An odd AR10 build?

gdcpony

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Jun 20, 2011
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Sherrodsville, OH
I was yesterday looking at a forlorn 98 action and adding up the price to turn it into the heavy barreled varmint rifle I dream of. Boy, did it add up! I figure I will have over $1200 in it by the time I am done.

So I look over at my current project- another Mauser- in my favorite caliber of .257Roberts. I plan to re-barrel it to the Ackley version upon wearing the current tube out. Beside it sat my trusty AR15 which as a Marine I love dearly. I love both actions (Mauser and AR) and calibers (.223 and .257)! I have both a bolt and semi .223, if only I could get the same in .257.

Then it struck me: Why spend so much to pretty much duplicate a rifle I already have? Why not look into an AR in .257? I swear there were angels singing! (ok not really, but you get it).

So I have begun researching the possibility of an AR10 build in .257Roberts. I have come up with these issues:

1. The .257 is a "mid length" cartridge. In my preferred 90-100gr bullets it is 2.780 which should be fine against the AR10's intended cartridge (the .308 I loathe so much) which is around a 2.8 OAL. So the mags should work. I just will have to keep away from the heavier 120gr bullets. Can anyone out there add anything to this? Will the longer case of the .257 eject cleanly?

2. Most people and even a few gun makers (particularly those I would have to deal with) don't even know what the .257Roberts is. So the barrel will be hard to source. This is especially true since I would want it to start out in the Ackley version. Any ideas there?

3. My wife will kill me once she finds out I plan to get rid of this derelict action and the Boyd's stock I have for it in favor of another black gun! How do you all convince your wives you NEED to build a gun just to be different?

It will probably be a couple years before I can complete this rifle, but the lower will be gotten before it is illegal to do so (I fear a second term!). The workings of a standard AR10 would be fine given the rim of the .257 is a match and case size
 
Re: An odd AR10 build?

Big catch: Anyone care to post the measurement of the ejection opening on an AR10? I have been told the .257 case won't fit out through it. Big hindrance.

I would like to build a SASS clone as I got to train on it on my last deployment. Maybe a longer tube though to burn the powder held by the a fore mentioned case.
 
Re: An odd AR10 build?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: RufDog</div><div class="ubbcode-body">If the 257 cartridge was worth putting in an AR10 it would have been done by now. Now, if this is a romance cartridge brace yourself for a long drawn out money pit.</div></div>
Consider it a favorite cartridge. Money? Sure bit by bit, part by part, I can watch it come together.
 
Re: An odd AR10 build?

257 Roberts has a bit too much body taper, you'll have better chances of making an AI version work.

Port location, port size, gas volume, etc. are going to be working against you with a 257 Roberts AI. Making an AR run is a whole different ball game then a bolt rifle.

Possible, yes. Would I do it, no. I've done plenty of ARs in different flavors but as you go down in bore, slower burn rate powders the dynamics of the system change a fair bit.

If you are going to do a 257 Roberts AI, I'd go with somebody who has messed with ARs in "odd" calibers.
Might be worth talking to these guys.
http://www.ar-10-rifles.com/index.php
 
Re: An odd AR10 build?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: McCourt Munitions LLC</div><div class="ubbcode-body">257 Roberts has a bit too much body taper, you'll have better chances of making an AI version work.

Port location, port size, gas volume, etc. are going to be working against you with a 257 Roberts AI. Making an AR run is a whole different ball game then a bolt rifle.

Possible, yes. Would I do it, no. I've done plenty of ARs in different flavors but as you go down in bore, slower burn rate powders the dynamics of the system change a fair bit.

If you are going to do a 257 Roberts AI, I'd go with somebody who has messed with ARs in "odd" calibers.
Might be worth talking to these guys.
http://www.ar-10-rifles.com/index.php</div></div> Thanks! I saw .257Roberts listed so the Ackley version should be available.
 
Re: An odd AR10 build?

You might try getting a .257 barrel blank in an AR profile From Lilja Precision. They did a .284 barrel blank for my MA-TEN I built in 7-08. Then, call Dave Kiff at Pacific Tool and Guage and tell him what you are building and that you need the appropriate chamber reamer. If the .257 Roberts AI can be made to work in an AR platform its likely Dave will have the plans for the chamber reamer. Once you have the barrel blank and the chamber reamer, the location and size of the gas port will have to be determined. This last step will probably make or break the project. Find a good gunsmith and get his opinion and then find another one and get a second opinion. Good luck. Hope it works for you.
 
Re: An odd AR10 build?

Well, in light of recent tragic events, I went out and picked up a whole rifle. Carbine actually, a DPMS base model in 7.62. I know something is coming and wanted to make sure I had one before then.

I figure it will be used as is for a year or so (wife rule) then I can mod the living crap out of it. I have a list of parts already. Maybe a few will sneak in earlier. I just need to find the barrel!

I will put some rounds through it sometime this week hopefully and post the groups up. I hope it surprises me. Right now the trigger seems creepy but not heavy. The 16" barrel doesn't worry me, but it not being free floating does. None the less I should be able to wring some good groups out of it.

Handloading .308Win for a 16" barrel? Maybe light bullets and a powder on the fast side? Hmm... The horrible thought of loading for a new rifle. LOL
 
Re: An odd AR10 build?

.257 Roberts was my grandpa's favorite caliber. He built his own rifle chambered in it, a Japanese Arisaka action. He was a master machinist and foreman for Douglas Aerospace before it became McDonnell Douglas.

Anyway, he was always talking to me about .257 Roberts when I was a kid, before I could even read and write, pulling out micrometers and calipers, showing me cases and their measurements. I wish I could talk with him about reloading now.

My thought was also about the case taper causing issues with feeding from 7.62 mags. If you AI it with a more straight taper, that might help with feeding, as mentioned.

I would look at AR10 gas port diameters for the .243 Winchester and .260 Remington, and see what the smallest one is, then work up from there with your preferred powder and bullet weight range, or go a little on the large side and control with an adjustable gas block.

I have also thought about a .257 Grendel, but gave up on it since I like 6.5mm so much more.
 
Re: An odd AR10 build?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: LRRPF52</div><div class="ubbcode-body">.257 Roberts was my grandpa's favorite caliber. He built his own rifle chambered in it, a Japanese Arisaka action. He was a master machinist and foreman for Douglas Aerospace before it became McDonnell Douglas.

Anyway, he was always talking to me about .257 Roberts when I was a kid, before I could even read and write, pulling out micrometers and calipers, showing me cases and their measurements. I wish I could talk with him about reloading now.

My thought was also about the case taper causing issues with feeding from 7.62 mags. If you AI it with a more straight taper, that might help with feeding, as mentioned.

I would look at AR10 gas port diameters for the .243 Winchester and .260 Remington, and see what the smallest one is, then work up from there with your preferred powder and bullet weight range, or go a little on the large side and control with an adjustable gas block.

I have also thought about a .257 Grendel, but gave up on it since I like 6.5mm so much more. </div></div>
Yeah feeding is a fear, but as you mention, the port size will be key to this project.
 
Re: An odd AR10 build?

I am about to order a barrel in .257Roberts for my LR308. To do so the smith needs a bit of info on the gas port. He needs the length and size. I told him to go with rifle length, but size is a mystery to me.

I have been advised to look at other rounds similar and the best would be the Grendel. The same pressures, similar bullet, and such. The size for those was listed as .090? I though about erring on the large size with the adjustable block, but should I? If so, how much?

I am looking for any input as I am planning to build this next (actually order the parts) next month.

Thanks,

So you can better gauge this here is my build plan:
LR-308 Lower
collapsible stock ( Butt stock )
24" SS Bull fluted and threaded for brake ( Barrel Source )
Muzzle Brake ( Brake )
Extended hand guard ( Handguard )
Adjustable Gas Block ( Gas Block )
 
Re: An odd AR10 build?

I'm going to say do NOT go .090" to start with. You can't put metal back in that hole, can you? If you want to see where this comes from, go over to the NJ high power website and look up AR10 stuph, and yes that's how it's spelled:

Gas hole diameter - how big is too big (or small)
Here's some sizes that worked.

.243
4 inches down, .077, AR10 spring, H1000, R-22, IMR&H 4831
1.8 inches down, .067 to .073, AR10 spring, powders between IMR4350 and H1000

6mm 22/250
1.8 inches down, .082 - .089, AR15 spring, powders like Varget

Gas hole timing and barrel length
One case: "barrel lengths were 26" and 27". Either length worked fine and there was no noticeable velocity difference between the two lengths. We even chrono tested one at 27" then cut it off to 26" and there was no change in velocity - which told us that by the time the bullet got through 26" of barrel, it was going about as fast as it was going to go. The 30" bolt gun barrels we chrono tested were also no faster than the 26" AR-10 barrels. As far as gas port location, we settled on 1.8" further forward from the standard location. We tried 4" further and it worked fine (with a larger barrel port size) but in cold weather, and with some powders, there was some minor vapor condensation noticeable around the gas tube inside the receiver. The condensation did not affect anything but we figured some people might not like the way it looked."

Gas Tube - fabrication
Gas tubes have been fabricated from seamless stainless brake line, but these proved to be soft and corrode. Use a gas tube drawn for an AR15 spacegun from another manufacturer (try Krieger). This will put the hole 1.8 inches farther down the pipe. UPDATE: John Holliger (who else?) supplies Krieger with 2" over gas tubes. He informs me that 4" over ones will be available soon, too. Contact him directly at White Oak Precision.

I would suggest you call John Holliger or Scott Medesha, etc. Or just send the barrel to one of these guys that have done this type of work before. There's certainly a couple of good AR builders on this board.

EDIT: I had to put this in after the fact. Do you know what kind of killed the .257 Roberts? Chambering it in short action rifles had a lot to do with it. Which is exactly what you're about to do, as long as you realize what limits you're imposing on yourself, go for it. I don't see using the Grendel as a comparison for pressure either.
 
Re: An odd AR10 build?

I always just called them. I have bought a number of custom uppers from them over the years. Before the craziness, customs took 6 to 8 months. I doubt too many people will order from them out of reaction to the proposed gun bans. They are too specialized. If you decide to use them I don't think you will be disappointed. Every upper I have bought from them has sub MOA accuracy.
 
Well, the project is on hold for a bit while I am adopting my two stepdaughters. I have the block and tube in route, but the barrel- which will come from Blackhole weaponry- and the Handguard will have to wait until the DOD reimburses me for the adoption expenses.

Might be a long delay, but the girls have been mine for 8 years now, time to give them my name. Besides, look at them! What could be better?

http://sphotos-b.xx.fbcdn.net/hphotos-ash3/530836_495260803858373_2045840685_n.jpg

The younger (12 now) with her little "assault" rifle putting a hurting on a couple Marines in a clay pigeon busting contest

http://sphotos-a.xx.fbcdn.net/hphotos-prn1/46746_494991060552014_328510173_n.jpg
The oldest borrowing the rifle to be dissected for this project for a 500yd shot on a metallic. She did ok 6/10.

http://sphotos-a.xx.fbcdn.net/hphotos-ash4/398113_494990430552077_1771446723_n.jpg
Kala with her own rifle putting more holes in the "gong" (an old for rent sign). not so hot 3/10

edit: I guess images don't post? Oh well, if you want just click the links.