Modifying AR15 Bolt

Im in the thinking process of a ludicrous idea. I want to build a light weight bolt action upper that will fit on any standard AR-15 lower. The bolt design lends itself well to a straight pull bolt action design, imo. No reason for making it other than for fun.

I want to chamber it (here is the ludicrous part) in 256 Winchester Magnum. The 256 WM, for those unfamiliar, is a 357 magnum case necked down to .257" and plugged with a 60 grain bullet. Slings it down range at about 2500 fps out of a 16" barrel.

This leads to my title question. The 7.62x39 and Grendel bolts have the correct .44x" rim diameter but the face is anywhere from .125" to .136" and the 256 WM has a rim thickness of .060".

My initial thought was to weld the face up a bit then cut the bore back to .060" on my lathe. But that will destroy the heat treatment on it and i have no means to restore that hardness. And i would need a .060" longer firing pin and ejector.

My second thought was to make up the .065" difference on the face of the breech. Essentially facing .065" off all but a small ring around the chamber that would fit into the bolt face with the cartridge rim. But that leaves me with a small ring only .032" thick around the base of the case. This sounds much easier than the first method.

I thought about taking .060" off the front of the bolt, lugs included, and spinning the barrel in another .060". But with a 50k psi cartridge rating, im not so sure thats wise. This seems like the easiest route

Thoughts? Criticisms? Am i fool and should i just build one in a standard caliber?

Jim Watson said:

Single shot? Or what magazine?

I'd wildcat it, .25 Blackout.

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I was gonna try a 7.62x39 mag, due to the curve, and try to modify it with a spacer block to keep the cartridges to the rear. I figured i could mill a feed ramp of sorts into said spacer. Might have to make some spacers for the sides to make it single stack.

Hmmm 25 blackout... I wonder how the 256 WM would do with a 120gr partition... The 256 WM is about a half inch shorter than 223 afterall.

The whole reason for this is to use the 256 WM as i have a set of dies and projectiles but no gun. A bolt action AR is something anyone can do by simply flipping the gass block around and removing the gas tube. But in 256 WM and with a forward mounted left side bolt lever, thats a little different.

Lol i suppose a contender barrel would be cheaper...

So i was doing some reading last night and stumbled upon something intriguing. A certain guy has been experimenting with a 256 AR for about 10 years now, though very very slowly. It seems he has too many irons in the fire (like me lol) and hasnt done much with it. But he got me thinking. He has done what he calls a 250 Short Savage (257 ARS) made off a 7.62x39 case and a 256 WM Rimless, based off the .223 case. It just so happens that .223 has a case base diameter of .376" and the 256 WM has a case base diameter of. 381". So im now imagining running 223 cases into a 256 WM die after trimming a proper amount off while leaving the proper length of case below the die to make up for the bolt recess.

This would allow me to run with a standard bolt and use the easily accessed 223 brass as my parent cartridge. It would also negate the chance of rim-lock and lessen the chance of a FTE. Also would allow me to use standard mags.

Douglas can make me a barrel, pre threaded for the AR barrel extension and chambered for 256 WM. Finding a barrel maker that has a 256 WM reamer is not easy lol.

Im thinking 1-10 rifling twist. This would allow for some heavier bullets to be toyed with as opposed to a 1-14 twist.

gpb said:

I agree with post #8 by BigBlue 94. I would run some 223 cases into a 256 WM die and see what happened. Trim to length and possibly thin the case neck a little. This should produce basically a rimless 256 WM case.

Finding 256WM chamber reamer may be a challenge, but possible to do. Sounds like nice little project.

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Yeah i read about the rimless version and a 223 case parent. I looked up the specs of each and the little lightbulb above my head flickered on

4D has the rental reamer available, and Douglas actually has the reamer and will chamber it. I think a Douglas barrel will be my christmas present to myself lol.

David Hoback said:

I’m not a fan of Sub-Cal rifles. For me it’s pistol cartridges in a pistol, and rifle cartridges in a rifle! But….., I enjoy repairing, modifying/upgrading & most of all, BUILDING firearms as much (if not more) than shooting!

Quite an intriguing endeavor. Keep us informed as I’m interested with progress.

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A 256 win mag flings a 60 gr bullet at 2700 fps from a 24" barrel.
A 223 hits 3100 with a 60 gr bullet from a 24" barrel.

Not a ton of difference there, and im essentially adding .060 to the length of the 256 WM case body. Which will allow a bit more case capacity and velocity.

I just like to play with obscure calibers and build and tinker. Though as to the sentiment of your post, i sure as heck dont want a 9x19 rifle!

Ran a piece of Aguila 223 brass, untrimmed, into the 256WM die. First one had a dent in the case mouth which folded inwards. I picked one with a better case mouth, dipped it in Forster HP lube, and pecked it up into the sizing die. Each peck i inspected and went a bit deeper.

At the end i came out with a long necked, randomly placed shoulder, 256 WM Rimless. So in theory this should work just fine. After fireforming to the chamber, ill neck size only.

Here it is next to a .223 Rem

David Hoback said:

Ok, well, now I’m a bit lost… you said in the beginning the 257 is “357 Magnum” resized. Below I posted a pic of a standard 357 next to a 223. Your “resized” 257 WM is MUCH longer than a 357 case. The 357 doesn’t have nearly enough length to form the shoulder & smaller neck. So I think your previous statement was a little misleading.

View attachment 1106566

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256 WM is formed by running a standard 357 Mag case into the 256 die.

The case i show above next to a standard 223 case was an identical untrimmed 223 case that i ran through the 256 WM die to an unspecified and unrecorded depth just to see how the case would handle the reforming process. That modified case in no way represents the final case design. I will end up cutting the 223 case at the base of the shoulder before sizing in the 256 die then final trimming.

The purpose of said experiment was to see if a rimless 256 WM could be achieved for use with a standard AR bolt.

^^^ basically the same as 223. Chop and neck to 257.

I should add i may have a contender barrel in 256 WM coming my way. Which would kind of negate my need for this. But after staring at that odd case yesterday, the prior idea of a 25 "blackout" still intrigues me. And the trial case above looks about right after getting rid of half the neck, for a 120 or 130 gr bullet. I do have some slightly shorter than standard120 gr Nosler partitions that may prove useful.

Ive seen the 25-45 sharps and it seems okay, but is pretty well length limited to the 87gr HotCors.

We will just have to see how this turns out.

David Hoback said:

Ok, well that’s misleading. In other words, you didn’t finish size the 223 case.

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No i didnt and i clearly stated that in the post with the picture; long necked and randomly placed shoulder.

Im well aware of the approximate half inch length difference.

David Hoback said:

I meant only the picture itself being a “misleading look”. Not your action. That’s all. Something can be misleading without any wrongdoing after all. Sorry for confusion.

Guess I didn’t use enough smiley faces to express the jest?

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No worries, its hard to relay some stuff correctly over the internet, especially emotion and sarcasm.

A different forum ive been a member of for 15 years changed from the old BBS smilies to new emoticons a couple years ago and i still find myself wishing for the old ones. They were so perfect for relaying things unable to be done with text. Then they had to go and change crap. Who the heck needs a unicorn or Albanian flag emoji on a ford bronco forum!?!

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I did end up winning an auction for a 256 WM barrel for my contender.

Is there any cheap/free program out there where i can fiddle around with case dimensions and load approximations? The fact that 458 socom uses a .473 bolt face has me really pondering a more powerful 25 caliber wildcat for the bolt action AR. Like a blown out and shortened 250 savage.

Here we go, as close to .256 WM Rimless as i can get without a chambered barrel. I cut the 223 case off at the shoulder and trimmed to length. For the moment, that trimmed length was from the leading edge of the 357M case matched up with the leading edge of the rim recess of the 223. 60gr. Speer Spire Point loaded to what visually looked correct. IMO this is what the PS90 should have been chambered in


With a proper feed ramp added to a 5.56 mag, it will feed.

And that has led to this, 2.255" OAL, 100gr Speer SP. 223 case had the neck chopped off. New neck is .200" long and the bullet seated ~.240" deep