I've started work on a National Ordinance Co. aluminum/bronze 80% lower casting.

This is my first "manufacture" of a lower, but I've done a bit of aluminum /metal work before.
My first impression was that this lower is much heavier than a standard stripped aluminum lower.

My question is, "how hard is this alloy?"

My files that should just shred aluminum barely cut any filings off this thing. The same Dremel bits that grind away on steel just act to polish the surface. It took all day cleaning up the welded-in trigger guard (an upgrade),and  fiting the mag well, grip area, and takedown notches. I've yet to finish fitting the top deck and then drill most of the holes.

AND I've another just like it when this one is done.

It files like iron, so does anyone have any experience with these, and did you have the same difficulty, or am I just a wimp in need of sharper files?

Thanks for the input.
Cheers, Otto

Otto -
I have no idea why a company would resort to using a copper-based alloy to make a fine weapon like the one you're trying to build, but here's the skinny on your delima...
First off you need to know exactly what the alloys are and their content. For you, this means a phone call to whomever you bought it from or the company itself that manufactured it to that stage. My guess is they're one in the same, but...for know I'm going to give you info which will be very broad but you'll be able to understand it more after you follow-up with the mfg. and get the alloy specifics from them.

"Among copper alloys are the copper/aluminum alloys, of which the aluminum bronzes rank very high in aircraft usage".(My dept.) "They would find greater usfulness in structures if it were not for their strength-to-wieght ratio as compared to alloy steels. Wrought aluminum bronzes are almost as strong and ductile as medium-carbon steel, and they posses a high degree of resistance to corrosion to air, salt water, and chemicals. They are readily forged, hot- or cold-rolled, and many react to heat treatment."
"These copper-based alloys contain up to 16 percent of aluminum(usually 5 to 11 percent), to which other metals such as *iron, *nickle, or manganese may be added. Aluminum bronzes have good tearing qualities, great strength, hardness, and resistance to both shock and fatigue. Because of these properties, they are more commonly used for diaphrams, gears and pumps." In other words Otto, high wear parts that see alot of friction. Also note the asterisks near the words Iron and Nickle, above. This means HEAVY, and harder than most aluminum.

This info was taken out of my Airframe and Powerplant/ Mechanics General Handbook  Publication EA-AC 65-9A, as it was the only place I could find any information on aluminum bronze configurations. As you can plainly tell, it leaves alot to be desired, as you, nor I have ANY idea as to what exactly are the "ingredients" to the matrix/metal make-up of your reciever. The only thing I can tell you is what you already know...

1. It's heavier than hell because of the amount of copper-tin and who knows what else is in there...(Note asterisks above)
2. It's harder than hell, more so than aluminum, due to the tempering of specific alloys contained inside.

So there you go for now. In short, your not a whimp nor do your files need replacing. The shit is heavy and hard. Call them and then let us know why they use bronze in normally light weight weapons recievers!

Semper Fi -
Cpl West

"The shit is heavy and hard."  Great summation! I'm still chuckling over that.

I did call National Ordinance in Houston and they were very helpful, offering to answer any further questions or issues. I was concerned about possible special metalworking practices for the harder-than-expected alloy.  They assured me that it should machine, drill and thread like any other metal. They did admit that they only use a mill when working in-house, so they couldn't comment on my filing method. If only I had a mill.

By way of background, I got these lowers second hand, so I didn't seek out the bronze/aluminum castings directly, but couldn't pass them up. BUT they are going to look great when finished, and definitely "different" for the alloy polishes up to be almost gold in color.
I can hear the Bling-bling comments already-- but I bet you don't have one! Build them up with pink Cav Arms furniture, or matching pearl pistol grips - Just kidding.

They did affirm that the tensil strength is over 150,000, the finished weight will be much greater than an aluminum casting (obviously), and the hardness is on the "C" scale as oppossed to only "B" scale for aluminum. Whatever that means.  I didn't ask about the exact material content-- I was more concerned with whether I needed to take any special precautions sizing and threading the receiver extension for the buffer tube. Nope. So I should be good to go.

Thanks for the input,
Semper Fi, Otto