4140 qt

[COUSIN DANIEL]

I am thinking of build a 308 bolt action rifle and was considering using 4140 qt to avoid heat treat as it is already 28-32 rock well.

I have no experience building rifles so of this is a disaster please let me know.

[Hick]

4140 in firearms is a very normal application-- but you might need someone to do a stress analysis to decide if you can really get away without heat treatment.

[COUSIN DANIEL]

4140 in firearms is a very normal application-- but you might need someone to do a stress analysis to decide if you can really get away without heat treatment.

I agree 4140 is an appropriate steel for a firearm.

I am trying to avoid having to deal with a heat treating facility as I fear they will not heat treat my part.

[IllinoisCoyoteHunter]

Why do you fear they will not heat treat your part? When you go pick it up have them show you on their hardness tester.

I would *guess* you’d be better off somewhere in low to mid 40s RC.
Don’t take my word for it. Maybe someone with more knowledge will come along.

[ulav8r]

What little info I have found on pre-hardened 4140 gives a range of 26-32 or so. 40 to 45 would be preferred. Seem to remember when I was looking for that info at least one source indicated that the pre-hardened in larger diameters was usually at the lower end of the range. If the bolt is also of 4140 it should be a little harder to lessen the chance of galling between the bolt and the action. Consider using S7 for the bolt and use a reputable heat treater. Study all the information you can find on various metals you might use and specify what hardness you want for each component.

[M-Tecs]

I know people are using 4140PH for some rifle action applications. The 28-32 RC is on the lower side. Too soft recoil lugs setback is possible. The larger the lugs the less of an issue. I cannot answer how soft is too soft.

https://www.africahunting.com/threads/action-rockwell-hardness.60866/

https://www.haverkampactions.com/technical.html

[Reg]

4140 in its normalized state is quite soft Rc 8-10 or so. It’s tough but very workable. It can be heat treated up to Rc46 but only needs to be Rc38 for a good receiver but at this Rc38 it’s quite difficult to work and most small shop machinery will find it quite a challenge.

[M-Tecs]

Why do you fear they will not heat treat your part?

It requires they have an FFL. Most HT'ers will not deal with HT'ing actions. There are places that will do actions. The only one I have used is Blanchard Metals https://www.bmproc.com/heat-treating

[IllinoisCoyoteHunter]

HT before machining and they will never know. You will also get your best results this way.

[Willbird]

IMHO you could design an action that was safe made from 4140 pre heat treat, but your lug area will need to be larger than if the action was at 44-46 rockwell. The whole action might end up 25% bigger ??

Bill

[COUSIN DANIEL]

I can't figure out how to post pictures of the blue prints.

If anyone knows how to post pics I will be glad to email them

My lugs will be 3/4 to 1 inch thick

[M-Tecs]

I can't figure out how to post pictures of the blue prints.

If anyone knows how to post pics I will be glad to email them

My lugs will be 3/4 to 1 inch thick

For setback thickness means nothing. Surface area is what will limit setback.

[Willbird]

For setback thickness means nothing. Surface area is what will limit setback.

Agreed. The thickness of the lug is what determines if the lugs will be ripped off of the bolt I suppose. At 28-32 they would need to be some % thicker than say a remington 700 lug, exactly what % is the question. More or less it will end up looking like a 50bmg sized action for a 308 bolt face made in 28-32 4140 I bet.

[IllinoisCoyoteHunter]

With my limited knowledge, I’d have to agree with what’s been stated about. Not saying it can’t be done, but I’m pretty sure most all action manufacturers are heat treating their actions for a reason. Even if you can get it in the upper 30s to lower 40s you will be better off. Very tough, a little hard but not brittle at all. Sounds like a very cool project. Please keep us in the loop with lots of pictures.

[John Taylor]

When I was helping to build 50 BMG bolt actions we were looking at 37C for the frame and 38C for the bolt. When you have two parts sliding together it is always best to have one a little harder than the other to help against galling.