Are You a Firearms Nerd? Take This Not-So-Simple Test to Find Out!

I noticed nobody has one that says “I love my job.” If you work where I think you do, I understand perfectly.

Of course it’s click bait. If hillary had won the election we’d have 10-12 postings a day with 500-1000 comments on each.

Trump, Hail Lord Trump, blessings upon his house and issue, is messing with RF’s bottom line cause he ain’t messing with our guns.

I know a bit about guns, but not everything. I won’t know even 25% of what there is to learn about guns before I die.

As I tell customers: Anyone who tells you they know everything about all guns is simply full of crap. The amount of knowledge and human inventive effort that has been sunk into guns is astonishing. If we’d devoted as much effort as we’ve sunk into guns as a species in the last, oh, 300 years, and put that instead into something like (eg) interstellar travel, we’d already be zipping around the galaxy in swank style. When one starts studying guns deeply, it boggles the mind to see how much effort has been invested by a large number of very smart people across 300+ years.

Guns helped spawn the machine tool industry in the US. The gun industry is responsible (in large part) for precision measurement instruments, gage blocks, interchangeable parts manufacturing, tolerances down to 0.001 before WWI, etc.

Ever hear of a company called Pratt & Whitney? You might have heard in recent years that they make jet engines. They used to make machine tools, including rifling machines to rifle barrels. In the Civil War, they made guns.

Mr. Pratt & Mr. Whitney both got their start as employees of one Mr. Samuel Colt. Here’s a nice bit of history on P&W and what they contributed to American industry & manufacturing:

http://www.prattandwhitney.com/images/customer-files/pratt_whitney_history_book2.pdf

MIL-spec receivers are made from 7075 aluminum. It looks like it welds OK, but it will stress crack, much as welding on cast iron will without pre/post heating, just outside the HAZ.

If you want to weld on a AR receiver, get one made of 6061 or 6063 aluminum.

1. You are correct – it is galling. When you have two pieces of steel of similar hardness sliding over each other, there is a tendency to gall, absent some very effective lubrication. Want to make a smooth, fast-cycling bolt on a gun? Make it harder than the receiver in which it is sliding. Gunsmiths and gunmakers learned this the hard way over 100 years ago. It took them a little while to nut this out; it wasn’t immediately obvious why two nicely finished pieces of steel sliding over each other gave such offense at the operation.

2. Correct again – but the effect was first used as a design element to slow down the bolt cyclic rate on the Thompson sub-machine gun – the original, civilian version with the comp and lovely (and expensive) machining.

3. BINGO! It was exactly that. Rather than use pyrometers, the old men who were in charge of heat treating were rather proud of their skills in judging critical temperature by eye. However, when we got into WWI and production of the 1903 ramped up, their “by eye” skills didn’t scale. As less experienced hands were brought on to ramp up production for our entry into WWI, the heat treatment suffered tremendously, with later investigations showing that the difference in the temp of the steel before quenching could vary by as much as 300+F between a cloudy day’s light in the shop and a sunny day’s light in the shop.

The ensuing investigation in the 1920’s found that some of the defective receivers had been so hot before quenching that the steel was actually burnt (ask any blacksmith to show you how this is possible – you see sparks start flying off the steel as it sits in the forge, and the next thing you know, there’s a whole lot less steel in your part), and after steel has gotten so hot as to be burnt, it is irrecoverable. There is no amount of annealing or normalization that will recover the steel to a useful state; it’s gone, toss the part.

jwm above makes note of the danger of ‘kabooms’ with the pre-800K 1903 receivers, and that is also correct. The entire episode is recounted in loving detail in Hatcher’s Notebook. Springfield Armory (the real one) and Frankfort Arsenal both recommended destruction of the sub-800K 1903 receivers because they found there was no way to correct the heat treatment issue.

4. Geoff PR has the answer correct above. The Garand and M14 receivers were made from War Department equivalent 8620 steel or AISI 8620 (respectively) and their outer case was in the Rc 52-55 range, with their interiors being in the Rc 42 range. To this day, if you want to case harden (or color case harden – the Garand and M14 receivers are case hardened, but not color case hardened) – you can achieve case hardening by heating the part to above-critical temps in a sodium cyanide salt bath, hold it there for a period of time (more time means a deeper case layer) and when the time needed for the specified case thickness has passed, quench in oil. It’s fast, reliable and effective. 8620 is also often used for transmission gears due to this ability to case harden quite nicely.

I should NB here that no one should attempt to case harden their own receivers (or anything else) in a cyanide salt bath; rather, just send what you need done out to a professional heat treatment shop. There are several who have FFL’s and do gun work and do nice work. The fumes coming off cyanide salt baths can prove fatal for those who are trying to do this as an ad-hoc heat treatment without sufficient ventilation hoods. Better to use charcoal pack case hardening in the small shop, and get some colors to boot.

5. You’re correct again – 6061 welds better than 7075. 7075 is sneaky stuff – it looks like it welded up just so nicely… until you put some actual stress on the joint, and then it breaks out, very much like the way welds break out when you’ve used something like 7018 rod to weld up an engine block.

The mechanism of failure is due to the particular alloy that is 7075, which creates micro-cracking in the HAZ as it cools off, rather than the precipitation hardening at the edge of the HAZ, as happens on cast iron. The net:net result appears to be similar – cracking out at the edge of the HAZ.

The only application where I’d weld 7075 is perhaps a mold.

The practical implication of this (as I’ve had to explain to two AR-15 owners) is that if you damage your AR-15 receiver to a point where it starts to crack, and you want to have a TIG guy (like yours truly) “just weld it up before the crack spreads,” well, you’re out of luck. The crack is going to spread, and there’s not much I, or anyone else with a TIG welding rig, can really do about it. Matter of fact, I believe that as soon as I lay an arc on it, the crack is likely to spread faster, not slower, than if I’d left it alone, but that’s just my personal opinion at this point; I don’t have statistically sufficient data on which to make that statement, just a hunch from a couple of results.

6061 welds up quite nicely, but the HAZ won’t machine well. Matter of fact, it will machine like cold, damp, chewing gum. The heat treatment of 6061 is part of what makes it machine so nicely.