Gas Port questions

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11 years 10 months ago #15989 by gdcpony
Gas Port questions was created by gdcpony
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. Should I make it a standard .308 size? What is that even? 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 ( Buttstock )
24" SS Bull fluted and threaded for brake ( Barrel source )
Muzzle Brake ( Intended Brake )
Extended handguard ( Handguard )
Adjustable Gas Block ( Gas Block )

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11 years 10 months ago #15994 by LebbenB
Replied by LebbenB on topic Gas Port questions
Size-wise, .257 is very close to 6.5mm. I would look at cartridges like 6.5 Grendel to get an idea of what the gas port size should be. IIRC, the Grendel gas port on a 20" barrel is around .90 inches or so.
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11 years 10 months ago #16006 by gdcpony
Replied by gdcpony on topic Gas Port questions

LebbenB wrote: Size-wise, .257 is very close to 6.5mm. I would look at cartridges like 6.5 Grendel to get an idea of what the gas port size should be. IIRC, the Grendel gas port on a 20" barrel is around .90 inches or so.


You mean .090 right?

The 257Roberts+P generates about 50k pressure, also about where I estimate my reloads are. The Grendel is the same. Good call!

With an adjustable block should I error on the large size and if so how far? .1? I am just making certain since I won't be able to afford a second go at it.

I am hoping to get this ready by May with an eye towards the VHA Jamboree. That is 20rds at 4-7" balloons out to 400+yds in a time limit.

Regardless, it ought to be a fun, almost recoil free, and one of a kind rifle to shoot. I can't wait to see it come together.

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11 years 10 months ago - 11 years 10 months ago #16011 by LebbenB
Replied by LebbenB on topic Gas Port questions
I would lean toward a gas port on the smaller side, simply because material taken away when drilling the port can't be put back. Since you're going to run an adjustable bock, why not size the port in the middle of the size range and use the block for fine adjustment? It'll also give you room to adjust downward when gas port erosion rears it's ugly head.

It just occurred to me - You might also want to re-look using a .308 buffer and spring. It might be too heavy and stiff for .257/6.5mm.

On a different tangent, why did you select the Mako buttstock and Cooley brake?
Last edit: 11 years 10 months ago by LebbenB.
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11 years 10 months ago #16022 by 13fcolt
Replied by 13fcolt on topic Gas Port questions
I second erring on the small side for gas port size. These rifles tend to be over gassed in the first place, so getting a head start on working up to the ideal port size is a plus.
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11 years 10 months ago #16098 by gdcpony
Replied by gdcpony on topic Gas Port questions

LebbenB wrote: I would lean toward a gas port on the smaller side, simply because material taken away when drilling the port can't be put back. Since you're going to run an adjustable bock, why not size the port in the middle of the size range and use the block for fine adjustment? It'll also give you room to adjust downward when gas port erosion rears it's ugly head.

It just occurred to me - You might also want to re-look using a .308 buffer and spring. It might be too heavy and stiff for .257/6.5mm.

On a different tangent, why did you select the Mako buttstock and Cooley brake?


The list is constantly evolving as people give me input. The buttstock will remain as is for now so the Mako is not being ordered. The hand guards are going to from JP Ent now (15.5"). The brake is still the same as I have heard nothing bad about it yet. I am hoping I can call my own shots with a gun that will likely be 14# and in a caliber that kicks about as much as a .243.

For a number on the GP size. In my constant- and mostly fruitless reading- I did find .07 thrown around a good bit. This was referencing the .243 I believe. Since my bullet target weight in 90-100gr, might it make sense to start there?

You all make good points, but I am not knowledgeable enough to use the info wisely in this case.

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11 years 10 months ago #16116 by LebbenB
Replied by LebbenB on topic Gas Port questions

For a number on the GP size. In my constant- and mostly fruitless reading- I did find .07 thrown around a good bit. This was referencing the .243 I believe. Since my bullet target weight in 90-100gr, might it make sense to start there?

Makes sense. I can't recall the chamber pressure that .243 generates in an AR, might wanna e-mail the 50 pound brains at Armalite and ask them.

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11 years 10 months ago #16517 by gdcpony
Replied by gdcpony on topic Gas Port questions
Well, I am sending off my barrel specs today.

Chambering .257 Roberts
Length- 24"
Stainless Steel
Standard DPMS heavy contour (.936 under block/ .925 muzzle)
Rifle Length Gas Port- .070 size
5/8 24 Threaded Muzzle
Fluted

We'll see their lead times and price on this.

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11 years 10 months ago #16523 by LebbenB
Replied by LebbenB on topic Gas Port questions
Hope it works out well for you. Post some "in progress" build pictures. I'd be interested in seeing them as well as accuracy pics once it's done.

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11 years 10 months ago #16524 by Sharkey
Replied by Sharkey on topic Gas Port questions

LebbenB wrote: Hope it works out well for you. Post some "in progress" build pictures. I'd be interested in seeing them as well as accuracy pics once it's done.


x2.. :thumbs:

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