Looking for a powder recommendation: 9mm subgun

[ATCDoktor]

I run 3.2 grains of HP38 and a 158 grain IMI FMJ 9mm bullet in my UZI with no problems:

http://www.youtube.com/watch?v=mC2196Pm5LU

The IMI 9mm 158 grain FMJ bullet is special tapered design that prevents bulging in the case.

[jmorris]

No matter what powder I use, I find that if I go below 5 grains, the action in my subguns, MP5A3 and K, will not reliably cycle.

Am I doing something wrong?

The problem with N310 is that it is about 33% more expensive than other powders. An 8 pound jug of AA or Hodgdon runs around $150, pre S&H and Haz mat. An 8 pound jug of any Vihtavuori powder runs around $200

There is more to it than just charge weight. Below 5 grains of what powder? What weight bullet?

Thats a lot more than I paid but I buy a lot at one time, also keeps you from paying "panic" prices if you have a few years supply on hand.

The regular MP5's (not an SD) run fine with the VV N310 load or the Titegroup loads I posted, both below 3.5 grains. TG is also less expensive than N310 but not as clean.

[testuser79]

An interesting thread. For what it's worth, I've been shooting 115 grain Montana Gold Hollowpoint bullets through a Vector Arms Uzi using 4.0 grains of Bullseye, OAL length 1.110 inches for at least four years. I've found the shorter overall length was necessary with this bullet for feed reliability in a variety of pistols and the subgun, it's a somewhat unusual profile.

No problems on full auto and never a jam. I've fired several thousand of these with out any issues at all. I think they usually chronograph at about 1100 fps from most handguns. I always heard that subguns need hot loads to function, but that hasn't been my experience at all. It seems like a myth to me, especially after firing 2k or 3k of these through my Uzi, most of them on full auto.

I've also tried Power Pistol and had great results with that, too. Recoil on full auto is more manageable with Bullseye and the rate of fire is slower, which I like. Plus, it's a lot more economical, which is nice when you're going through so much ammo at once.

[Machineguntony]

I am using 115 grain Berrys plated bullets. I have tried Longshot, AA5, and Bullseye. To get 100% reliability, or near 100%, I can not go below 5 grains.

Bullseye I can go to 4.5 grains, but one in 5 magazines shot will have some sort of cycling or feeding failure.

Longshot probably consumes the most, of the powders I use. To get 100% or near 100%, I need to use at least 5.5 grains.

I do prefer minimum loads. Slower rate of fire, and less wear on the guns.

[highrolls]

Originally Posted by Machineguntony

I do prefer minimum loads. Slower rate of fire, and less wear on the guns.

Wow. I do not believe those are compatible. I will come back and try to explain when it ain't so late. I am fading too fast to try being coherent again this evening.

[highrolls]

Wow again. Cant believe such a simple comment could be so hard to explain. I guess the start point is with the blow-back design itself. To make the design safe, you do not want the bolt to move the case back while it is under significant pressure, but at the same time, that pressure must impart enough force to assure the bolt will go back. So the design will rely on bolt inertia to hold the case in place until the pressure is dropping.

The time scale for this is rather short. Lets use testuser's velocity of 1100 fps to make a sanity check. To oversimplify, lets say our UZI barrel length is 12 inches (1 ft). Start velocity of zero, muzzle velocity of 1100 fps gives an average velocity of 550 fps across a 1 ft distance. Travel time is 1 ft / 550 fps = 1.8 mili-secs. At that time the bullet exits and pressure falls to zero. Actual peak pressure, depending on powder burn rate will be very close to the chamber and earlier in time than this 1.8 mili-secs. And the further back the better to allow bolt inertia to do its job properly. (That is the argument for a faster powder is better.)

If the pressure peaks too close to the chamber (and keeping the peak pressure safe) either there will be insufficient total pressure under the time pressure curve to overcome bolt inertia or insufficient to impart the necessary start velocity to the bolt for it to complete the cycle. (This is the argument that the powder is getting too fast, ergo a need for a slower powder)

OK another sanity check, using testuser's 115 grain bullet at 1100 fps. Muzzle momentum (is that a proper phrase?) will be mass times velocity = 126500 gfps. At that point in time the bolt momentum should at least be equal (ignoring a lot of other, smaller contributions and the real math). The UZI bolt I am familiar with is approximately 20 oz = 8750 grains. Solving for velocity gives 126500/8750 = 14.4 fps. as the bolt start speed.
Over the next roughly 0.25 ft (no I did not measure it) at an average velocity of 7.2 fps since it comes to a stop, we can estimate the first half cycle to take 0.25/7.2 = 0.035 or 35 mili-secs. The return trip of the bolt takes longer because the start velocity is zero and with a spring-accelerating force seriously reduced by stripping a new round from the magazine (which also adds mass to the bolt). Normally, I would sanity check by saying the decelerating force on the bolt to stop it roughly equals the same force to get it back but there are too many funky things going on with the return trip so I just double the time back to 0.07. Add that to the time to open and the cycle time estimate is approximately 0.1 secs. That easily becomes 10 rounds per second or 600 rounds per minute.

The above paragraph is intended to allow a simple look at a complex system to see the primary driving factors. The bullet powder is not a driving factor for the gun's cycle rate. At best, you can play with the bullet weight and velocity to try your best to impact the bolt start speed. In the example above, that impact is rather small, and note that you can only slightly impact a third of the cycle timing. The return bolt trip is a mechanical function independent of your bullet efforts. To significantly change the cycle speed will require altering the recoil springs into something they are not being asked to do, altering the blow back design itself, or doing something that will change the timing on the bolt cycle during both back and forward motion, like a significant change to the bolt weight. More weight will slow the cycle, less weight will speed it up. If you lighten the bolt too much, the inertia factor goes away, the case is pulled out before the bullet leaves the gun, and the gun goes boom. If you increase the bolt weight too much, it reaches the point where the cycle pressure is no longer safe with commercial powders.

Minimal loads are good if they reliably cycle your gun. This does not mean you have effectively changed the cycle rate of the gun. I am not sure how either of these things relate to wear of the gun, or even what that phrase means?

[jclayto]

Thank you for the detailed explanation HighRoll.

I think I have more than enough info to work with from this thread. I am stuck in a holding pattern until I can locate one or all of the powders mentioned so that I can start experimenting. Hopefully the powder shortage loosens up soon.

Thanks all!