the barrel to drive a piston at the front. It's a requirement that enough ENERGY is put into the gas system to operate the mechanism. Energy is pressure multiplied by time. If you don't have enough energy to drive the bolt carrrier you have to either increase the pressure or time the of the gas that's applied through the bolt carrier gas key. Opening the gas port diameter will help but you get rapidly diminishing improvement once that diameter is equal to the inside diameter of the gas tube.

I don't buy the argument that the cartridges are too heavy in the magazine stack. That's just silly. I've shot 50 Bewoulf with 600 grain bullets with no problem in standard AR magazines. A test is to shoot just one cartridge. If the bolt doesn't lock open you don't have an adequate gas impulse. It could still be inadequate even if it passes that test.

Lou,

Thanks for the reply. Since I make the barrels myself, it's not that hard to change the position of the gas tap. It's just a matter of setting up the lathe. As I mentioned, the gun functions fine in both full-auto and semi-auto with only a few rounds in the magazine. What we don't know is whether the problem is due to excess drag on the bolt carrier from a loaded mag or slow feeding due to the cartridge stack weight. That can probably be determined by slow-motion video.

As far as energy (work) applied to the bolt carrier, it really is more about power (work per time). As you pointed out, there is very little time in the bore (it's more like 175 ms according to QuickLoad) so the application of that work must be done at a higher power. In any heat engine driven by expanding gas, the power is directly related to the mass flow of gas as well as the pressure and temperature. Therefore, using larger powder charge (and therefore more gas generated) should improve the performance. Unfortunately since I want to keep the bullets subsonic, there's a practical limit on powder charge that's not simply related to chamber pressure.

I think maintaining or increasing the gas tube length is a bad idea because it will increase system inefficiencies. A shorter gas tube could be problematic because it would potentially unlock the bolt too early. The timing of this would be critical when designing a system to fire both super- and sub-sonic ammunition.

So I'm left with several options:

1. Increase the potential power generated by the gas
a. Increase powder charge (mass of gas available)
b. Increase pressure at the gas tap
c. Reduce system losses (hard to do--requires custom large dia. gas tube)

2. Reduce the amount of power required
a. Shorten the time the gas has to travel to the bolt carrier
b. Reduce the drag load on the bolt carrier
c. Reduce the spring force of the recoil spring (problematic for other reasons)
d. Reduce the mass of the cycling parts (bolt/carrier/buffer/spring)

3. Try changing the gas tap position
a. Further relative to the muzzle (longer time for gas to act on bolt carrier)
b. Closer relative to chamber (higher pressures available but possible trouble with bolt unlock timing)

Can you think of any other options I'm missing?

Thanks for the interesting replies!

[carcass]

Also to Lou:

Your suggestion to use heavier bullets makes sense except for what I note below, however I'm a cheapskate and would rather use the less expensive (and more available) 200 grain bullets. Of course, I'm spending a huge amount of time and effort on making them work, so the opportunity cost is rapidly reducing the bullets' cost competitiveness :)

Ironically, however, QuickLoad shows that the bore pressure for a 240 grain bullet at the gas tap position is actually about 800 psi lower than that for a 200 grain bullet, to drive both bullets to the same speed. And that requires almost 1 grain less powder, therefore the gas mass is lower as well. Believe it or not, the 200 grain bullet is just about at the sweet spot for the 300BLK/Whisper to get the highest bore pressure at the gas tap.

Strangely enough, the lighter the bullet, the higher the pressure at the gas tap location. This holds true all they way down to the 150 grain bullets (as low as I've tested). Of course, once you get down to those light weight bullets, the 300 BLK stops being competitive with other rounds like the .40 S&W and 9x19 at subsonic speeds.