Reciever Roughing allowances before HT?

Hello all, I'm looking for some experienced advice on a fabrication question. I'm building a single shot shell holder style 50 BMG (ala ULR 50) and incorporating a Remington 700 trigger take advantage of a couple spare Timney's I have lying around. I'll be using 4140 HTSR steel for the bolt and reciever.

Although it pains me to say this, I've been a lurker on this forum for too long not to throw out a few pre-emptive answers to the inevitable questions that follow a post about designing your own firearm haha.

- Yes, I am fully aware I can easily purchase a fully functional rifle from a store and it will shoot just fine. The point of this project is because I find it immensely enjoyable and satisfying.

- Yes, I have done the math on my initial design (utilizing the excellent article written by Dan Lilja on bolt lug strength and his formulas for calculating shear, flex, and yeild). The SF under a worst case pressure event, case head seperation of a proof load, is around 4.99 (don't have my math in front of me). To chase safety even further I will also throw my final design through a FEA upon completion.

- While I'm not a professional gunsmith / engineer / heat treater, I have been quite successful in all my blue printing, rebarreling, and trigger work so I'm confident in my skills to safely and accurately make my dreams a reality.

- Asking for real world advice is simply augmenting information I've already gathered from other sources. As any tradesman can tell you, real life can often depart from an engineer's well laid plans. No offense to engineers hahaha.

Now down to the real question I'd like to throw out there. How much extra meat should I leave on my work before I send it off to get heat treated (professionally)? I've read varying reports on warping that occurs during the process. Some say that you should leave 0.025 for final fitting, and others have claimed to see negligible warping after HT when using 4140 HTSR. Initial hardness claims to be around 32 RC, but I want to go to 40 on the bolt and 38 for the reciever.

For those of you that have been down this road what have you experienced for warp and how much have you allowed for finishing?
Should I finish my barrel threads to full depth?
Should I bother tapping the other smaller holes (scope rail, trigger hanger etc.)? I'm worried that with the final RC I'll get into hot water trying to use a small diameter tap.

Since my design excludes an ejection port and full length raceways it will be fairly robust and simple. Thanks for your opinions!

Don't know about laws in Canada, but in the US a heat treater has to have a FFL to treat a receiver.

I suggest purchasing 4140 annealed, heat treating before any machining while it is just a bar and not a receiver. If you use already ht material, you have to have it sub annealed before it is rehardened, adding to the cost.
All that avoids warpage (mostly) and scrapping a receiver after all that hard work.

I make my receivers from 4140 hardened to 36 Rc, and they aren't hard to machine even with HSS, of course I use carbide when I can. But any of the oddball tooling is HSS because I can't make it from carbide.

https://www.practicalmachinist.com/...er-raceways-lugways-ya-someone-had-do-349085/
^ Thread on broaching 4140 36 Rc.

You might want a bit more of a spread in the hardness between the receiver and bolt, usually about 5 points or you can get galling.

I worked in a tool and die shop for a couple of months and .01 left for finishing was common with low distortion steels like A2 and D2. Do not know if that means anything for 4140.
Going with Doug's comment about a license for the heat treater a roughly turned tube does not look like a receiver yet. I would not think 40RC 4140 would be too difficult to finish machine.
MI Bill lurks here and has much experience. I would pay attention to his observations.

Thanks for the "intro" FredC. Sounds like OP and I have very similar backgrounds coming into our .50 projects. Although I do have an engineering background, I also have friends in the defense industry with relevant experience ("BIG guns"). I relied heavenly on their input during my project.

My action was cut from 4140 @Rc34-36 and the bolt head was Rc38-40. I preformed ALL operation in the hardened state so there was no heat treating required after machining. The vast majority of the operations were with HSS tooling. 4140 in both cases cut very well! I will use the same materials for ALL future actions I make.

As for DougW's comments on a greater spread in the hardness... The "boys" at GDLS said it was unnecessary... A few gunsmiths I talked to said "a couple points" was all that was needed to prevent galling... I never received advice of more than a couple points difference in the hardness of the steel before I started my project and have not seen any signs of problems. With that said I think it important to note that although MY receiver and bolt MY be no more than 2 points different, they may also be as much as 6 points different! Although here is nothing wrong with using 4140 @ Rc40, I don't see the need to go that that hard. Anything above Rc30-32 on a receiver (IF PROPERLY ENGINEERED) is fine. That can give you a little more room to get a few more points difference in the steels if you wish to follow Doug's advice. Most heat treaters will only give you a 2 point range. Committing to a single point of hardness when you drop off a part is something I have never seen any shop commit to. SO if you drop off your steel and ask for Rc38-40 for one part and Rc40-42 for the other there is a chance you could get back 2 pieces of Rc40... And I can not comment on how 4140 Rc42 may cut.

I don't pretend to know the laws in Canada, so I don't know if there would be an issue with doing a heat treat on a roughed out action. In the U.S. it is not LEGALLY an issue. However, very few places want anything to do with preforming ANY operation on firearms parts! I ran into a problem having magazine floor plates anodized because of that. IF you still choose to go that route, you may want to make sure you heat treater will do the job on a rough action.

The ONLY part I had heat treated after machining was the cocking piece. It is a small part I made from D2 and warping was NOT an issue.

Excellent! Thanks very much for the advice!

I haven't had the opportunity to work with 4140 in the higher RC range but if its machinablity isn't a deal breaker then I think I will rough out the IDs and ODs then complete the lug abutments, and other critical points after HT.

And as far as I can tell there are no restrictions on firearm fabrication in Canada if you are not selling the firearm, and it does not fall under a restricted/prohibited classification (extreme short barrels, Overall lengths, full auto etc.) when completed or at any time during the build process.
That is a bit of a vague statement (as with many of our firearm laws) so don't analyze it as gospel haha. The real point is that you can build a bolt action long gun in Canada for your own personal use. Surprisingly it would also appear that no inspection by a CFO (chief firearms officer) is required, and no registration numbers are required. I will making a serial number so as to identify in case of theft. Also it is rather rewarding if your end product looks legit enough that people ask "Which company makes that?"

Thanks again! Once I start making chips I will make a build thread with some pictures. I've been pretty miserable with cataloging my various previous works so perhaps this will hold me accountable