Project - 300AAC Blackout Mini-14!!

Preliminary Range report:

IT'S ALIVE!!

We've been playing with a single gun to tweek out the gas system to work with our 300BLK supersonic handloads, and there's been alot of back and forth from the range to my small hobby shop in the garage. I have not been bringing the camera, as there is not much to take pictures of when the outcome of what's going on is either CR*P-not-enough-gas-to-cycle or WOOHOO! IT WORKS! It's taken a bit of monkeying for sure, but now we've got a gun that cycles so when we make the minor modifications to the remaining two guns I'll bring the camera and include some of those pictures in the official range-report writeup. We've got a small amount of tweeking/optimizing left to do and are waiting on materials, so it might be a week or two before we get to official performance testing status.

As a starting point we set up the test gun's gas system to be analagous with with the gas system in the 7.62x39 mini-30 as it comes from the Ruger factory. The gas port diameter was 0.080 and the gas bushing hole was 0.100. We left the gas piston unaltered.

Well...

The 300AAC Blackout is pretty much specifically designed to work in the AR15 platform with either pistol or carbine length gas systems. The gas port on an AR15 Pistol Length barrel is located approximately 5.5 inches from the bolt face, and on an AR15 Carbine length barrel it's about 7.5 inches from the bolt face. On the mid-length system it's about 9.5 inches from the bolt face, and on a full length system it's a whooping 13.25 inches. They all use different port diameters, but because we're playing with a .308 instead of a .223 it's a bit meaningless.

AAC states in their 300BLK FAQ that the cartridge will work with either the pistol-length or carbine-length gas systems, but not mid-length or full-length systems. This is because the quick burning magnum pistol powders used in the blackout loads develop their peak pressure much quicker that standard rifle rounds, and consequently the ideal pressure/volume required to do the work of cycling the gun occurs closer to the breech that it does when using slower burning rifle powders.

Now for the kicker - the gas port on the mini-14 is located approximately 10 inches from the breech - it's even further away than that of the mid-length gas system AAC says won't work with the blackout!

Compounding this problem is the quantity of gas itself....the shortened case of the blackout does not hold much powder. The .223 parent case has an internal volume of 1.85cm3. Our other reference, the 7.62x39 has an internal case volume of 2.31cm3. Measuring 10 of my re-formed 300BLK cases, they average 25.0 grains of water capacity, or 1.62cm3 assuming water density = 1.000. Full-power .223 loads use approximately 26 to 27 grains of powder. Full power 7.62x39 loads use approximately 27 to 29 grains of powder. The little blackout case takes a maximum of about 20 grains of powder. Ignoring the burn rate and applying the law of conservation of mass, the blackout can only generate approximately 75% of the gas in comparison to the other two.

So here we have the double whammy. Only 75% of the absolute gas quantity in the bore to play with, and a burn rate/pressure curve that puts the optimum pressure about 4 to 5 inches closer to the chamber. This is probably why you don't see any mini-14's chambered in 300BLK, and in fairness to others I was warned that getting enough gas to cycle would be a challenge.

How does the saying go? Something like "don't let fear or common sense get in the way of a good time"?? Besides, we are 8.5 months pregnant so let's have a go.

Function was woefully anemic with our initial gas system confiruration. So we gradually increased the diameter of the gas port, reamed out the gas bushing to match, and even milled out the eventually resulting orfice in the super-hardened factory gas piston. Because we did this carefully and in small steps, it took FOREVER with multiple trips to the range interspersed between minor machining sessions.

Each time that operating rod came back just a bit more. Soon, it was cocking the hammer, but not ejecting the spent case. A bit more machining, and it was now ejecting the spent case, but not picking up the next round in the mag. Then, finally, with almost no more room to go in the opening-up department, she CAME ALIVE!! WOOHOO!

CLANK CLANK CLANK CLANK CLANK! (only those who know and love the mini will appreciate that sound...:))

I briefly played around with slower burning powders, however abandoned that route as I want the gun to work with factory spec supersonic 300BLK ammo.

We've since been playing around a bit with the recoil spring, and have increased the reliability of the cycling and general function to acceptable mini-14 standards (ie: it WORKS!! Everytime!). Rather than use the now-basterdized factory spring, we've ordered up some custom made compression springs that will have the proper length and proper compression force to work with the Blackout Barrel.

Once the springs arrive, we'll have one more range session to test it out, and then we'll finish setting up the other two guns. I'll bring the camera for the last machining session so I can post pictures of the gas system setup, and then we'll hit the outdoor range to stretch her legs a bit and report back on things like accuracy and empirical trajectory.

And then this fall...WHITETAIL!! <- for those wondering "why not an AR"?!? Eat that HERETIC boy! I guess that's what your blackout bolt gun is for though....

Cheers,

Brobee
 
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very very cool!

If you had to put a dollar amount on what you've done to this point, what would you ball park it at including machining? Just curious.
 
GIGGIDY said:
very very cool!

If you had to put a dollar amount on what you've done to this point, what would you ball park it at including machining? Just curious.
Click to expand...

That is not an easy question to answer. From a raw materials perspective, the stainless barrel blanks were worth about $300 each, and we're into the various springs, bushing materials, and barrel vice/receiver wrench for maybe another $50 or $100 bucks. We bought all three guns at the same time, brand new, for around $850 each.

But as you touch on in your post, the real value is in the machining work. I'm very very fortunate that my good shooting buddy owns/operates a hardcore CNC production machine shop - the value of the shop is likely somewhere in the 5 to 7 MM$$ range. Just the equipment we used for the barrel project is likely in the 500k$ range. What is the value of having it all under one roof, as well as the very skilled labour that goes into running it all?

In terms of time, I figure he is into the project for somewhere between 10 to 15 hours, so multiply that by whatever you think the shop rate would be worth. My guess is that it is reasonably in the $300 to $400/hr range, including both the machinery and the skilled labour. We converted three guns, however the time required to convert just one would be about the same.

All this also excludes my time - driving around to collect the guns/barrel blanks, making the minor tweeks and modifications to the gas system, and the many many trips to the range to test out the tweeks. I would estimate I have about 15 to 20 additional hours in for this part of the effort.

So....final "commercial value" to the 300BLK Mini-14 project? It hurts my head to even think about it, but sure illustrates the power of a good friendship. We have a tonne of fun together clanking away out at my range on the farm not too far from town, and will have even more fun together wringing out the the empirical range testing on the blackout guns. Also, I was fortunate enough to guide my buddy through his first season of deer hunting last year, and this season I'll be taking both him and his brother. For sure I will be hunting my *ss off to set them up on deer as I don't want either of them to come away dissapointed.

The icing on the cake will be when we use our 300BLK mini-14s to do the deeds...:)

Cheers,

Brobee
 
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