gunsmithing: which steel for blowback bolt?

Ask on the gunbroker.com forums lots of gunsmiths post there including me and somone would surely know. Theres probably quite a few choices for what you could use as for hardness I don't know off hand I could probably find out but it would take time, which I currently don't have. Sorry.

P.S. have you thought about just buying a new bolt? Would probably be cheeper time wise in the longrun.

Don't know if this is any help but Hatcher's Notebook shows a M1 Garand bolt of 8620 treated to Rockwell C 55 to 59 and the M1 Carbine bolt of 4140 treated to Rockwell C 48-54. I thought I remembered Thompson info in the book also but was incorrect.

Steve.

I'll take a look over there, thanks.

Well, cheaper isn't the point. It's a fairly complex part, and I'd like to test my skills by making one. By using something that's easy to test, I can spend some quality shop time and make something useful in the process. The finish quality on the Kahr manufactured bolts is not very good, and I'm hoping to improve on that as well.

Thanks, Dave

Ah. Of the two, the carbine's is probably more relevant - I'm not locking any lugs into anywhere, And, I have 4140, so it's a good starting point. Thank you.

Yes, I thought he wrote something about it also. I've got I think all of his books, so it may take some digging.

Dave

I think you would be golden with 4140

Gunner

"The importance of morality is that people behave themselves even if nobody's watching. There are not enough cops and laws to replace personal morality as a means to produce a civilized society. Indeed, the police and criminal justice system are the last desperate line of defense for a civilized society. Unfortunately, too many of us see police, laws and the criminal justice system as society's first line of defense." --Walter Williams

Good to hear. Any thoughts on where I should shoot for on the hardening?

Dave

Have you developed your methodology?

In my recollection, the 1927A1 bolt is a bar stock part , rectangular in cross section with a round turned projection in the front about 2~3" long that accomodates the ctg head at the end.

If you heat treat after machining you will be susceptible to warping, dimensional change, and scaling of your machined surfaces.

So, are you planning on surface , slot, & cylindrical grinding after HT?

I don't believe the factory bolt was heat treated, really no need for it. It's an unstressed part for a low pressure ctg. I also believe the extractor is staked in place, furthering the need for metal that is suitable for deformation.

I think any un-heat-treated steel available from 1018 to 4140 would fit the bill.

Tony

50-52//just as a gut feeling.

Is the sear carried on the bolt?

Gunner

"The importance of morality is that people behave themselves even if nobody's watching. There are not enough cops and laws to replace personal morality as a means to produce a civilized society. Indeed, the police and criminal justice system are the last desperate line of defense for a civilized society. Unfortunately, too many of us see police, laws and the criminal justice system as society's first line of defense." --Walter Williams

Yes.

Looking at it further, I was considering making the cylindrical part seperate, and inserting it into the rectangular part with a low-clearance fit and pins.

In the current production at least, the extractor is held by it's own spring tension into a t-slot milled into the bolt. I'm trying to think where I've seen another like it but can't think of one offhand. But to remove the extractor from the bolt, you merely lift it "outwards" with the head of a cartridge and pull it forward. It doesn't come out in use because the outward motion is constrained by either the receiver, and/or the spring tension of the extractor itself.

Any thoughts about the 2-piece fabrication idea? Or, of I won't be heat treating, then really could just do my offset with the 4-jaw; everything else other than that extractor slot is pretty straightforward.

Thanks, Dave

(4140)

No, but of course that leads to the followup question. The firing pin on the 1927a1 rides with the bolt, and is a piece of 1/8" flatstock cut to an interesting contour. I made a new one (broke the firing pin tip off of the original one due to my sear spring reduction experiments). However, after only a few hundred rounds, the firing pin where it mates with the sear, is showing rounding. It's unknown alloy. An point in trying to harden that, at least in the part which meets the sear? I'm completely ignorant of heat treating, which is pretty obvious at this point, but I'm thinking get that local area of the firing pin up to cherry red or so, and quench in oil?

Without knowing the alloy, I know it's a crapshoot. But knowing the person I got these materials from, if it's newer than 1970 technology I'd be shocked. Any thoughts?

I would make it from one piece. A press fit is susceptible to seperation from the slamming / reciprocating motion of the bolt.

Tony

The firing pin is a piece of flat stock affair with the tip slightly rounded where it strikes the primer. I would use mild flat stock & case harden for

45min~1 hour in Kas-en-it @ 1600d F. You will need to harden the part since it is subject to high wear, especially where the sear catches.Since this part is relatively simple, you can straighten out any warpage that occurs. And the soft core will reduce the possiblilty of a sudden failure during usage, an important feature in any firearm ignition system.

Of course you can get any of these parts from GPC or other sources, but that might not be as fun.

Tony

Dont bother with Mystery Metal. Get a piece of O1 or similar and make a firing pin. If you need one..Ill dig though the stacks.

Btw...as I recall..the Thompson was never meant to have a light trigger. You yank it, not squeeze it.

Gunner

"A prudent man foresees the difficulties ahead and prepares for them; the simpleton goes blindly on and suffers the consequences."

- Proverbs 22:3

Yup. I came to that conclusion over the weekend myself. I don't owe this metal anything, and if I don't know what it is, or how it behaves, spending an afternoon on something complex only to have it be unusable (the bolt, not the firing pin) would be silly, when known material is readily available. Relative values and all that.

So what do I do with this accumulation of unknown steel then? And, more importantly, why am I asking _you_? (written with a smile, I assure you)

can't/won't train myself to do that to a trigger. It's reasonable as reworked which is as good as I can expect to get. I learned enough making the first pin out of "mystery metal" that I now know how to jig it, which order of operations, and so on. So I'll get some of the right stuff and make one just like it but better. (and save this one as a spare, because it does work).

I'll post some pics if there's anything picworthy.

Dave,

I bought some of this steel from Brownells about 10 years ago and have fabricated firning pins from it that have survived many 1000's of rounds in half a dozen rifles:

It's pricey, but it works. I had to replace the all firing pins in my Rem 788's because the originals were flat sided and kept eating up the firing pin springs and jamming up the actions (wouldn't open). This is good stuff and a little goes a long way. It machines very well.

FWIW, I tried drill rod and had breakage problems with it. Probably because of my lack of skill at heat treating. This stuff requires no heat treatment.

Highly recommended.

Mike Eberle> > >

Ah, thank you. The bolt is about half finished and I'm getting ready for the firing pin, so this advice is timely. The firing pin in the

1927a1 semi-auto tommygun is a piece of 1/8" flatstock, bandsawn or milled to a bizarre contour. I'm not happy with the limited engagement with the sear, also it has vertical travel which I've identified as the source of the trigger creep. As designed it's unavoidable.

So, of course, I'm redesigning it. I plan to use a firing pin assembly very much like the 1903 Springfield's. If I can adapt any of those parts without modifying them, so much the better. Wider contact between the firing pin assembly and the sear will give me better wear characteristics - after only 1000 rounds or so, the wear is noticable on that narrow spot.

Excellent. I'm having good luck getting a nice finish with the 4140 Prehard that I'm using for the bolt, but it's some chewy, stringy stuff. Finish cuts are very light with sharp tools and it's not horrible, but heavy cuts get really fugly really fast.

I have neither the equipment, nor the knowledge for heat treating, so this sounds perfect.

Dave Hinz